Food-Borne Infection

Sujatha Panikker. Cambridge World History of Food. Editor: Kenneth F Kiple & Kriemhild Conee Ornelas. Volume 1. Cambridge, UK: Cambridge University Press, 2000.

Numerous infectious diseases are acquired by the ingestion of contaminated food, milk, or water. Such illnesses have a worldwide distribution, although, predictably, the incidence is greatest in those countries where deficiencies exist in the provision of adequate sanitation and hygiene.

Yet outbreaks of food poisoning and other food-borne infections occur with frequency even in the most developed countries of the world. Here increasing demand for vast quantities of readily available food has been met by increased reliance on intensive farming techniques, industrialization of food production, introduction of various methods for preservation, and expanding networks for transport, storage, and distribution.

Many of the stages involved in the commercial processing and production of food provide ideal opportunities for the entry, maintenance, and multiplication of microbes. Consequently, several outbreaks of food-borne infection occur each year in developed nations, with the size of the outbreak reflecting the extent of the distribution of the product. Containment measures rely heavily upon early diagnosis and close cooperation between clinicians, medical microbiologists, and officials of public-health departments and are aimed at rapid detection of the source of infection and prompt action to curtail the outbreak. The measures taken must include immediate withdrawal of the incriminated article of food from sale to the public, dissemination of clearly worded information to consumers defining the source and mode of spread of the infection, and—where appropriate—guidelines on methods of preventing cross-infection between individuals.

Outbreaks of food-borne diseases occurring in Britain in recent years have included (1) salmonellosis spread via infected poultry meat, powdered baby milk, hens’ eggs, imported chocolates, and spaghetti; (2) Escherichia coli infection transmitted through hamburger meat; (3) botulism from yoghurt that was flavored with contaminated hazelnut puree; and (4) listeriosis from soft cheeses. Recognition of the phenomenal cost entailed in the investigation and control of such outbreaks has led to a heightened awareness of the need for effective measures to curtail the incidence of food-borne diseases. Accordingly, recommendations have been formulated for improving the microbiological quality of food. However, the subject is a complex one and problems continue to arise.

Natural History of Infection

Food and water may transmit infectious diseases when they contain critical numbers of live pathogenic microorganisms that are adapted to invade and colonize the body of the victim through the gastrointestinal tract. Typhoid fever and viral hepatitis are typical examples of such diseases. In some cases the mere presence in food of products of microbial growth, such as toxins, is sufficient to induce disease even though the microbes themselves may have been destroyed. These microbial toxins are relatively heat-stable, retaining their activity even after the food has been heated before consumption, as, for example, staphylococcal enterotoxin and botulinum toxin. Strictly speaking, these are examples of food-borne intoxications in contrast to infections, but it is customary among microbiologists to include them under the rubric of infections.

A characteristic feature of food-borne infection is that, in general, the food concerned looks and tastes appetizing, with no obvious signs of spoilage. This is because the large numbers of bacteria required to cause symptoms in humans would also cause visible signs of spoilage in the food. And that food is unlikely to be consumed.

Thus, in most cases of food-borne infection, the causative agent does not come from spoiled food but from contaminated food and water that have been infected by traveling from host to host via the fecal–oral route. That is to say, the microbes discharged in the feces of an infected human or animal find their way via food, water, or unwashed hands into the mouth of a new host. The organisms are swallowed and proceed to establish themselves on, or in, the mucus membrane lining the gut, or they may penetrate the gut wall to invade other parts of the body, traveling in the bloodstream and lymphatic channels.

Multiplication of the organisms in their chosen sites is often followed by symptoms of the disease, although symptomless or subclinical infection may also occur. In either case, large numbers of the organism leave the patient (voided in feces, vomit, urine) ready to repeat the cycle in a new and susceptible host. Such diseases may belong to one of two categories: (1) diseases of the gastrointestinal tract, in which the symptoms may include diarrhea, vomiting, and abdominal pain, such as cholera, dysentery, or bacterial and viral gastroenteritis; and (2) diseases with symptoms not necessarily related to the gastrointestinal tract, such as toxoplasmosis, poliomyelitis, brucellosis, and typhoid fever.

Source of Infection

The meat of an animal infected with the parasite causing toxoplasmosis, sarcocystis, or tapeworm is infective when eaten undercooked or raw. Eggs laid by hens that are infected with Salmonella enteritidis often transmit infection when the egg is eaten raw or undercooked. The question of whether bovine spongiform encephalopathy may be readily transmitted to people eating beef from infected cattle is still hotly debated, but it is becoming more widely accepted that this transmission can occur. Milk from infected animals, when consumed without prior heat treatment, transmits diseases like tuberculosis, brucellosis, and Q fever.

Infected fish may also transmit infection (fish tape-worm) when eaten raw or undercooked. Shellfish contaminated with the organism Vibrio parahaemolyticus, which normally grows in the salty water of warm seas, cause food poisoning when ingested. Some fish and mollusks acquire powerful heat-stable toxins through their food and are then able to transmit diseases like ciguatera fish poisoning and paralytic or neurotoxigenic shellfish poisoning to people eating them. These animals feed on toxin-containing dinoflagellates and concentrate the toxins in their flesh. The fish, however, appear to be unharmed by the toxin. In contrast, scombroid fish poisoning is acquired by eating a preparation of the scombroid group of fish, like smoked mackerel, that has been subjected to substandard processing and/or improper storage conditions prior to smoking. Here, bacteria that are commonly present—such as species of Proteus and Klebsiella—act on, and break down, the histidine present in the musculature to yield histamine and other substances (scombrotoxin). The symptoms produced on ingestion mimic those of histamine poisoning.

Contamination in the Slaughterhouse

Meat is often contaminated with organisms that normally inhabit only the lumen of the gut of animals but escape when the intestinal wall is damaged during slaughter and evisceration. This occurrence seems to be the most common route of transmission of such infections in Britain. Typical of this group of pathogens are Clostridium perfringens and species of Salmonella, Campylobacter, and Yersinia. Some species of Salmonella are able to cause a disseminated septicemic illness in their hosts, as, for example, S. enteritidis in poultry, which spreads to the heart and other organs that are used as giblets.

Food that is contaminated in this way may then contaminate other clean food by direct contact, as happens when raw meat comes into contact with cooked food, either directly as in refrigerators, or indirectly through use of the same preparation surface or knife.

Fecal Contamination of Food and Water

Both human and animal feces, which contaminate food and water, are major causes of diarrheal diseases. These diseases are especially prevalent in countries lacking adequate facilities for the safe disposal of sewage or protected water supplies. The role of flies, rodents, and birds in the transfer of pathogens from infected feces to food is well documented. Moreover, the washing of salad vegetables or fruit in water contaminated with human or animal sewage results in the transfer of diseases like typhoid, cholera, and bacillary or amoebic dysentery, as well as gastroenteritis caused by E. coli, Campylobacter, and Salmonella infections.

Bivalve shellfish (oysters, clams, and mussels) that are harvested from shallow inshore coastal waters adjacent to an outflow of sewage often contain large numbers of fecal pathogens. These animals are filter feeders, filtering large amounts of water each day to extract their food. The microbes present in fecal particles are concentrated in the flesh of the mollusk, which—when eaten raw or undercooked—may transmit diseases like typhoid fever, hepatitis A and E, and viral gastroenteritis. In addition, whereas the process of depuration (leaving the shellfish in tanks of purified water for over 24 hours prior to sale) removes most of the bacteria from the mollusks, viruses remain intact. Heat treatment is therefore required to inactivate them.

Waterborne infections commonly occur in areas lacking chlorinated water supplies. They are also associated with wars and other causes of sudden, uncontrolled movement of populations that disrupt normal services. Waterborne infections occur from time to time in developed countries, resulting in explosive outbreaks in the area of distribution of the water supply concerned.

Generally, such outbreaks follow a sudden or unforeseen breakdown of chlorination or filtration in the water purification plant, but they are also seen in institutions that are situated in remote areas and maintain their own water supply. It is notable that the cysts of some gastrointestinal protozoan parasites, such as species of Giardia, along with Entamoeba histolytica, are resistant to the standard chlorination process used for drinking water. Improperly filtered but adequately chlorinated tap water has resulted in several recent outbreaks of cryptosporidiosis in Britain and the United States.

In developing countries, where infective diarrheal diseases are common, debility arising from the illness compounds the effects of malnutrition, and childhood mortality rates are excessive. Up to 15 out of every 1,000 children in some areas die before the age of 5 from diarrheal disease transmitted by water.

Food Handlers

Food may be contaminated due to poor standards of hygiene among those who handle and prepare it. These food handlers may be: (1) “carriers” who harbor and continue to excrete pathogenic microorganisms and are thereby able to transmit them to others, even if they themselves no longer suffer from symptoms of the disease or have never had any symptoms; (2) people who are not yet symptom-free following gastrointestinal illness; and (3) those who are caring for patients with such diseases. The danger lies in the transmission of microorganisms to food via unwashed hands and poor toilet hygiene, or by aerosols caused by sick food handlers vomiting in preparation areas. Food handlers typically cause outbreaks of staphylococcal food poisoning, viral gastroenteritis, and enteric fever, which is particularly significant in the commercial or bulk preparation of food. Staphylococcal food poisoning is especially relevant here since the toxin produced can resist heat treatment.

