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Diagnostic dilemma: "I know the pet food was responsible" (Proceedings)


When a pet exhibits signs of gastrointestinal disease, the owner often concludes that its food must be the culprit. Pet owners are particularly sensitized to the issue of pet food safety as a consequence of the large pet food recall of 2007 in which imported feed commodities intentionally contaminated with melamine, cyanuric acid, ammeline and ammelide were incorporated into pet foods.

When a pet exhibits signs of gastrointestinal disease, the owner often concludes that its food must be the culprit. Pet owners are particularly sensitized to the issue of pet food safety as a consequence of the large pet food recall of 2007 in which imported feed commodities intentionally contaminated with melamine, cyanuric acid, ammeline and ammelide were incorporated into pet foods.

In the past, when pets relied on table scraps, carrion, garbage, and improperly cooked pet foods for sustenance, this conclusion was reasonable. Today, foodborne disease in household pets is rare. The 1990 annual report of the American Association of Poison Control Centers (AAPCC) indicated that of the 41,854 cases of dog and cat poisonings reported, foodborne illnesses accounted for only 1.7% of the total. This phenomenon can be attributed to the fact that most pets in developed countries depend on processed commercial pet foods for their daily diet, instead of fresh food or table scraps. Present-day commercial pet foods are much safer than those in the past because of modem manufacturing methods and a high degree of governmental regulation. There is an incentive for pet food manufactures to provide a safe and nutritious product since procedural breakdowns or accidental contamination during the production or storage of a pet food product can have a catastrophic effect on company profits and reputation and even the future viability of a company.

Modem pet foods are not composed of a single ingredient but are formulated from multiple ingredients, including grains, meats, meat by-products, vegetables, eggs, dairy products, fish, and other added nutrients. The use of many and varied ingredients tends to dilute any contamination in a particular commodity or ingredient. Commercial pet food manufacturers commonly use manufacturing techniques such as extrusion for dry foods and cooking in a commercial retort for canned foods to produce temperatures high enough to destroy most pathogens and heat-labile toxins. Improved packaging materials and a better knowledge of proper warehousing also help protect raw materials and finished products from moist conditions and possible contamination during storage. Furthermore, manufacturers use sensitive analytical techniques to verify that ingredients and final products are high quality and free from contaminants

Pet foods and individual pet food ingredients that are shipped across state or international boundaries are regulated by the U.S. Food and Drug Administration (FDA) under the authority of the Federal Food, Drug and Cosmetic Act (FFDCA). Section 402 of the Act states that foods, including pet foods, shall be considered adulterated when they contain an added substance (e.g., bacteria, mycotoxins, drugs, pesticides, metals) that may render the food injurious to health. The Act further empowers the Secretary of Health and Human Services to promulgate regulations and tolerances that limit the quantity of these unintentional added substances. The FDA monitors pet food and individual pet food ingredients for pesticides, mycotoxins, and heavy metals as part of its Feed Contaminants Program. Pesticide tolerances for food ingredients and food products are unique in that they are not set by the FDA but instead fall under the jurisdiction of the U.S. Environmental Protection Agency (EPA) under the authority of the FFDCA and the Federal Insecticide, Fungicide and Rodenticide Act (FIFRA). They are developed by combining the results of crop and animal field trials with laboratory animal toxicity data. The EPA establishes and publishes pesticide tolerances for the various plant and animal commodities in 40 CFR 180. The U.S. Department of Agriculture (USDA) and the FDA jointly enforce the EPA pesticide tolerances. For contaminants not covered by a specific tolerance level, the FDA may choose to issue either an action or an advisory level. Both are considered maximal allowable levels; however, an action level is generally supported by more definitive safety data than is an advisory level. Action and advisory levels constitute nonbinding FDA guidance that the agency uses in exercising enforcement discretion when considering product adulteration. There may be circumstances that warrant enforcement below an action or advisory level or in which enforcement is not warranted even though an action or advisory level has been exceeded. For contaminants not covered by any of previously discussed limits (tolerance, action, or advisory level), the maximum remains theoretically at zero. However, the sensitivity of present-day analytical methods has progressed to the point that minuscule amounts can now be detected. Fortunately, the FDA is able to exercise discretionary power when a contaminant is detected at a low level not considered to be a safety concern. Finally, intrastate pet foods are less subject to federal scrutiny and fall under the authority of local and state officials.


The bacteria of major concern for foodborne illnesses in people include the following: Clostridium perfringens, C. botulinum, Staphylococcus aureus, Bacillus cereus, Salmonella spp., Listeria spp., Yersinia spp., Aeromonas spp., Campylobacter spp., E. coli, Vibrio spp., Enterococcus faecalis, Enterobacter cloacae, and Klebsiella ozaenae. These same organisms have the potential to cause disease in animals but usually at a lower incidence rate. The infective dose can vary greatly, depending on the animal species, food substrate, patient's immunologic status, and resistance of the normal intestinal flora. For example, dogs are apparently tolerant to a common cause of human food poisoning, staphylococcal enterotoxin. Healthy adult dogs and cats are also fairly resistant to the pathogenic effects of Salmonella spp. However, the presence of these bacteria in food or water is indicative of poor hygiene and inadequate cooking. Racing greyhounds have been infected with salmonella by diets supplemented with large amounts of raw meat from rendering. Although dogs are less susceptible to the effects Clostridium botulinum toxin than are people, naturally occurring canine botulism has occurred. Toxins of Escherichia coli, strain 0l57:H7, have been incriminated in an unusual clinical syndrome in racing greyhounds termed "Alabama rot" or "Greenetrack disease".


