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Commercial Pet Food Contamination

Bacterial, Fungal and Endotoxin Contamination of Pet Foods

Bacterial and Fungal Contamination

Bacteria and other microorganisms contaminate commercial pet foods and can be responsible for digestive tract diseases. The bacteria found consist of microorganisms associated with food ingredients, acquired during handling and processing, surviving any preservation treatment, and contaminating food in storage. Most pet foods are exposed to many potential sources of microorganisms. They include sources of contamination during production, harvest, handling, processing, storage, distribution, or preparation for consumption. Contamination can be by bacteria in soil, water, air, living plants, feed or fertilizer, animals, human beings, sewage, processing equipment, ingredients, and packaging materials. Contaminated final products can also be a source of contamination for other products.

During the final process of dry food manufacture the product is coated with a digest of animal proteins and liquid fat (or with lactose for Purina Puppy Food). The digest contains proteins such as chicken viscera. Although cooking destroys any bacteria, the final product loses its sterility during subsequent drying, fat-coating, and packaging stages of the manufacturing process.

The number of bacteria in food varies. It depends on original contamination, increases or decreases of bacteria during processing, recontamination of a processed product, and growth or death during storage, retailing and handling.1 The usual number of bacteria in most animal products used for food is 1,000 to 10,000 per gram. Ground meat is more contaminated than whole cuts of meat because of the type of meat used, extra handling during grinding, and release of meat juices that promote bacterial growth. Bacterial numbers are lower in heated than unheated foods. But poor-quality ingredients, poor sanitation, unsatisfactory heating, recontamination, or poor handling and storage cause some heated products to have high bacterial numbers.

There is great concern today about food wholesomeness. A focus of this concern is contamination of animal-food-sources by disease-causing bacteria. People have become ill and died after consuming meat products contaminated with some forms of Escherichia coli. If human foods are very widely contaminated, problems of bacterial contamination are likely to be as great for pet foods. The quality of ingredients in pet foods is poorer than for human consumption. In addition, animal-protein foods rejected for human consumption are ingredients for either pet foods or fertilizers. Bacterial contamination of rejected food is likely because it is a common reason for rejection. To change this, it would require the industry to completely change the source of important ingredients for making pet foods.

Microorganisms Involved
A study was recently completed to determine the numbers and kinds of bacteria that could be cultured from many commercial dry pet foods.2 It is surprising that all such foods are contaminated with bacteria. Although only some of those in the following table consistently cause disease, the widespread contamination of the popular brands indicates that feeding a dry food exposes a pet to many different bacteria.

The numbers of bacteria may not affect most animals that consume a meal quickly. It is predictable that dry foods will cause problems when some of the label feeding instructions are followed. One large pet food manufacturer recommends the following on feeding puppies:

During weaning, it is best to keep moistened Purina Nutrient Management available at all times.

Bacteria in the moistened dry food multiply rapidly so for puppies that consume the food later, severe vomiting and diarrhea can follow. This is not a recommendation that should be made for a product contaminated with bacteria.

Bacterial Contamination of 40 Dog and 40 Cat Foods

Bacterial Contaminant

Percent of Dog Foods

Percent of Cat  Foods


Bacillus cereus*



other Bacillus species**



Enterococcus faecalis***



10 other bacteria**



* Causes digestive tract disease

** Some can cause digestive tract disease

*** Sign of contamination by fecal material

The numbers of bacteria may not affect most animals that consume a meal quickly. It is predictable that dry foods will cause problems when some of the label feeding instructions are followed. One large pet food manufacturer recommends the following on feeding puppies: During weaning, it is best to keep moistened Purina Nutrient Management available at all times.” Bacteria in the moistened dry food multiply rapidly so for puppies that consume the food later, severe vomiting and diarrhea can follow. This is not a recommendation that should be made for a product contaminated with bacteria.