Contamination from the Environment

Microorganisms found in the environment may contaminate food and water. The spores (resistant forms) of Clostridium botulinum survive for long periods of time in the soil and mud of lakes and rivers. They may contaminate vegetables and fish and secrete a powerful neurotoxin into the food. Listeria monocytogenes, found in the gut of cattle, sheep, and other animals and in soil and grass, may contaminate foods such as unpasteurized milk products. Moreover, the ability of listeriae to grow and multiply at temperatures prevailing in most domestic refrigerators contributes to the problem of preventing infection.

Canned food has often been found contaminated with organisms that have gained entry through tiny undetected faults in the seal. During manufacture, cans of food are first heat-sterilized, then cooled by immersion in water before the labels are attached. When the cans undergo sudden cooling, material from the outside gets sucked into the interior, and infective agents gaining entry at this stage may then multiply freely in the food. If the food is to be eaten uncooked or only lightly cooked, it presents a hazard. A can of imported corned beef was implicated as the source of the outbreak of enteric fever in Aberdeen in 1964. At the canning factory in South America, the cans were cooled in untreated water taken from a river close to the factory. Further investigation revealed that raw sewage from a neighboring settlement was discharged directly into the river upstream from the factory. Enteric fever was endemic in this community.

Back in Aberdeen, the infected corned beef was sliced and sold to the public through a retail outlet, resulting in the outbreak. The causative agent was identified and traced back to the retail premises, revealing the corned beef as the source of the outbreak. All cans belonging to the same batch as that causing the outbreak were withdrawn from sale. However, in spite of this action, the outbreak was observed to continue, with the diagnosis of more new cases in the area. It later became apparent that other items of precooked food sold in the shop, like ham, had been cross-contaminated when they were sliced on the same machine as the contaminated meat. Following this observation, the outbreak was brought under control by enforcing measures for (1) preventing further sale of contaminated food and (2) thorough cleaning and decontamination of all equipment and work surfaces involved. Canned food has also been implicated in outbreaks of botulism, staphylococcal food poisoning, and other diseases. In Great Britain, the most common food-borne infections today are campylobacter and salmonella gastroenteritis acquired from poultry and meat. Among the salmonellae, the incidence of S. enteritidis phage type 4 is now increasing steadily in England and many other countries.

Bacterial Food Poisoning

The food-poisoning group of organisms include Staphylococcus aureus, Salmonella species, Clostridium perfringens, Bacillus cereus, Clostridium botulinum, and Vibrio parahaemolyticus.

The term “food poisoning” is generally used in a restricted way to exclude clinical entities like typhoid and cholera, but it includes those conditions in which intestinal disturbances commonly occur, resulting in diarrhea and vomiting (with or without other symptoms). It is caused by ingesting either live bacteria or products of bacterial growth—the latter arising after the microbes have multiplied in the food for some hours before it is eaten.

This vital initial period of bacterial multiplication in food is an essential feature in the natural history of all bacterial food poisoning, as it ensures that a vastly increased number of bacteria are present in the food. The “challenge dose” of bacteria (the number that have to be ingested in order to cause disease) is much higher in the food-poisoning group than it is with other diseases, like typhoid and shigellosis, where food merely acts as a vehicle for transmission of the infection. C. botulinum is included in the food-poisoning group, although the symptoms produced here are neurological rather than gastrointestinal, because this pathogen from the soil must grow and multiply for several hours in food before it is able to cause disease.

Crucial factors in the evolution of an outbreak of food poisoning include the type of food involved, the way in which it has been prepared, and the duration and temperature of storage prior to eating. As with most other food-borne infections, the food looks and tastes normal.

Bacterial food poisoning may belong to three different types, depending on the mechanism of pathogenesis observed:

  1. In the infective type, as caused by Salmonella, large numbers of live organisms must be consumed. They then invade the cells lining the small intestine, where further multiplication occurs, causing inflammation, diarrhea, and vomiting. The time interval between ingesting the food and developing symptoms (the incubation period) is between 14 and 36 hours.
  2. In the toxic type, caused by S. aureus, B. cereus, and C. botulinum, the phase of multiplication in food results in the release of toxins. When ingested, these toxins (preformed in the food) are responsible for producing the symptoms. It is not necessary to ingest the live organisms. Rapid heating of the food may destroy the bacteria but allow heat-stable toxins to remain active. In contrast to the previous type, the incubation period here is fairly short—a few hours—because the preformed toxin is able to act immediately on the gut.
  3. The intermediate type, caused by C. perfringens, has an incubation period of 8 to 20 hours. The toxin is released not in the food but in the gut of the host, following ingestion. The release of toxins here coincides with the formation of spores (a resistant form of the bacterium). Prolonged low-temperature cooking or inadequate reheating of infected, leftover meat stews and casseroles increases the occurrence of this disease, because it stimulates sporulation of the C. perfringens in the gut of the host.

It is apparent that three important criteria have to be satisfied for the successful development of food poisoning. These are as follows:

  1. Introduction of organisms into the food at some stage during its preparation.
  2. Inadequate cooking. Most microbial agents in food are destroyed by cooking at high temperatures for the prescribed length of time. When frozen food, like meat, is cooked before it is completely thawed out, the temperature reached in the center of the meat may not be sufficient to kill the bacteria even when the outside looks well done.
  3. Leaving food standing at an ambient temperature for several hours, enabling the bacteria to multiply and attain the infective dose. Refrigeration of food stops multiplication of most of the harmful pathogens, with the exception of species like Yersinia enterocolitica and L. monocytogenes, which are particularly adapted for growth at temperatures of 0 to 4 degrees Celsius (°C). Freezing suspends replication of all pathogens but fails to kill them. After frozen food is allowed to thaw, bacteria may start to multiply again if kept standing at room temperature. Hence, careful attention is important at all stages of food preparation to ensure adequate cooking and prevent contamination. Furthermore, if food is not to be eaten without delay, it should be rapidly chilled before freezing for storage.

Clearly, even contaminated food is not harmful if properly cooked and eaten right away. But when eaten raw, inadequately cooked, or after remaining for a long time in a warm environment, contamined food may transmit disease. Bacteria in food replicate rapidly by binary fission, doubling their numbers progressively at intervals of less than 20 minutes by a simple process of division in which each cell gives rise to two daughter cells. Food provides the ideal nutrient for bacteria, which continue to multiply, so that in just over 3 hours a starting population of 1,000 organisms (which is too low a dose to cause food poisoning) may reach the level of millions and constitute an effective challenge dose. In most outbreaks, food has been left for longer than 3 hours and the starting population may be higher than 1,000. Organisms like Campylobacter, Shigella, and the typhoid bacillus are infective at a much smaller dose level than the food-poisoning group, and in this case it is not necessary for the infected food to remain at room temperature for several hours in order to be able to transmit infection. Indeed, organisms like Campylobacter and viruses do not multiply in food. As already mentioned, in this case, food merely acts as a vehicle for transmission of these agents.

Most pathogenic bacteria are unable to grow in acidic food (pH less than 4.5) or in food with a low moisture content. Similarly, high salt or sugar concentration in preserved food inhibits many bacteria. However, they may survive for long periods of time in dried food products.

Staphylococcal Food Poisoning

The food-poisoning strains of S. aureus are usually harbored by human food handlers, either in septic skin lesions (boils and whitlows) or as a part of the normal resident microbial flora of the nose or skin. Transfer of such organisms to food may be restricted by the use of a no-touch technique in its preparation. This is especially relevant for large-scale processing of food. Food poisoning does not occur if the contaminated food is either eaten before the bacteria have a chance to multiply, heated before the bacteria multiply and produce toxins (S. aureus are readily killed by heat), or refrigerated promptly and served chilled, to prevent multiplication of surviving bacteria.

If contaminated food is first refrigerated and then left standing for a considerable length of time in a warm room, the bacteria are able to multiply again. S. aureus multiplies rapidly over a wide range of temperatures, liberating its heat-stable enterotoxin. The type of food involved here is usually that which requires little or no cooking and has a high fat content, like cream cakes, trifles, and ham sandwiches. When eaten, the toxin acts on the host, causing symptoms of nausea, vomiting, and abdominal pain (but rarely with diarrhea) within 1 to 6 hours of ingestion. The toxin appears to act on nerve endings in the gut, transmitting its message to the center in the brain that controls vomiting.

Most cases are self-limiting. Recovery occurs rapidly, usually within 12 hours, and no treatment is required. If the infected food is heated to 70° C before serving, the S. aureus will be killed, but the toxin is still active, and even boiling for 30 to 60 minutes may not inactivate it. Moreover, S. aureus is not inhibited by a high concentration of salt. Therefore, foods like ham and other semi-preserved meat with a high salt and high fat content make an ideal medium for staphylococcal growth and multiplication. Prevention involves stringent measures for personal and environmental hygiene in the food preparation area, the covering of skin lesions with waterproof dressings, rapid refrigeration of prepared food that is not to be eaten right away, and thorough cooking of soups and meats before eating. Control of food handlers and use of the no-touch technique are important in the food manufacturing industry.

Salmonella Food Poisoning

Salmonellosis is a disease affecting both humans and animals and may be transmitted between them (a zoonosis). There are over 200 species of salmonellae, and they are widely distributed in nature, inhabiting the intestines of wild as well as domestic animals and household pets. They frequently contaminate meats, especially chicken, along with eggs, milk, and other products. Intensive farming techniques have resulted in widespread infection in the food chain, and mechanized mass processing of poultry carcasses ensures the cross-contamination of most poultry sold in shops.