Mycotoxins such as aflatoxin, deoxynivalenol, rubratoxin, and penitrem A can also be present in pet foods. These agents occur in raw ingredients and, in turn, the final product. Aflatoxins are heat stabIe and are not destroyed by boiling, autoclaving, or other food-manufacturing methods. Therefore, the FDA has established an action level of 20 parts per billion (ppb) for total aflatoxins in pet food. Prevention strategies involve identification of raw materials with unacceptable levels (>20 ppb), maintenance of proper storage conditions, and assay of final feeds. Vomitoxin, also known as deoxynivalenol or DON, is a mycotoxin produced by members of the genus Fusarium. Vomitoxin can be found in many grains but most commonly affects wheat, corn and barley. Like most other mycotoxins, it is heat stable. Dogs are among the most susceptible species to the effects of vomitoxin and are affected at relatively low concentrations. The clinical signs include feed refusal, vomiting, and diarrhea. The 1993 FDA advisory level for DON in grains and grain by-products used in pet foods is 5 ppm with the added recommendation that these ingredients not exceed 40% of the diet (i.e., 2 ppm in the complete pet food). However, feed refusal in dogs has been reported at concentrations below 2 ppm. Therefore, a more practical maximal level is probably 1 ppm.


Pet foods may also be contaminated with metallic agents. They can become contaminated during commercial manufacturing and in the home by the inadvertent addition of metal shavings, grease, oils, and other chemicals. Acidic foods can leach paint, soldered joints, or plating agents from the food container or food bowl. Most foodborne metal intoxications in dogs and cats involve lead, zinc, cadmium, and arsenic. These agents cause a variety of clinical syndromes, depending on age, dose ingested, and length of exposure. Several studies have been conducted in an attempt to quantify such exposures from commercial pet foods. Although the studies confirm that low, nontoxic levels of metals may be present in pet foods, their presence at detected concentrations would not cause metal intoxication.


Ionophores are antibiotic agents produced by Actinomycetales bacteria. They are commonly used in livestock as anticoccidials and growth promotants. Intoxication of cats and dogs as a result of pet food contamination with salinomycin and lasalocid, respectively, has been reported. Clinical signs associated with intoxication included muscle weakness, lameness, limb paralysis and respiratory failure.

Diagnostic Approach

If a diagnosis of food borne illness seems feasible, extensive questioning about the animal's diet should be initiated. First, the veterinarian should discuss all possible food sources, including commercial foods and home preparation, feeding amounts, use of table scraps or raw meat, and the availability of unintentional food sources (e.g., garbage). This is also a good time to request that the client bring the food to the clinic for testing purposes. It is important that the entire food container be brought and not just a sample selected by the client. Sample collection, shipping, and analysis should follow the rules of physical evidence, even if the possibility of litigation seems remote, so that testing results are admissible in court if circumstances change. Common questions about commercial foods should include the brand name, manufacturer, lot or date code, feeding method (e.g., meal fed, free choice), length of time that the pet has been consuming this brand of food, length of time the pet has been fed from the present container of food (e.g., bag or can), and storage method and whether water is mixed with the food. Any recent change in either the food source or food preparation should be queried further. The amount of food consumed should be compared with the manufacturer's recommended amount for an animal of similar size. If the amount consumed is markedly less than the calculated amount, it could mean that the animal does not like the food and may be foraging other food sources or garbage. Decreased intake may also indicate feed refusal typical of vomitoxin contamination.

Contamination of a major brand of pet food usually produces an epizootic of sick pets with a wide geographic area. If other animals in the same household are consuming food from the same bag or container and are asymptomatic, implication of the food is diminished and other possible etiologies should be investigated. Commercial pet foods are often purchased from a veterinarian, so that individual would have first-hand knowledge of other animals consuming the same lot or batch of food. If the other patients are asymptomatic, the commercial food may again be largely discounted as the causative agent. However, even if it appears that the commercial food has been exonerated, one should not end the investigation there because the commercial food could still be involved if a supplier or client has compromised the product's integrity by improper storage or usage. The clinician should contact the manufacturer to determine whether similar cases have been reported. Company technical personnel can also aid in the differential diagnosis process by supplying key information about product testing, additional areas of investigation, and beneficial laboratory tests.

The relationship between microbial populations and the quality of pet foods can only be estimated and must be viewed with caution, especially when one considers that most sampling and microbial counting procedures possess inherent inaccuracies. However, measurement of microbial populations may yield information that is valuable in comparing one sample of a pet food to another sample of the same product. For example, it would be valuable to know whether bacterial numbers had increased dramatically while the food was in the pet's bowl. This knowledge could yield information about the hygiene and timeliness of the pet's feeding routine. In summary, the presence of an organism in a food alone does not establish the diagnosis but must be considered as one piece of the diagnosis puzzle.

If the client is feeding a commercial pet food, following label directions, and using proper storage procedures, the likelihood of pet food-borne illness is quite low. As stated previously, the failure to follow proper preparation, food hygiene, and storage procedures can increase the risk of food borne disease for pets that consume commercial pet foods. A proper and complete laboratory evaluation of the patient, food, food containers, and food utensils helps establish or eliminate the possibility of pet food contamination. A thorough knowledge of the pet's environment also helps quantify the patient's exposure to other sources of toxicants and microbial agents. If the pet is allowed to roam freely outdoors, the risk of exposure to other foodborne pathologic agents increases greatly.

Due to the sophistication of modem analytical methods, the isolation of a microorganism or detection of a toxic agent in a food sample is not a sufficient basis for diagnosis. A diagnosis with a high rate of confidence needs to be based on a strong correlation between all factors related to the case: history, clinical signs, clinical laboratory values, food analytical results, pathology, microbial isolations, response to therapy, and the simultaneous elimination of other similar or complicating diagnoses.

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