Different species of Salmonella bacteria are best documented as the cause of diarrhea in dogs and cats. Salmonella can be cultured from the feces of up to 30 percent of dogs.3 Many are normal and show no signs of disease. Infection by Salmonella usually follows ingestion of contaminated food or water. The food is of animal origin or contaminated by foods of animal origin. Poultry and poultry products are the most frequent source. Other meats and milk are also sources. Salmonella has been found in commercial pet foods, something the public never learns. It should not be surprising that salmonella is found in pet foods. A notice in the American Veterinary Medical Association Journal reported that:

The results of an FDA survey of vegetable and animal protein ingredients used in animal feed, presented at the recent US Animal Health Association meeting, show that nearly 57% of animal protein samples and 36% of the vegetable protein samples tested positive for Salmonella. In other data, more than 60% of animal-protein processing plants and 37% of vegetable-processing plants tested positive for Salmonella.

Pet foods are cooked to kill Salmonella bacteria but where the processing plant is contaminated, food is exposed to contamination after cooking. The chances of contamination with Salmonella are so great that some companies routinely culture their final products. Finding contamination, the batch of food is not sold for animal consumption. Unfortunately, the assumption is made that commercial pet foods are not importantly contaminated with other bacteria; manufacturers do not culture for others. Other bacteria can contaminate pet foods and be a potential cause for problems, however.

Disease caused by foods contaminated with Staphylococcus aureus is the second most commonly identified form of foodborne bacterial disease. These bacteria most commonly are found in contaminated meat and an important source is food processors. The bacteria produce toxins that cause vomiting and diarrhea. The toxins are exotoxins that directly damage the intestinal mucosa and enterotoxins that stimulate the intestine to secrete large amounts of fluid that cannot be completely reabsorbed.

Staphylococcus aureus contamination can be prevented by using clean food sources. Since food processors are an important source of contamination, adequate hygienic measures should be followed. That might include removing contaminated handlers from the processing operation. Optimum cleaning and sanitizing procedures are necessary. Maintaining food sources under proper refrigeration is essential. Once the food is contaminated, the product can be cooked to destroy the bacteria but heating does not destroy any toxins that have already been formed so that the food remains a source for gastrointestinal problems.

Clostridium perfringens is the third most common bacterial cause of food-borne illness in humans. This organism is a well-documented cause of gastrointestinal disease in dogs and cats.4 It is normally found in the intestinal tract of small animals and causes no problems. Under certain conditions it produces spores containing enterotoxin that is released when the spores break apart. Food contaminated with the bacterium can be cooked to kill the organism but the spores usually survive. Therefore, cooking a commercial pet food during its processing kills contaminating bacteria but spores containing enterotoxin are still present to cause illness. When diagnostic testing is done enterotoxigenic Clostridium perfringens is a common finding in dogs presented with gastrointestinal disease; it can be responsible for chronic diarrhea. Testing is by examining fecal samples for Clostridium perfringens spores and by identifying its enterotoxin in feces.

Clostridium perfringens infections are associated with environmental contamination that results in transmission of enterotoxin-producing strains. Many cases develop after boarding at a kennel or during a hospital stay. Whether most of these animals become infected from the premises or from contaminated pet food is unknown. Decontamination is difficult because Clostridium perfringens spores are resistant to disinfectants and heat. Clostridium perfringens infection requires treatment with antibiotics, and long-term therapy may be required.5 Dietary management with fiber supplements are reported to be of benefit.

Food-borne infection by Escherichia coli is seldom documented in dogs and cats because complicated and expensive testing is required for its identification. Escherichia coli are found in the large intestine of normal animals, in a site where they cause no problems. Some of these organisms are not pathogenic. Others can invade intestinal mucosa and the body, produce an enterotoxin, or produce an exotoxin that directly destroys the mucosa and causes hemorrhagic diarrhea.6

The disease producing strains may be prevalent, as shown by the outbreaks of Escherichia coli infection in people eating inadequately heated meat from fast-food restaurants. Is this an important problem? In the summer of 1994 a panel of experts met and concluded the following:

A new, sometimes fatal form of E. coli bacteria is now so prevalent in beef that is is no longer possible to guarantee the safety of hamburger, a panel of experts said Wednesday.