Chickens infected with S. enteritidis are able to pass the infection on in their eggs. Infected humans acting as food handlers may also transmit infection unless scrupulous attention to hygiene is observed with regard to hand-washing after using the toilet and before handling food. Milk that is not heat-treated may also transmit salmonellae. The organism is readily killed by boiling for half an hour, so poor culinary practices are largely to blame for outbreaks of salmonellosis in humans. Examples of such practices include cooking inadequately thawed large frozen joints of meat, low-temperature cooking, allowing cooked food to come in contact with uncooked meat or its juices, and prolonged storage at room temperature after inadequate cooking.

This last factor is the most important one because it enables an effective infective dose of salmonellae to develop in the food. Normal gastric acidity is sufficient to kill many ingested pathogens, but this defense may be breached when the microbe is taken in large numbers along with food that temporarily neutralizes the protective mechanism. Frequent outbreaks of salmonella food poisoning continue to occur in many developed nations because of (1) infected manufactured food; (2) the practice of adding raw egg to food that is to be eaten uncooked; and (3) importation of food products from areas with a high disease prevalence.

The strain of Salmonella responsible for an outbreak of gastroenteritis often points to the species of animal from which the food derived, as many strains have preferred hosts. S. enteritidis and Salmonella typhimurium are commonly found in poultry and cattle, Salmonella dublin in cattle, and Salmonella hadar in turkeys. However, at times, certain strains appear to sweep through an entire population of livestock, probably because of contaminated feed and intensive farming techniques involving overcrowding and animal stress. The infection is often asymptomatic in humans and other animals, but continued excretion of the pathogen in feces enables transmission to occur.

This excretion is especially marked when animals are maintained in overcrowded situations and is reflected in the high incidence of infection in flocks of poultry and in beef cattle. During slaughter and evisceration, gut contents often contaminate the meat. Cross-contamination of other carcasses then occurs either through direct contact with the meat or by contact with contaminated surfaces and equipment. Thus, all retailed meat may be regarded as potentially contaminated.

Consumption of an infecting dose of 105 to 106 organisms is required in order to establish infection. This amount is readily achieved if improperly cooked meat is left to cool at room temperature for 3 or more hours. Fewer cells (10 to 100) may cause illness in very young children or in the elderly, especially if the pathogens are carried in high-fat foods such as cheese, cheesecake, salami, or hamburgers. Low numbers may also be infective when waterborne. A high fat content in food is believed to protect the salmonellae to some extent during cooking. This may be relevant to the survival of S. enteritidis phage type 4 in eggs and meat.

Since 1985, this particular strain has appeared with increasing frequency among the poultry population and has now spread to epidemic levels among them. In birds, the infection is not confined to the gut but passes into the bloodstream to give a severe septicemia, and the eggs are infected before they are laid. Thorough cooking of the eggs or pasteurization of bulked eggs would destroy the pathogen. Pasteurization entails heating the product to a temperature of either 63 to 66° C for a period of 30 minutes or 71° C for 15 seconds. This form of heat treatment kills all vegetative pathogens like salmonellae in milk and eggs.

Human infection follows the ingestion of approximately 1 million organisms, which then invade the intestinal mucosal cells and cause local inflammation. The incubation period lasts for 14 to 36 hours and is followed by diarrhea and vomiting, with or without abdominal pain. Most infections are self-limiting and do not require any treatment, but in more severe cases, supportive therapy with rehydration is implemented. In very young babies or in elderly or debilitated patients, for instance, the disease may follow a septicemic course, giving rise to a serious life-threatening condition that necessitates the administration of appropriate antibiotics as well as rehydration. Following recovery, a small proportion of people may retain organisms in the gut for a varying period of time and continue to excrete them intermittently in the feces. If these “carriers” are employed as food handlers, they may transmit the organism to foods unless scrupulous attention to hygiene is observed.

Salmonella organisms may be introduced into manufactured food products at various stages in the production line. For example, they may enter after the heating stage and cause widespread outbreaks. Intercontinental outbreaks have been traced to chocolates and cheeses. Critical monitoring of production procedures to detect and control the possibility of micro-biological contamination at every stage is of vital importance. Although it would be ideal to reduce salmonellosis in the livestock by measures like heat treatment of all animal feed and improvement of farming techniques, this aim is not readily achievable.

Legislation to enforce heat treatment of all milk sold from retail outlets has helped reduce milk-borne outbreaks in humans. However, the occasional undetected breakdown of a pasteurization unit has resulted in the sale of contaminated milk that has caused outbreaks. Countries lacking basic sanitation and hygiene tend to have a higher prevalence and transmission rate of salmonellosis. Foodstuffs imported from such areas (for example, spices like pepper) may require more than the usual decontami-nation. Alternatively, spices and the like can be added to food before it is cooked rather than afterward; this, too, prevents the problem.

Clostridium Perfringens Food Poisoning

This food poisoning is associated with reheated leftover meat dishes, like stews. The causative organism is a normal commensal of the gut of humans and other animals. It is a strict anaerobe able to grow and multiply only in an environment devoid of oxygen. Clostridia are able to produce tough, resistant spores that survive in adverse environments, and these spores germinate into vegetative cells. Multiplication can occur only in the vegetative state and not in spore form.

Inadequate cooking of meat enables the spores to survive. C. perfringens type A has particularly heat-resistant spores that can survive prolonged boiling, and these strains are associated with outbreaks of food poisoning, usually in institutions such as nursing homes. The type of food involved is, generally, meat dishes such as large joints cooked at a low temperature, or stews and broth when there is insufficient heat penetration to kill off all the spores.

Furthermore, the spores are protected by the protein-rich food. If the cooked food is eaten right away, it is generally safe, but when such food is left for between 3 and 5 hours at room temperature, it provides the ideal anaerobic conditions for the spores to germinate and the vegetative cells to multiply. Multiplication is rapid, with a mean doubling time of only 7 minutes at 41° C. The infecting dose of 108 organisms per gram of food is soon attained. If, at this stage, the food is thoroughly reheated (to over 70° C), the vegetative cells are killed and the food is rendered safe.

If the food is only partially reheated (up to or below 60° C), though, the cells survive, and when ingested, they sporulate in the gut and shed the vegetative fragment, which releases a heat-labile enterotoxin. This acts on the small intestinal mucosa, damaging the brush border of epithelial cells at the tip of the villous processes. There is an outpouring of fluid and electrolytes resulting in violent, watery diarrhea and abdominal colic, occurring 8 to 20 hours after ingestion of the food. Vomiting and fever are uncommon, but nausea may be present. Recovery occurs within 24 hours in most cases, and no specific treatment is required. However, in elderly, debilitated individuals, severe dehydration may result, necessitating supportive therapy with rehydration.

Prevention involves the high-temperature cooking of meat in small amounts, rapid refrigeration if it is not eaten immediately, and reheating of all leftover meat dishes to a temperature higher than 70° C to destroy the vegetative cells. Cooking methods like frying and grilling have an advantage in that they deliver very high temperatures directly to contaminated meat surfaces.

Bacillus Cereus Food Poisoning

B. cereus produces spores that help it to survive in adverse conditions such as drying and heat. Unlike the clostridia, this organism is an aerobe (growing in the presence of oxygen). It is found widely distributed as a saprophyte in the environment and on cereals like rice. In fact, outbreaks of B. cereus food poisoning are frequently associated with reheated rice dishes such as fried rice and take-out fast food from Chinese restaurants. Bacterial spores survive the short cooking period involved, and if the rice is left at an ambient temperature for several hours to dry, germination of the spores occurs. The bacterial cells then multiply rapidly in the rice and liberate various enterotoxins. When the precooked rice is stir-fried to make fried rice, the heat-stable toxin is not destroyed by the short exposure to heat and may cause symptoms shortly after ingestion. When boiled rice is stored in a refrigerator, it goes lumpy, and when subsequently stir-fried, it fails to present the aesthetically acceptable appearance of well-separated grains of rice. This probably accounts for the tendency, especially in fast-food outlets, to leave precooked rice at room temperature for several hours and reheat it as required.

Two distinct types of disease symptoms are observed with B. cereus infection, depending on the type of toxin formed:

  1. In the emetic type, vomiting is common. The incubation period here is 1 to 5 hours, and symptoms include nausea, vomiting, malaise, and, sometimes, diarrhea, lasting for 6 to 24 hours. This type is caused by a heat-stable toxin and is associated with cereals, especially the cooked rice just mentioned. The condition may, of course, be prevented by eating freshly cooked rice while it is hot; but if this is not possible, then the food should be maintained at over 60° C or stored at temperatures of about 8° C.
  2. The diarrheal type is the rarer of the two and is due to the production of a heat-labile enterotoxin associated with lightly reheated meat sauces, puddings, and vegetables. Symptoms include profuse watery diarrhea, abdominal cramps, and, sometimes, vomiting occurring 8 to 16 hours after eating the food. Recovery is generally complete in 12 to 24 hours.

Botulism

Botulism is a neuroparalytic disease caused by the ingestion of very small amounts of botulinum toxin. An oral dose of 0.005m g may be lethal for humans. C. botulinum is a spore-producing organism that can grow only in the absence of oxygen. The tough spores are found worldwide in the soil and in the mud of lakes and rivers. When food like fish, vegetables, and meat is contaminated with soil containing the spores and is then left at room temperature, the spores germinate and multiplication of the organism occurs.

Some strains grow in temperatures as low as 3.3° C. During the growth process, a powerful neurotoxin is produced, which, when ingested, results in symptoms of botulism, following an incubation period of 18 to 36 hours. The preformed toxin causes a muscular paralysis with hoarseness, visual disturbances, headache, nausea, vomiting, and difficulty in swallowing and speaking. If left untreated, death from respiratory or cardiac arrest is a real danger. Symptoms may last from several weeks to many months.

A less severe disease is caused in infants ingesting spores of C. botulinum. These spores germinate in the infant’s gut, releasing toxin in the intestine. Symptoms of flaccidity and weakness, with difficulty in feeding, are the presenting symptoms of infantile botulism. Treatment includes administration of antitoxin antibodies to attempt to neutralize the toxins.

Prevention relies on the destruction of spores by thorough heating of all preserved food. This is especially relevant to home-preserved food—the cause of many outbreaks. The high acid content of certain foods prevents the growth of this organism, but pre-formed toxin retains its activity when added to acid food. As mentioned previously, in an outbreak in England in 1989, the source was contaminated hazelnut puree used to flavor yoghurt. The toxin in food may be destroyed by heating at 85° C for 5 minutes.

Vibrio Parahaemolyticus Food Poisoning

This form of food poisoning is related to the ingestion of undercooked or raw shellfish or fish. V. parahaemolyticus has a predilection for high salinity and grows well in the warmer seas. The shellfish and fish taken from such sites may be contaminated with the organism. When undercooked shellfish are eaten, symptoms of diarrhea and vomiting, fever, and abdominal cramps appear within 16 to 48 hours, resolving in 2 to 7 days. Other halophilic (salt-loving) bacteria, like Vibrio vulnificus, may cause a septicemic infection in people eating raw oysters. Prevention depends on the adequate cooking of seafood.

Other Bacterial Food-Borne Infections

Apart from the food-poisoning group of organisms, many other bacteria transmitted in food may also cause infection.

Campylobacteriosis

Campylobacteriosis occurs throughout the world and is the most common cause of acute infective diarrhea in most developed countries. The organisms Campylobacter coli and Campylobacter jejuni are widely distributed in nature, infecting the gut of domestic and wild animals, poultry and wild birds, and also humans. Poultry and cattle are important sources of human infection, because in addition to developing acute infection, they may become symptomless excreters of the organism with long-term carrier status. This is a classic example of a zoonosis (a disease transmitted between animals and humans) and, therefore, as is the case with nontyphoid salmonella, cannot easily be eradicated.

The curved, spiral-shaped organism is found in large numbers in the gut contents of infected animals and is commonly transferred to poultry and animal meat during slaughter and evisceration. Most poultry carcasses sold in shops are consequently contaminated with the organisms. They may also be transferred from the feces of infected animals, wild birds, or humans to unprotected drinking water supplies or to food. Outbreaks occur most frequently after the consumption of improperly cooked poultry but have also taken place after the drinking of milk that was not heat-treated and unchlorinated water.

The infective dose is small (a few hundred bacteria), and the incubation period is approximately 3 days. The disease produced is an acute enterocolitis, and lesions are seen in the intestinal mucosa similar to those produced by shigellae or salmonellae. A characteristic feature of campylobacter is that the incubation period is followed by a period of a few days when the patient feels ill with headache, malaise, fever, and abdominal pain (the prodromal period). This is followed by a dysentery-like illness with frequent stools, sometimes containing blood and mucus. The prodromal symptoms help differentiate this illness from that caused by salmonellae or shigellae. Furthermore, the symptoms of abdominal pain and diarrhea seen here may be more prolonged. In fact, the pain may be so severe as to lead to a misdiagnosis of acute appendicitis and to the patient undergoing unnecessary abdominal surgery.

In most cases, recovery occurs within a week, and there is no need for antibiotic therapy. However, in debilitated or immunodeficient patients the organism may spread through the blood to cause a severe systemic infection, requiring appropriate antibiotic therapy.

Prevention of infection is achieved by measures that break the routes of transmission, such as the following:

  1. Thorough cooking of all meat.
  2. Observance of good principles of kitchen hygiene, like preventing contact between cooked food and raw meat, including contact with surfaces on which the raw food was placed first.
  3. Heat treatment of milk by pasteurization or sterilization.
  4. Adequate chlorination of the water supply or boiling of untreated drinking water.

Because the campylobacter organisms are so widespread in the animal kingdom, it would be unrealistic to hope to eradicate them entirely. It would, however, be useful to reduce the incidence of campylobacteriosis in poultry stocks and farm animals by good principles of farming and processing, while preventing human infection by observance of the simple measures recommended. This is also true of nontyphoid salmonellosis. Unlike salmonellae however, the campylobacter do not appear to multiply in food that is kept warm. The infective dose is low, and heavy contamination of food ensures transmission. Any food may be contaminated by direct or indirect contact with the feces of infected domestic pets, other animals, birds, or humans.

Shigellosis (Bacillary Dysentery)

Shigellosis is a common disease seen throughout the world, but unlike nontyphoid salmonellae and campylobacter, shigellae infect only humans—there is no known animal host. The species involved are Shigella dysenteriae, Shigella flexneri, Shigella boydi and Shigella sonnei, and the severity of the diseases they produce decreases in this order.

Shigella sonnei, the mildest of the four forms, causes outbreaks mainly among young children in institutions like daycare centers and elementary schools. Poor toilet hygiene is the main cause of transmission in such cases. Outbreaks also occur in military barracks, institutions for the mentally handicapped, and in populations subjected to major social upheavals with a breakdown in the normal pattern of life.

Shigella dysenteriae, which causes the most severe of the four infections, is commonly seen in Asia. A high level of transmission is maintained in those countries with unprotected water supplies, a lack of facilities for the safe disposal of sewage and poor environmental conditions and personal hygiene practices.

The infective dose is small, and the disease is readily transmitted from person to person. There is no need for the organism to multiply in food in order to cause disease, as is the case with salmonella food poisoning. Following ingestion, the organism invades superficial layers of the cells lining the intestinal lumen, causing ulceration and inflammation that results in bleeding, secretion of mucus, and exudation of pus. The symptoms are diarrhea with loose stools containing blood, mucus, and pus. The severity of the illness varies with the species involved. The infection is localized to the intestine and does not invade deep tissues or the bloodstream except in very severe forms.

Antibiotic therapy is restricted to the serious forms of illness, like S. dysenteriae, or any severe form of dysentery occurring in the very young or debilitated patient. In most cases antibiotics are contraindicated as they may prolong symptoms and excretion of shigellae in the feces.

Prevention in the developed countries involves improving toilet hygiene, especially among children in institutions and schools. In developing countries lacking environmental hygiene and safe water supplies, travelers are well advised to drink only bottled or boiled water and to avoid uncooked or salad vegetables and fruit that is to be eaten unpeeled.

Typhoid Fever (Enteric Fever)

Tyhoid fever is a severe febrile septicemic illness that is endemic in countries with poor sanitation and affects only human hosts—animals are not involved. The causative organism, Salmonella typhi, is transmitted from one host to the next, usually through water supplies contaminated with human sewage. Food handlers also play an important role in the transmission. Recovery from this fever may be associated with the carrier state, which may last for a lifetime, with intermittent excretion of S. typhi in feces and urine. Such a state occurs when the organism continues to live in the gallbladder or kidneys of the host but no longer causes adverse symptoms. However, it is excreted from time to time in the feces of the carrier and may, therefore, be transmitted to other hosts. It is important to identify such carriers, educate them on food hygiene, and exclude them from following a career in the food industry.

Food may act as a vehicle for the transmission of this disease, which is by the fecal–oral route. Symptoms appear after an incubation period of about 10 to 14 days, with fever, headache, malaise, and, sometimes, diarrhea or constipation. Such symptoms are severe, and fatal complications may occur if untreated. Treatment with an appropriate antibiotic is important in all cases of enteric fever. Prevention requires the provision of protected, chlorinated water supplies and safe sewage disposal. In endemic areas it is wise to boil all drinking water or to treat it with chlorine tablets. Travelers to endemic zones are advised to drink only bottled water and not to eat salad vegetables, uncooked food, or unpeeled fruit. A vaccine containing heat-killed S. typhi is available but gives only partial protection, so the prevention of infection remains the main aim.

Paratyphoid fever caused by Salmonella paratyphi A, B, or C is a milder form of the infection just discussed. Salmonella paratyphi B is the strain transmitted in Europe, whereas S. paratyphi A and C are endemic in other areas.

Escherichia coli Gastroenteritis

This form of infection is possibly the most common cause of diarrhea in the world. Specific strains of this large family of organisms are involved in causing human gastroenteritis. Their natural habitat is the gut of humans and animals, and transmission is from person to person or by the intake of contaminated water or food. The infective dose is small, and there is no need for the organism to multiply in food for several hours before becoming infective.

The disease is prevalent in areas with poor environmental hygiene and frequently causes diarrhea in travelers to these areas (appropriately termed Aztec two-step, Rangoon runs, Delhi belly, Tokyo trots, and gippy tummy). In developed countries, outbreaks of infantile gastroenteritis have occurred in hospitals, and more recently, outbreaks of hemorrhagic colitis and hemolytic uremic syndrome have been reported where the causative organism was E. coli serotype 0157, transmitted in undercooked hamburgers. It is possible that some strains cause zoonotic infection, whereas with others, human hosts are the sole source of infection.

The organisms have been classified according to the way in which disease is produced: Hence there is enteropathogenic E. coli (EPEC), enterotoxigenic E. coli (ETEC), enteroinvasive E. coli (EIEC), and verotoxin-producing or enterohemorrhagic E. coli (VTEC or EHEC).