The panel of scientists, policymakers and meat industry representatives urged some new methods for treating meat before it is sold to the public. They recommended that all ground beef be radiated to kill bacteria. That has not happened yet. Is this applicable to pet foods? All the meats they contain are ground and are probably more contaminated than ground beef prepared for human consumption.

This acknowledges that Escherichia coli are commonly found in meats used for human consumption. Heating reduces their ability to cause gastrointestinal upsets by killing the organism and destroying some toxins they produce. Some toxins are resistant to heat, however. It is not known whether any commercial pet foods ever contain any heat-resistant Escherichia coli toxins that survive heat-processing, however.

Escherichia coli in food represents "fecal contamination" and that is more common for some ingredients in pet foods than in foods for human use.7 For example, the meat meals found in many pet foods are prepared from dead animals very contaminated with fecal types of bacteria (coliforms). It is possible that Escherichia coli are an important cause of infectious gastrointestinal disease in animals. This would make it important to feed pet foods made only from "wholesome" foods.

Other bacteria with the ability to cause gastrointestinal disease include Bacillus cereus, Campylobacter jejuni, Streptococcus species, and a variety of others. Their importance for dogs and cats is unknown.

Fungi are a cause of food-borne illness. They cause disease by invading intestinal mucosa, allergic reactions, and toxic chemicals produced by fungi. Many toxins are produced by fungi with aflatoxin produced by Aspergillus flavus the most studied. Many foods and feeds contain aflatoxin. Peanuts and corn have been the most important source. The toxin can cause acute and sometimes fatal problems. The toxin can also cause chronic disease including cancer for which the cause is often not recognized. Aflatoxin production in pet food depends on harvest, production, and storage procedures for ingredients and finished products. Prevention is mostly by selection and use of wholesome ingredients, and proper handling and storage to minimize fungal growth.

In August 1995 Nature's Recipe, a pet food manufacturer, recalled seven of their dry dog and cat foods made over a two-month period. The products caused many animals to sicken. The company did not state a specific cause but indicated that the products may have contained mycotoxins. The manufacturer blamed the contamination on their source of raw grain. This outbreak shows the importance of using only wholesome ingredients for feeding pets.

Foodborne infectious diseases are not commonly caused by viruses. That may be because little has been done to investigate such problems.

Chemical By-products of Microorganisms

If heat has been used to destroy bacteria contaminating food, it is still possible to identify the contamination by chemicals left behind. For example, bacteria acting on the amino acid histidine convert it to histamine, which remains after the bacteria are killed by heat treatment. Measurement of histamine levels in cat foods show high concentrations in some, reflecting bacterial contamination of ingredients. Other foods rich in histamine include fermented cheeses (very high amounts), pork and beef sausage, some liver, canned tuna, meats, spinach and poorly stored fish such as mackerel. Foods can also stimulate the release of histamine. They include egg white, shellfish, chocolate, fish, strawberries and tomatoes as well as alcohol.

Many other chemicals formed from bacterial action on nutrients could be used to evaluate food quality. Unfortunately, little is done to assess food quality other than subjective means of sight, smell, taste, or touch. Measurement of endotoxin levels can evaluate a food quality by indirectly determining numbers of endotoxin-producing bacteria in a food before any heat treatment to destroy bacteria.

Bacterial Toxins

Bacteria produce many toxins that cause diarrhea and generalized signs of illness. Three types of toxins are produced. One remains in the intestinal tract, attaches to mucosal surfaces, and stimulates secretion of fluid. This enterotoxin can cause massive losses of fluid and death in untreated cases. Cooking kills enterotoxin-producing bacteria and usually inactivates their toxin.

Cytotoxins are a second type that can kill mucosal cells directly. Some Clostridia bacteria produce this toxin.