In developing countries, diarrhea and dehydration brought on by such infection is a major cause of infant mortality. Adults are also affected from time to time, but they appear to develop a measure of immunity from constant exposure to all the different strains in the community. Travelers to infected areas, however, are unprotected and extremely susceptible to all the new organisms in the environment.

EPEC strains belong to certain known serogroups and, in the past, have caused outbreaks of gastroenteritis in infants and young children. Symptoms include diarrhea and vomiting, and severe dehydration may occur. Thus, unless supportive treatment is initiated rapidly, mortality rates can be high. Adults may also be occasionally infected. The mechanism of pathogenesis of EPEC strains is not clearly understood, but the organisms remain within the lumen of the gut and produce changes in the appearance of the cells lining the small intestine. Antibiotic therapy is restricted to the severely ill and debilitated cases.

All ETEC strains secrete enterotoxins, of which there are two types: heat-labile toxin (LT) and heat-stable toxin (ST).These toxins act locally on the cells of the intestinal mucosa, stimulating them in such a way as to result in a net outpouring of fluid and electrolytes from the bowel wall into the lumen.

The disease rarely occurs in temperate climates. In developing countries, however, it is common in children and is an important cause of travelers’ diarrhea. Sewage contamination of water supplies is important in transmission. ETEC strains do not invade the body but remain in the gut lumen closely attached to the surface of the mucosal cells by means of short fibrillar processes (colonization factors).The toxin is secreted in the gut lumen and is internalized by the mucosal cells. Recovery occurs in a few days when the cells of the small intestine are replaced by fresh ones. Cells in the small intestinal mucosa are normally shed and replaced very rapidly. Because there is no inflammation in the gut wall, the stools contain no inflammatory cells. This is in sharp contrast to shigellosis and salmonellosis. Treatment is strictly supportive, with rapid correction of dehydration a lifesaving remedy.

EIEC strains (like E. coli serotype 0124) cause invasive disease similar to shigella dysentery, with ulceration of the intestinal mucosa that results in diarrhea with blood, mucus, and pus in stools. Food-borne infection is common, and cross-infection can occur.

EHEC (VTEC) strains have caused outbreaks of bloody diarrhea in people eating undercooked hamburgers. When meat is ground, it enables the organisms to grow better and cause greater contamination. The hemorrhagic colitis may be followed by hemolytic uremic syndrome (kidney failure and anemia due to the destruction of the red blood cells and destruction of thrombocytes). Many patients suffer only mild diarrhea, whereas others are seriously ill. Escherichia colis erotype 0157 is particularly associated with such outbreaks. The incubation period is 2 to 9 days. A contagious spread of the disease may also occur and is important in hospital outbreaks. Prevention is similar to the prevention of shigellosis with the added incentive to ensure that hamburgers are properly cooked.

Cholera

Cholera is a purely human, intestinal form of disease, characterized by profuse watery diarrhea (rice-water stools), that frequently leads to dehydration and death in untreated cases. It spreads along the fecal–oral route, with waterborne transmission most responsible for explosive outbreaks. Transmission from person to person also occurs. Flies can spread it, and the disease may also be food-borne. Fecal pollution of seawater can contaminate seafood which, if eaten raw or undercooked, may transmit the disease. No animal host has been detected for the cholera vibrio. Normal levels of gastric acid provide some defense against such infections in humans. The disease is more likely to occur in those with no or low levels of gastric acid.

Vibrio cholerae remains in the lumen of the gut attached to the surface of mucosal cells. The pathogenesis is mediated by a toxin that is secreted into the gut by the organism. This toxin is internalized by the cells and stimulates an outpouring of fluid and electrolytes, resulting in profuse diarrhea with the potential of causing death from dehydration (up to 20 liters per day of fluid loss).The mode of action of the cholera toxin is similar to that of E. coli LT. Indeed, the two toxins are similar in structure. There is no damage to the mucosa, and with modern medical treatment, recovery occurs in a week’s time in most cases.

That treatment involves rapid rehydration. Oral rehydration with frequent small drinks of an electrolyte and glucose solution is usually used. In severe cases intravenous fluid infusion is necessary. Orally administered tetracycline is useful in reducing the period of excretion of the vibrio and also the severity of symptoms.

The latest cholera pandemic is due to the El Tor variant of V. cholerae, which causes a milder form of cholera than that seen in the nineteenth-century pandemics that swept the globe. It began in the 1960s in Southeast Asia and was spread by travelers, first to the Middle East and Africa and then to South America. Outbreaks, common where water supplies and environmental sanitation are poor or nonexistent, are especially likely in camps for refugees from wars or natural disasters. Cholera prevention. Careful disposal of human sewage is essential, along with the provision of safe chlorinated water supplies or, failing that, boiled or bottled water. Vibrio eltor is more difficult to control because it causes a milder disease, meaning there are more ambulant cases to disseminate it more widely. A vaccine is available, but its protection is of short duration, and there is considerable doubt as to its efficacy.

Yersiniosis

Yersiniae are part of the normal gut flora of domestic and wild animals. Human infection is acquired by the ingestion of inadequately cooked meat containing sufficient numbers of the organism. Y. enterocolitica induces a typical food-poisoning-type syndrome with mild or moderate diarrhea, fever, and abdominal pain. Nausea and vomiting are rare, but mesenteric adenitis (inflammation of the abdominal lymph nodes) may occur and lead to misdiagnosis as acute appendicitis. Symptoms appear within 16 to 48 hours after the meal and may last for a period ranging from 24 hours to 4 weeks. Reactive arthritis is a complication that may occur following this infection, especially in people of HLA type B27. A heat-stable enterotoxin is produced by the organism that may contribute to the pathogenesis in the gut. However, the infection is not confined to the gut, as the organism invades the body to cause inflammation in the mesenteric lymph nodes.

A characteristic feature of this organism is its ability to grow and multiply in the cold at 0° C; consequently, refrigeration of contaminated food does not prevent the development of an infective dose. The bacteria, however, are readily inactivated by adequate cooking. Another strain, Yersinia pseudotuberculosis, is also found in the gut of animals and infects humans eating contaminated meat. It often causes a febrile illness with mesenteric adenitis, and sometimes a fulminant typhoid-like condition ensues. The yersiniae are associated with outbreaks of apparent acute appendicitis among young schoolchildren.

Listeriosis

The listeriae are pathogens of humans and animals that have the unusual property of being able to grow well at refrigerator temperatures as low as 4° C.They are widely distributed in the environment as well as in the guts of animals and humans. Contamination of food is therefore likely unless particular care is taken at all stages of food preparation and production to prevent contamination. The types of food associated with recent large outbreaks include unpasteurized milk products (like soft cheeses), cook-chilled chicken from supermarkets, and coleslaw made with cabbage that was fertilized by sheep manure. The organism frequently contaminates animal feeds, increasing its prevalence in the environment, but it does not grow at a low pH in the presence of organic acids.

When ingested by humans, listeriae may bring on a mild form of influenza-like illness, but in neonates, the elderly, and debilitated persons they may cause severe septicemia or meningitis. In pregnant women a mild influenza-like illness, followed by abortion or stillbirth, is often reported. Newborn babies may acquire the organism from the maternal birth passage and develop a fulminating septicemia or meningitis.

The organism is destroyed by the pasteurization of milk or the thorough cooking of food. Prevention of the contamination of food with environmental organisms is an important consideration.

Plesiomonas Shigelloides

This organism is another environmental bacterium that is widely distributed in nature and has been occasionally implicated in sporadic cases and outbreaks of diarrhea in many countries. It is found in freshwater (rivers and ponds), mud, fish, dogs, cats, and other animals. Adequate cooking of food destroys the organism.

Brucellosis (Undulant Fever)

Brucellosis is also called undulant fever because the disease often presents with bouts of fever alternating with afebrile periods. It is a zoonosis and is primarily a pathogen of goats, cattle, pigs, and camels. Humans acquire the infection by ingesting live brucellae in unpasteurized milk from infected animals, as well as through infective secretions from animals. Veterinary surgeons, farmers, and abattoir workers are especially at risk.

The main species of importance to humans are Brucella abortus from cattle, Brucella melitensis from goats, and Brucella suis from pigs. Brucella melitensis causes a more severe infection (Malta fever) than the other two species. The symptoms follow the ingestion of infected goats’ milk or goats’ cheese, and the condition is endemic in Europe, Africa, and the Far East. Brucella suis causes infection in America. In cattle, B. abortus causes abortion because of the organism’s predilection for the placenta due to the high concentration of erythritol in bovine placenta and in the fetal fluids. Vigorous multiplication, therefore, occurs at this site, and the cow’s milk will contain live brucellae, making it highly infectious, as are products made from it like butter, cheese, and cream.

In humans, the infection causes a protracted systemic illness. The organism is distributed in the bloodstream to various tissues, causing joint and muscle pains, headache, malaise, and depression. In many cases the disease runs a mild course, but in others a prolonged and disabling form of presentation is seen, especially when untreated.

Routine pasteurization of milk stops this mode of transmission so that only those in direct contact with infected animals remain at risk because the bacillus lives inside the cells of the host. Treatment with antibiotics must be continued over a long period and repeated to ensure recovery.

Tuberculosis

Tuberculosis is generally transmitted by a respiratory route, but it can also be acquired by drinking unpasteurized cows’ milk. The organism Mycobacterium bovis infects cows and passes in the milk to humans. It usually causes enlargement of lymph nodes in the neck or abdomen; fever, loss of weight, and can cause pulmonary symptoms. Treatment with several antibiotics is required. The infection may also spread in the bloodstream to involve other sites like meninges, bone, joints, and other organs.

Tuberculosis prevention. Pasteurization, boiling, or heat sterilization of milk renders it safe for use. Infected animals may be identified by tuberculin skin testing. They are then slaughtered to prevent spread to other herds. In humans, the BCG vaccine may be used to confer protection against this disease.

Q Fever

Q fever is an influenza-like febrile illness associated with patchy pneumonic consolidation of the lungs that humans acquire from animals by drinking untreated milk or by inhaling their infective secretions. The causative organism Coxiella burnetti has a worldwide distribution, affecting various domestic and wild animals and birds.

This organism is particularly resistant to desiccation and can survive heat-treatment temperatures only slightly less than those recommended for pasteurization of milk. The infection in humans may occasionally be severe, with complications like osteomyelitis, endocarditis, meningoencephalitis, and hepatitis. Treatment requires antibiotics, and prevention involves adequate pasteurization or heat treatment of all milk.

Food-Borne Viruses

Any virus transmitted by the fecal–oral route may be transmitted in food. Viruses can contaminate food at its source or during preparation. Mollusks and shell-fish growing in coastal waters that are polluted with fecal material often contain human intestinal viruses. Oysters, clams, and mussels, eaten raw or incompletely cooked, can result in outbreaks of infection by the hepatitis A virus and small round structured viruses (SRSVs) such as those seen recently around the Gulf of Mexico. Outbreaks of virus infection acquired by drinking unchlorinated water have also been reported in other countries (poliomyelitis and hepatitis A and E). Chlorination of water prevents waterborne virus infection. Depuration of oysters and bivalve mollusks in clean water does not remove viruses as easily as it removes bacteria. Therefore, adequate cooking, by raising the temperature to 85 to 90° C and maintaining it there for between 1 and 5 minutes, is recommended.

Viral infections may also be transmitted to food by food handlers who are excreting the virus. A characteristic feature of these viral food-borne infections is the frequent occurrence of secondary cases following the outbreak. Here, person-to-person transmission occurs after the initial food-borne outbreak.

Viral Gastroenteritis

The viral agent most frequently found to cause food-borne outbreaks of gastroenteritis in Great Britain belongs to the group of SRSVs also known as Norwalk and Norwalk-like agents. SRSVs cause an acute short-lived attack in which vomiting is a prominent feature. The disease usually affects adults and is associated with outbreaks of gastroenteritis in restaurants, canteens, and institutions like hospitals. Food handlers are implicated in the transmission, especially when they have had a bout of projectile vomiting in the food preparation area. But any kind of food may also be involved in this form of transmission; raw oysters, for example, have been the source of outbreaks in recent years. Secondary cases usually follow the primary cases and are a result of transmission by airborne aerosols. The clinical picture presented is of a flulike illness with nausea, vomiting, diarrhea, and abdominal pain, following an incubation period of 1 to 2 days. Symptoms resolve within a further 1 to 2 days.

Control measures include scrupulous personal and environmental hygiene, the use of chlorine-based compounds to disinfect contaminated surfaces, and adequate cooking of shellfish.

Rotavirus

Rotavirus infection affects mainly schoolchildren, though adults may also be infected. It has a worldwide distribution, causing enteritis following a fecal–oral or airborne spread—usually in winter months. The incubation period is 48 to 72 hours. The symptoms presented are diarrhea and vomiting. The disease is acute and self-limiting, but moderate dehydration may result. Malnourished and debilitated children in developing countries may be severely affected. Treatment is mainly supportive (rehydration), and recovery is seen in a few days.

Viral Hepatitis

Hepatitis A virus. Hepatitis A affects mainly children and young adults in schools, colleges, and military camps. The virus is excreted in feces 7 to 10 days before the onset of symptoms and for a few days after that. Transmission occurs by the fecal–oral route during this period. Food handlers are important in this transmission since they are infectious before they become ill. Transmission also occurs by water, from flies, and in food, especially shellfish. The incubation period is from 3 to 6 weeks and is followed by symptoms of anorexia, fever, nausea, vomiting, and abdominal pain. Jaundice appears 3 to 10 days later, and the patient then starts to feel better. Prevention includes standard measures of hygiene and well-cooked food. A vaccine is now available to protect against hepatitis A infection. Normal human immunoglobulin, to provide passive protection over a period of 4 to 6 months, has been given to those traveling to endemic areas.

Hepatitis E virus. Waterborne outbreaks of hepatitis E virus infection (a recently recognized entity) have been reported from many countries. It is transmitted by the oral–fecal route, and food-borne infection (especially by shellfish) is a distinct possibility. Preventive measures include the provision of safe water supplies and protection of food from fecal contamination.

Poliomyelitis

Poliomyelitis is a disease affecting only the human host and is transmitted by the fecal–oral route, mainly through contaminated drinking water, although food may also act as a vehicle of infection. The infection usually causes only mild intestinal or systemic disturbances. Occasionally, however, a serious form of the illness is seen with involvement of the meninges, severe paralysis, and even death. In these cases recovery may leave permanent disabilities. A good vaccine is available to prevent the disease, which, it is hoped, will mean its eradication all over the world through a mass vaccination campaign. Meanwhile, improvement in environmental and personal hygiene reduces its transmission.

Bovine Spongiform Encephalopathy (BSE)

BSE is a recently recognized disease of cattle caused by a transmissible agent (prion) that resembles the agent of scrapie in sheep. Cattle develop spongelike areas in the brain, with vacuoles appearing in the gray matter. The infectious agent is extremely resistant to heating and standard sterilizing temperatures, and its incubation period is very long, ranging from 2 to 8 years. It is thought that the scrapie agent from infected sheep may have adapted itself to a new host (cattle), possibly as a result of the practice of feeding sheep protein to cattle. Whether BSE can be transmitted to humans eating infected meat has been a topic of debate for some time, but recent concern over outbreaks of “mad-cow disease” in Britain has prompted a wide belief that this is so. Although as yet unproven, BSE infection from food is suspected to be the cause of a number of human cases of a new type of the rare, deadly brain malady known as Creutzfeldt-Jakob disease.

BSE prevention. Precautions to be taken include the destruction of all BSE-infected animals and their milk, banning the feeding of ruminant-derived protein to cattle, and prohibiting the sale of bovine offal from organs like the brain, intestine, and spinal cord, where the agent may replicate. In recent years, the British beef industry has been badly hurt by these and other efforts to control the disease.

Protozoal Infections

Amebiasis

Amebiasis is a disease with a worldwide distribution that occurs mainly where sanitation and hygiene are lacking. Transmission is mediated by the viable cysts of Entamoeba histolytica, which are tough-coated, rounded forms of the parasite, excreted in the feces of infected humans.The incubation period is between 2 and 6 weeks. Following ingestion of the cysts in contaminated water or food, excystation occurs in the intestines, and the liberated vegetative cells or trophozoites establish themselves in the large bowel. Ulcers are produced in the bowel wall, resulting in symptoms of amebic dysentery, namely diarrhea with blood and mucus in the stools. Infection may spread in the blood to cause abscesses in other organs like the liver, lung, and brain.

In many cases, noninvasive, symptomless infection continues for years, making carriers of people who excrete cysts in their feces and who are, thus, a potential threat to others, but these cases may convert to the symptomatic state when conditions are appropriate—as, for example, in the immunocompromised patient. The vegetative form, called trophozoite (feeding stage), is only occasionally seen in the feces (in diarrheal stool), is noninfective, and is rapidly destroyed in the environment. But the presence of a tough wall around the cyst (resting stage) renders it highly resistant to adverse environmental conditions. Cysts can survive for 2 to 4 weeks outside the host. They develop from trophozoites in the lumen of the gut and are passed out in the feces. Nuclear division and further development occurs within the cyst, even after it has been voided from the body.

Travelers often develop symptoms following ingestion of contaminated food or drink in countries where transmission is high. Oral or anal sex is also a possible route of transmission of this disease (for example, the “gay bowel syndrome” occurring among homosexual people). Prevention is achieved by the provision of good-quality water supplies or by boiling all drinking water (cysts are not killed by chlorine), safe disposal of sewage, screening food handlers to exclude cyst passers, and treatment of already infected individuals.

Cryptosporidiosis

Occurring in both humans and animals, infection by Cryptosporidium parvum is transmitted to the former mostly in rural areas, following contact with animals or consumption of contaminated milk, water, or food that has not been heat-treated. Explosive outbreaks in urban areas (Milwaukee, for example, where in 1993 more than 400,000 people became infected) are usually the result of waterborne spread. The cysts are not destroyed by chlorination but may be removed from drinking water by filtration. Indeed, a malfunction in the filtration of water in sand beds is believed to have caused several outbreaks of this disease in Great Britain. Outbreaks have also arisen from consumption of unpasteurized milk. The pasteurization process destroys this pathogen.

The incubation period is between 3 and 11 days. Following ingestion, the organism emerges from the cyst and penetrates the epithelial cells lining the intestinal lumen, coming to lie just beneath the membrane within a vacuole. Here, organisms multiply to form the oocysts that are released in the feces. The symptoms include impaired digestion, malabsorption, profuse watery diarrhea, vomiting, and weight loss. In the immunocompetent host, recovery occurs within 7 days. Immunosuppressed patients, however, develop severe and protracted diarrhea with dehydration and prostration, as is seen when AIDS patients are infected.

There is no effective treatment for this infection apart from supportive measures like rehydration. Prevention of infection involves preventing oocysts from coming in contact with food or water, provision of safe drinking water, and proper cooking of all food.