Endotoxin is a third type of toxin produced by some bacteria. Endotoxin is formed from part of the cellular structure of gram negative bacteria, such as Escherichia coli, normally found in the colon. Endotoxin is released on bacteria death. As mentioned, coliform bacteria live primarily in the colon but little endotoxin is absorbed there. When coliforms live in the small intestine, endotoxin produced and released can be absorbed and enter the body. Small amounts of endotoxin cause shock that can lead to death.

Endotoxin Effects on the Intestine
Processed foods contain endotoxin.2 Endotoxin levels are not measured in pet foods, however, possibly because the amounts are thought to be low. Endotoxin given orally makes normal animals sick, however.8 Small amounts of endotoxin cause diarrhea in humans with inflammatory bowel disease.9 From the few studies that have examined this, endotoxin's mechanism of action is not entirely clear. Endotoxin is known to cause illness by reducing colonic salt and water absorption. It also increases colonic mucosal permeability by releasing mediators of inflammation. Increased intestinal permeability makes it easier for the absorption of endotoxin as well as other substances which can stimulate allergies. Endotoxin acts synergistically with poor protein nutrition to permit the entry of increased numbers of intestinal bacteria into the portal circulation. Thus, endotoxin can initiate and perpetuate damage to the intestinal mucosal surface, and it can perpetuate inflammatory diseases of the digestive system, including allergies.

Endotoxin Effects on the Liver
Endotoxin entering the body is carried to the liver where it is inactivated. Increased endotoxin levels can damage the liver.10 Moreover, when the amount of endotoxin reaching the liver is normal, the presence of another potential toxin can interact with endotoxin to damage the liver.10 The other substances are not necessarily toxins. They include vitamin A, copper and iron, and many drugs. Thus, any level of endotoxin can damage the liver. Exposure to endotoxin should be minimized as much as possible.

Control of Endotoxin Absorption
Antibiotics given orally are helpful in the management of many digestive tract and liver problems in dogs and cats. One beneficial effect of antibiotics is from their activity to reduce numbers of intestinal bacteria and therefore reduce endotoxin production. The potential of endotoxin from the intestine acting with normal levels of substances such as vitamin A or copper to damage the liver is less after giving antibiotics orally.10 Reduction in endotoxin production also helps protect the lining of the digestive tract. In addition, antibiotics reduce numbers of invading bacteria to which an animal might be allergic.   

The diet affects endotoxin production. When nondigestible cellulose or hemicellulose fiber accumulates and ferments in the colon, aerobic bacteria (mostly coliforms) increase 100 to 1,000 times.11 Most pet foods have high concentrations of undigestible matter, thereby supporting aerobic bacterial growth better than other foods. Polished white rice contains low amounts of nondigestible fiber and is more completely digested than other sources of carbohydrate in pet foods. Rice's inclusion in the diet reduces growth of colonic endotoxin-producing bacteria. Thus, it is the carbohydrate of choice in diets for treatment of digestive tract diseases in dogs.

Cultures of lactobacillus are of no value in reducing the numbers of endotoxin producing bacteria in the digestive system of dogs and cats.

Food as a Source of Endotoxin
In addition to using measures for minimizing the production and absorption of endotoxin in the digestive system, attention should be paid to reducing endotoxin levels in pet foods. In a recent study the level of endotoxin was measured in commercial pet foods.2 All food examined contained endotoxin. Some contained very large amounts, amounts that were 50 times higher than foods with smaller levels of endotoxin.

Meat Meal Contaminants
Greater than 50 percent of meat meal can be contaminated with Salmonellae.4 These bacteria produce endotoxin. Processing pet foods containing meat meal by the expansion-extrusion process kills bacteria. Cooking contaminated meat meal does not destroy endotoxin, however, so the final product contains a toxin that can cause disease. Contaminated meat meal is also likely to contaminate other ingredients including the final product after cooking. Salmonellae sometimes contaminate the premises to the extent that some pet foods containing no Salmonella-tainted ingredients are contaminated before the products leave the factory. Recommendations are made to produce animal-protein sources that are Salmonella-free and to keep them clean.4 Other recommendations are to eliminate meat meal materials from feed formulas. It is much cheaper to use meat meal than other protein sources, however. Meat meal is even cheaper than plant proteins.

Meat meal is one dietary source of endotoxin that can make an animal sick. Manufacturers do not analyze pet foods for endotoxin, but it can be present. The fact that coliforms can be cultured from meat meal shows that it contains endotoxin. Meat meals are highly contaminated with bacteria because their source is not necessarily slaughtered animals. Animals that have died because of disease, accidents, or natural causes are a source of meat meal. The animal carcasses may not be rendered or cooked until sometimes days after death. During this time even if the carcass is kept at cool temperatures, bacteria leave the intestinal tract and spread throughout the body. Thus, the carcass is contaminated with bacteria and high concentrations of endotoxin accompany such contamination.

The pet food industry uses inexpensive sources of protein in order to produce an inexpensive product. Thus, meat-meal protein continues to be used. Some argue that dumping this source of protein is a waste of nitrogenous products that could serve some useful purpose. Some suggestions include the use of meat-meal protein to make fertilizers instead of pet food. Such fertilizers would contaminate soils with disease causing bacteria. Food for human use could become contaminated by this practice. The solution is to produce animal-proteins that are Salmonella-free.

Coliform bacteria such as Escherichia coli have been isolated from meat products that cause severe and sometimes fatal disease in humans. This problem is solved by cooking meat for longer times and maybe at higher temperatures. Because Escherichia coli is a coliform it is another source of endotoxin, however. Heat does not destroy endotoxin and it can be a cause of illness. So Escherichia coli can produce disease by more than one means, with one still potent after thorough cooking. This shows the important need to produce clean meat for both pet and human consumption.

Miscellaneous Food Contaminants
Ingredients for pet foods can contain other disease-causing agents. Aflatoxin is one such that comes from mold or fungi growing on foodstuffs. Mold growth does not occur with proper drying and storage of crops. Pet food plants should control the moisture and humidity of storage areas to prevent mold damage. Proper plant sanitation is also necessary to minimize fungal contamination of pet foods. Heating does not destroy fungal toxins so they remain in the cooked final product. Ingredients most likely to be contaminated with toxin are peanut meal, cottonseed meal, and fish meal because they originate from areas of the world where there is a high risk for aflatoxin. Pet foods made from such products can be dangerous to feed.

In August 1995 Nature's Recipe, a pet food manufacturer, recalled seven of their dry dog and cat foods made over a two-month period.12 The products caused many animals to sicken. The company did not state a specific cause but indicated that the products may have contained mycotoxins. The manufacturer blamed the contamination on their source of raw grain. This outbreak shows the importance of using only wholesome ingredients for feeding pets.

Decontamination of Pet Foods
Food is cooked for several reasons. Cooking improves digestibility and kills bacteria that might cause disease. Currently there is great interest in some Escherichia coli that can cause severe digestive tract problems and sometimes death. This problem appeared because the cooking of meat was inadequate to kill bacteria. The problem was solved by cooking meat longer. The presence of these bacteria showed that meat was contaminated with a coliform and killing the bacteria took away the possibility of sickness caused by a toxin destroyed by heat. Cooking does not eliminate the problem caused by endotoxin released from the dead bacteria, however. Endotoxin persists in cooked food when it has been previously contaminated by coliform bacteria.

Drinking water can also be a source of endotoxin released from coliform bacteria that are killed by chlorination. Coliform bacterial numbers reflect the degree of contamination by fecal material in water as well as in food.