Giardiasis

Giardiasis is an infection principally of the small bowel, with symptoms of diarrhea, passage of bulky, pale, offensive stools, abdominal cramps and distension, malabsorption, weight loss, and fatigue. The mal-absorption involves mainly fats and fat-soluble vitamins. The disease is characteristically seen as severe bloodless diarrhea in children. The trophozoite form of Giardia lamblia/intestinalis inhabits the duodenum and upper ileum, closely attached to the mucosa. Transmission is by the cyst forms, which are passed in stools that contaminate water and, sometimes, food. Cysts are destroyed by boiling but not by the routine chlorination of drinking water. Hence, the importance, as with cryptosporidium, of providing efficient preliminary sand filtration. The infective dose is said to be as low as 10 cysts. Following ingestion, excystation occurs, and the trophozoites are responsible for the intestinal pathology. Heavy infestation damages the mucosa and atrophies the villous processes, but there is no penetration into intestinal cells. The disease has a worldwide distribution and is found in relation to poor hygiene and the substandard quality of drinking water. The incubation period is between 1 and 3 weeks, and the disease, if untreated, lasts for 4 to 6 weeks. Symptomless carriage is a common feature. Prevention involves the use of boiled or filtered water and protection of food from contamination.

Other Intestinal Protozoa

Balantidium coli. This is a normal inhabitant (commensual organism) of the pig bowel. It has a worldwide distribution but rarely causes disease in humans. Those ingesting the cysts may develop a symptomless carrier status, or invasion of the colonic mucosa may lead to symptoms of colitis.

Isospora belli. This infection is particularly found in association with diarrhea in patients with acquired immunodeficiency disease (AIDS). In normal individuals it is possibly a cause of symptom-free infection or of mild transient diarrhea. Little is known about the life cycle. The organism inhabits the small bowel, cysts are passed in the stools, and transmission is believed to be by direct person-to-person transfer or via food or water.

Microsporidium. This group of organisms is found in a range of host species. The organisms have also occasionally been isolated from patients with AIDS and are now believed to be opportunistic pathogens.They are possibly transmitted during the spore stage, voided in the stools of an infected host, and they contaminate food or water. When swallowed by the new host, the organisms enter the gut mucosal cells and multiply there, producing symptoms of intractable diarrhea.

Toxoplasma gondii

Toxoplasmosis is a food-borne systemic infection with a wide geographical distribution. Serological tests indicate that a large proportion of the human population has been infected at some stage in life.

Toxoplasma gondii has a complex life cycle, with its definitive or final host (in which a sexual stage of development occurs) being the domestic cat and some of its wild relatives. The sexual stage takes place in the intestine of the cat, which then excretes oocysts in its feces, although the cat is rarely ill with the infection. Indeed, this relationship typifies a successful parasitization, where neither the host nor the parasite appears to suffer. The intermediate host, in which the asexual life cycle occurs, may belong to a wide range of species, including Homo sapiens, who is infected by ingesting the oocysts from cat feces. The parasites then travel via the bloodstream to infect cells in diverse parts of the body, where they multiply asexually (by fission), producing infective cysts. The incubation period may range from 5 to 20 days.

In the healthy individual, the infection is controlled by the immune response, although the parasite may evade immune surveillance by encysting in various tissue cells, surrounding itself with a tough coat, and continuing to multiply at a slow rate within the cyst. Damage to tissues may occur at these sites in immunocompromised individuals like AIDS patients. Pregnant women acquiring the acute infection may transfer the infection transplacentally to the fetus, which may be seriously damaged as a consequence. As in the immunocompromised patient, lesions may occur in the fetal brain, lung, liver, and other tissues with a fatal outcome.

The majority of people infected (often in childhood) develop no symptoms, but some develop a glandular fever-like syndrome, with fever and generalized enlargement of lymph glands. The disease may sometimes present with inflammatory lesions of the eye (choroidoretinitis). Avoidance of infection involves rigorous attention to hygiene, prevention of contamination of food with cat feces, the cooking of all meat before consumption, and advice to pregnant women to avoid contact with cat litter and feces.

Sacrocysts

Sarcocystis hominis causes a disease in humans (the definitive host), with mild symptoms like nausea, diarrhea, and malaise appearing a few hours after the ingestion of raw, infected meat from animals like cattle, sheep, and pigs (intermediate hosts) that contain the encysted infective forms. It is a common infection in animals but rare in humans. This is perhaps because of low transmissibility, inactivation by cooking or freezing, or the frequent occurrence of subclinical infection. The oocysts are passed in the feces of infected humans, and animals ingesting these oocysts develop the tissue cysts.

Trematodes (Flukes)

Adult trematode worms may inhabit either the intestinal tract, the bile ducts, or the lungs. Trematodes are flat, leaflike organisms having an alternating sexual cycle in the final (definitive) host and an asexual multiplication cycle in the intermediate host. Many trematodes have, in addition, a second intermediate host.

Fascioliasis

The liver fluke, Fasciola hepatica, is a common parasite of sheep and cattle kept in damp pastures in many parts of the world, and it is only occasionally found in humans. The adults live in the bile duct of the host and produce eggs that are passed in the feces of sheep and cattle. The developing form (miracidium) leaves the eggs and enters a suitable snail host. Here it multiplies asexually, and in 4 to 5 weeks the infective form leaves the snail and encysts on vegetation such as grass, watercress, or radishes growing at the site. Humans acquire the infection by eating the raw, infected vegetables. The young fluke then excysts and penetrates through the duodenum, goes to the liver, and settles in the bile ducts.

Symptoms of dyspepsia, nausea, fever, and abdominal pain occur in the acute stage. In the chronic stage, pain, enlargement of the liver and obstructive jaundice, and anemia may occur. Prevention involves protecting vegetation to be eaten raw from cattle or sheep feces and eliminating snails by draining pastures or using molluscicides.

Fasciolopsis

Fasciolopsis is the intestinal infection involving snails that occurs in China and some other parts of Asia.The large-sized fluke Fasciolopsis buski attaches itself to the wall of the small intestine of its host and passes its eggs in the feces. When the eggs find themselves in freshwater ponds, they hatch, and the larvae infect freshwater snails, in which they develop further. On leaving the snails, they encyst on the fruit and roots of water plants, like the water calthrop, water chestnut, and lotus. Pigs and humans are common hosts of the infection, which they acquire by eating the infective encysted forms on raw aquatic vegetation. Children in endemic areas often peel this material with their teeth, thus enabling entry of the parasite. Following ingestion, the parasite emerges from the cyst, attaches itself to the wall of the intestine, and develops into an adult.

In most cases the infection is asymptomatic, but where there is heavy infestation, symptoms include diarrhea, vomiting, and gripping pain. Diagnosis is confirmed by finding the eggs in the feces. Prevention involves careful peeling or cooking of water-grown vegetables before consumption in endemic areas.The cysts are killed by the drying of water plants or by a few minutes of boiling. It is necessary to keep human or pig feces away from ponds.

Heterophyiasis and Metagonimiasis

Heterophyes and Metagonimus species are small flukes that attach themselves to the wall of the small intestine between the villi. Eggs are passed in the feces into water, and larvae emerge to enter freshwater snails. The larvae emerge from the snails in due course to infect freshwater fish. Human infection follows ingestion of infective cysts in raw fish. The larvae excyst and establish themselves in the gut where they grow to adults, causing nausea, diarrhea, and abdominal colic. Adequate cooking of fish and prevention of fecal pollution of freshwater are important preventive measures.

Clonorchiasis and Opisthorchiasis

Infection with Clonorchis species occurs mainly in the Far East. Infection with Opisthorchis species occurs in the Far East, in Poland, in the Ukraine, and in Siberia. Adults are small flukes that live in the bile duct. Eggs, passed in the feces, infect snails in slow-flowing streams.

After a stage of development, larvae leave the snail and encyst themselves beneath the scales of freshwater fish. When raw or undercooked fish is eaten, the flukes excyst in the intestine and migrate up to the bile duct. Light infection may cause only vague symptoms related to the biliary tract. However, signs of biliary obstruction, portal hypertension, and enlargement of liver and spleen may occur in heavy infection. Dogs can also be infected and act as reservoirs of infection. Prevention involves the cooking of fish and treatment of feces before their use as fertilizers.

Gastrodisciasis

Human infection occurs in India (Assam), Bangladesh, Vietnam, China, and Russia, where adult Gastrodiscoides hominis attach themselves to the large intestine (colon or caecum), and eggs pass out in the feces to enter and develop in pond snails. The infective form then leaves to encyst on vegetation, like water calthrop. Eating infected raw vegetables leads to human infection and superficial inflammation of the intestinal mucosa. The presenting symptoms are diarrhea and passage of mucus in stools. Pigs act as a reservoir of infection. Prevention includes the careful peeling and washing of all water vegetables and keeping pig feces away from the water. Dried water-plants can be fed to pigs because drying inactivates the parasite.

Paragonimiasis

Paragonimiasis is seen in Asia,Africa, and America. The lung fluke—Paragonimus westermani—infects humans and animals that eat crustaceans like freshwater crabs and crayfish. The fleshy adults live in pairs in the lungs, resembling large coffee beans in size and shape. Eggs may be coughed up in sputum or swallowed and passed in feces. When they reach fresh water, the larvae emerge and penetrate snails. After a stage of development, the larvae leave the snails to penetrate and encyst in crabs and other crustaceans. When these are eaten uncooked, they can transmit the infection. Pickling processes that fail to kill the larvae are also often involved in transmission of the infection. Excystation in the small intestine enables the young fluke to emerge, penetrate the gut wall, and migrate across the diaphragm and into the lungs. Granulomatous changes occur in the lung surrounding the fluke, and symptoms include dry cough, pain in the chest, difficulty in breathing, and the coughing up of blood. Prevention involves the proper cooking of crustaceans and care in handling and washing them so as not to cause cross-contamination with other food.

Cestodes (Tapeworms)

Adult tapeworms all have flat, tape-like bodies and live in the intestinal tract. The body is segmented and the eggs are contained in the terminal segments, which are passed out in the feces. These eggs are infective.

Taeniasis

Taenia saginata (beef tapeworm). Human infection with this tapeworm occurs when raw or slightly cooked beef is eaten. Cattle are infected by eating infective eggs and develop cysticercosis, whereby infective larvae are found encysted in the muscles. When beef is eaten raw by humans, the larvae emerge and attach themselves to the walls of the small intestines and grow into adults of 5 to 20 meters in length. The terminal segments containing eggs drop off, emerging with feces to remain infective on grass and vegetation contaminated by human feces. The eggs are eaten by cattle and hatch out in the duodenum. The parasites then penetrate through the intestinal wall to reach the bloodstream and are carried to the muscles, where they form fluid-filled cysts (cysticerci). Each of these contains an infective larva that, when ingested raw by humans, can grow into an adult tapeworm in the gut. Cysts are killed by cooking at over 56° C.

The symptoms involved are vague abdominal complaints, with segments of worm occasionally wriggling out of the anus. Prevention includes cooking beef adequately, inspecting all meat intended for sale, and the condemnation of any “measly” meat containing tiny pinhead-sized cysts.

Taenia solium (pork tapeworm). People are at risk of contracting this tapeworm in countries where much pork is eaten, including Eastern Europe, China, Indonesia, South Africa, Mexico, and Chile. The infection is carried in pigs as infective cysticerci in the muscles. People acquire the infection by ingesting undercooked or raw pork. The infective form then leaves the cyst and attaches itself to the mucosa of the small intestine, where it proceeds to grow over the next 10 weeks to its full length of between 2 and 10 meters. The terminal segments containing the eggs are passed out in the feces. Pigs eat the infective eggs in soil or feces, and the larvae hatch out, penetrate the gut wall, and travel by the bloodstream to encyst in the musculature, causing cysticerci. As in cysticercus bovis, each cyst contains the infective cephalic end of the worm, which, when ingested by the definitive host (human), excysts and attaches itself to the small intestine, where the worm develops to the adult form. Symptoms are abdominal pain, diarrhea, and passage of segments through the anus.

Humans may also develop cysticercosis when they ingest the eggs of Taenia solium along with raw vegetables or drinking water. The larval forms migrate to and encyst in muscles, brain, and other tissues. Auto-infection may possibly cause cysticercosis in humans when reverse peristalsis carries eggs up to the stomach and duodenum, enabling hatching and migration of larval forms. The larvae in the cysticerci die off within a year and become calcified. Symptoms produced are related to the site of the lesion; for example, Jacksonian epilepsy can occur when the cysts form in the brain.

Prevention of transmission is achieved by (1) eating only well-cooked pork; (2) inspection of pig carcasses in the slaughterhouse to eliminate any that show signs of cysticercosis infection; and (3) safe disposal of human feces to interrupt the transmission to pigs.

Hymenolepiasis

Hymenolepiasis is cosmopolitan in distribution. Children in institutions and the immunodeficient or mal-nourished are particularly prone to it. The dwarf tape-worms of Hymenolepis species do not require an intermediate host. The adult worm lives in the small intestine, passing its eggs in the feces. When these eggs are ingested by other humans along with contaminated food or water, infection occurs. Autoinfection also takes place when eggs hatch out in a patient’s gut; they develop into adults in 2 weeks.

Hydatidosis (Echinococcosis)

Hydatid disease occurs in the sheep- and cattle-rearing areas of the world, mainly those in South America, Kenya, Vietnam, and China. The adult Echinococcus granulosus is small, less than 1 centimeter in length, and inhabits the small intestine of carnivorous animals like dogs—the definitive hosts. The terminal segments of the worm, which contains the eggs, are passed in the dog’s feces and can survive for months on pastures. When eaten by the intermediate host (humans, cattle, sheep, goats), the eggs hatch and the larval form penetrates the intestinal wall to be carried by the bloodstream to various parts of the body where hydatid cysts develop. The sites affected are often the liver, lung, brain, or bone, with the slow-growing cyst consisting of a laminated cyst wall filled with fluid and containing many infective forms (hydatid sand). When the tissue form is eaten raw, as happens when dogs are fed infected offal in farming regions, adult worms develop in the dog to repeat the cycle.

Symptoms of a slow-growing, space-occupying cyst are related to the site of the lesion. Treatment often requires careful surgical removal of the cyst. Prevention involves personal hygiene to avoid the ingestion of eggs from dogs, with regular worming of dogs to eliminate the tapeworm, and sanitary disposal of infected offal and viscera of slaughtered animals.

Diphyllobothriasis

Diphyllobothriasis occurs in temperate and subarctic countries where fish is eaten raw, as in Finland, the former Soviet Union, Japan, Canada, the United States, and Chile. The adult Diphyllobothrium latum (fish tapeworm) measures 3 to 10 meters long and remains attached by the head end to the mucosa of the small intestine. Eggs are discharged on their own (not within the segments as in taeniasis), pass out in feces, and hatch when they reach fresh water. The parasite now waits to be ingested by a freshwater microcrustacean (cyclops), in which it develops further. If this tiny freshwater cyclops is ingested by a freshwater fish, the larva emerges and penetrates the intestinal wall, developing into its infective form in the muscles of the fish. People ingesting infected raw fish acquire the disease, the symptoms of which include diarrhea, vomiting, fatigue, abdominal pain, dizziness, and megaloblastic anemia. Diagnosis is based on looking for eggs in stools. Prevention means avoiding raw fish, for the infective larvae can be killed by cooking, freezing, or thorough pickling procedures. Human sewage should be treated before discharge into water.

Nematodes (Roundworms)

Ascariasis

Ascariasis is a common disease in areas with poor sanitation, especially in the tropics. Infection with Ascaris lumbricoides (roundworms) is achieved by ingesting embryonated eggs on food, like salad vegetables. The larvae hatch, penetrate the wall of the duodenum, and travel in the bloodstream to the lungs, where they develop further. They then penetrate the alveoli and come up the trachea to be swallowed down into the intestines, where they settle in to mature in the ileum. Adults live in the lumen of the gut for up to 2 years, and their eggs are passed in the feces. The eggs are thick-walled and able to resist drying and the standard procedure for sewage treatment. They are, however, killed by heat over 60° C. These eggs require a further period of 10 to 50 days to develop in warm moist soil before they become infective. They may be consumed on raw salad vegetables or unpeeled fruit.

The majority of cases remain symptomless although respiratory symptoms occur when the larvae are traversing the lungs (cough, chest pain, pneumonitis with eosinophilia). When a heavy worm load is present in the intestine or when allergic reactions to the worm occur, symptoms of digestive disorder or intestinal obstruction may be observed. Diagnosis may be made by demonstrating eggs in feces. Prevention is accomplished by thorough washing of all salad vegetables, improvement of sanitation, safe disposal of sewage, and treatment of existing cases.

Trichuriasis

The adult nematode Trichuris trichiura (whipworm or thenoworm) has a narrow anterior whiplike portion and a broader posterior end. The whole worm is 40 centimeters long and lives in the colon with the anterior whiplike end embedded in the mucosa. Eggs are passed in feces, develop in warm moist soil, and are ingested by another human host. Larvae hatch to penetrate the wall of the large intestine where they develop into the adult form. They then return to the lumen of the colon, embed their whiplike ends by tunneling through mucosa, and repeat the cycle.

In children, following heavy infestation, the disease may present as retarded growth and malnutrition. Sometimes a prolapsed rectum may occur. Prevention measures are the same as those for ascariasis.

Enterobiasis

Enterobiasis is a common disease in most parts of the world, including temperate countries. The adult Enter-obius vermicularis (pinworm) is about 11 millimeters long and lives in the lumen of the large bowel. Females lay their eggs on the skin around the anus, which causes intense itching. When scratched, the eggs are transferred on fingers or through dust to food. After they are ingested, larvae emerge to develop into adults in the bowel. The disease is common among children, can affect whole families, and is seen in institutions.

Diagnosis involves the demonstration of eggs in skin around the anus, whereas prevention involves hand washing and treatment of whole families or institutions. The cleaning of bedclothes and rooms to remove eggs is also essential.

Trichinosis

Infection with Trichinella spiralis in animals is worldwide, but human infection is mainly seen in Eastern Europe, the Arctic regions, South America, Asia, and East Africa. The domestic pig is the main source of infection, and the infective larval form is found encysted in the muscles. When eaten in raw or under-cooked pork, it develops into the adult worm, which lives embedded in the mucosa of the small intestine. The adult is viviparous, producing many live larvae, which penetrate the intestinal wall to be carried in the blood to muscles, where they coil up forming cysts.

Symptoms include diarrhea, abdominal pain, and pain in the muscles. Heavy infection causes severe illness and may be fatal due to the involvement of the nervous or cardiac systems.

Diagnosis is confirmed by looking for larvae in muscle biopsies, and prevention is accomplished by cooking meat at over 60° C or deep freezing it to kill the larva. In addition, improving their feed prevents pigs from contracting the disease.