Endotoxin contaminates ingredients in pet foods. Meat meal with or without bone is often made from animals that died, not slaughtered for meat. In many cases the animals are dead for hours to days without being refrigerated. They decompose before being processed as meat meal. Shortly after death the bacteria that are in very large numbers in the colon migrate out into the rest of the carcass. Coliform bacteria contaminate the carcass used for producing meat meal. Meat meal is processed by cooking to render out the fat and to kill bacteria. Killing large numbers of coliform and other gram negative bacteria releases large amounts of endotoxin. The level of endotoxin in pet foods reflects a food's degree of bacterial contamination before cooking. The level of endotoxin reflects the quality of the foodstuffs that went into the final product. The amount of endotoxin decides whether a pet is at risk for illness when it consumes the food.

Commercial Pet Food Recalls

Pet foods were involved in a widely-publicized recall in 2007 because of toxic chemicals. The reasons for the failures of these products included the incorporation of ingredients to reduce the costs of the final product. Formulations to reduce costs have been and will probably continue in producing pet foods.14   
Pet food contaminated with pathogenic bacteria such as Salmonella prompt recalls on a disturbing basis.15
The magnitude of all recalls is documented by the Food and Drug Administration with listings of products recalled over the recent years.16


Government and industry call for pet food ingredients to be "wholesome." The definition of wholesome is "good for one's health" or "healthful." Foods with predictably high levels of microorganisms or toxins can produce signs of gastrointestinal disease and cannot be considered wholesome. They should not be used to produce food for pets.

Bacteria live in the small intestine and they represent the retention of orally ingested bacteria. The kinds and numbers of bacteria living there change as the population of ingested bacteria change. Bacteria are contaminants of food and/or environment. The contamination of the small intestine can be controlled by using wholesome foods and proper processing for producing pet foods. Recent studies show that the bacterial population of the small intestine can be reduced slightly by the addition of some novel complex nondigestible carbohydrates to pet food.13 Use of such substances to prevent the consequences of bacterial contamination of pet foods should not replace efforts to produce a product by using wholesome ingredients.


1. Banwart GJ. Basic Food Microbiology. New York: Van Nostrand Reinhold, 1989.

2. Cullor James S. 1995. Unpublished data.

3. Borland, Edith D. 1979. Salmonella infection in dogs, cats, tortoises and terrapins. Veterinary Record,  96(18):401-402.

4. Anonymous. Food notes on immunosuppression; Salmonella JAVMA vol 206 P. 935. April 1, 1995.

5. Twedt DC. Canine Clostridium perfringens Diarrhea. Proceedings of 17th Waltham/OSU Symposium, 1993, p 28-32.

6. Strombeck Donald R, and Grant Guilford. 1990. Small Animal Gastroenterology, 2d ed. 320-327. Davis: Stonegate Publishing.

7. Hollingsworth, Jill, and Bruce Kaplan. 1997. Zero Tolerance for Visible Feces Helps FSIS Fight Foodborne Pathogens. Journal American Veterinary Medicine Association 211(5):534-535.

8. Cort, N., G. Fredriksson, H. Kindahl, L.-E. Edqvist, and R. Rylander. 1990. A Clinical and Endocrine Study on the Effect of Orally Administered Bacterial Endotoxin in Adult Pigs and Goats. Journal of Veterinary Medicine 37:130-137.

9. Ciancio, Mae J. 1994. Endotoxin-induced diarrhea in inflammatory bowel disease: cellular and molecular mechanisms. Progress in Inflammatory Bowel Disease 15(1):17.

10.Strombeck, Donald R, and Grant Guilford. 1990. Small Animal Gastroenterology, 2d ed. 519-521. Davis: Stonegate Publishing.

11. Strombeck, Donald R, and Grant Guilford. 1990. Small Animal Gastroenterology, 2d ed. 18-19. Davis: Stonegate Publishing.

12. Anonymous. 1995. Nature’s Recipe recall lost $20 million due to vomitoxin. Petfood Industry 37(6):37-38.

13. Willard, M. D., R. B. Simpson, K. Delles, N. D. Cohen, T. W. Fossum, E. D. L. Kolp, and G. A. Reinhart. 1994. Effects of dietary supplementation of fructo-oligosaccharides on small intestinal bacterial overgrowth in dogs. American Journal Veterinary Research 55:654-659.