Salmonella

Salmonella has gained the most attention regarding bacterial contamination concerns of raw food diets. In humans the national rate of reported Salmonella isolates in 2006 was 13.6 per 100,000 in the United States.⁹ A large proportion of Salmonella typhimurium isolates, the most commonly isolated subtype, have also been found to be resistant to multiple antimicrobials. In 2004 39% of nationally reported S. typhimurium isolates in the United States were resistant to one or more drugs, and 23% had a five-drug resistance pattern.¹⁰

Symptoms of salmonellosis are similar across species and include fever, vomiting, abdominal pain, and diarrhea. Severe cases, especially in previously immunocompromised individuals or in individuals with systemic disease, can be fatal. It is common, however, for Salmonella to be present in the feces of asymptomatic animals. According to published estimates, 1% to 36% of healthy dogs and 1% to 18% of healthy cats are asymptomatic fecal shedders. Fecal shedding of Salmonella can wax and wane after the first week of infection, and clinical signs are seen in conjunction with stress and immunosuppression.³⁵

Whereas most human Salmonella infections originate from food animals, food sources include eggs, meat, poultry, and produce. Human infections are also seen involving direct contact with animals and their environments.⁸ It is the combined risk of using raw meat and poultry while in close contact with pets and their environments that has raw food opponents concerned for human safety.

There are multiple published reports of pet dogs and cats testing positive for Salmonella isolates that were indistinguishable from isolates obtained from humans. In these cases both symptomatic and asymptomatic animals were linked to humans in multiple capacities. Some were staff in veterinary clinics, individual pet owners, children of owners, or neighbors of infected pets. A connection was also found between children attending a day care center with a child living with two asymptomatic positive cats.^11,36,53 Only one of these reports mentioned diets consumed by Salmonella-positive pets. In this case a symptomatic infant was positive for a Salmonella Virchow isolate that was indistinguishable from isolates found in two of three asymptomatic dogs living in the same home. All three dogs were fed a commercial diet supplemented with boiled chicken 2 to 3 times per week.³⁶ Salmonella isolates in all of the previous reports were also resistant to multiple antimicrobials, with flouroquinolones and third-generation cephalosporins included in some cases. This is significant because these classes of antimicrobials are commonly used to treat human salmonellosis.⁵³

Risk of zoonosis is an important consideration with salmonellosis, especially because in all the aforementioned reports, multiple pets in a household were infected with variation in the presence of symptomatic and asymptomatic pets.^11,36,53 After an investigation in one veterinary clinic, 5 of 43 hospitalized cats tested positive for Salmonella, including the asymptomatic blood donor cat. The floor of the boarding area and floor and door handles to the isolation ward in this clinic also tested positive.⁵³ In another veterinary clinic, three previously healthy pets developed transient diarrhea after prophylactic dental procedures performed by a veterinary technician later determined to be Salmonella positive. One of the three pets had a positive stool sample with an isolate indistinguishable from that of the veterinary technician.¹¹

These reports underscore the importance of training veterinary staff to recognize, address, and learn to avoid situations that can place themselves and others at risk of Salmonella infection. On the basis of a list of factors contributing to four of the aforementioned outbreaks, pertinent reminders for all veterinary staff can be made (Box 19-1).⁵³

One recommendation on the list involves environmental contamination concerns. A small study by Weese and Rousseau⁵⁰ evaluated survival of Salmonella spp. in pet food bowls using a Salmonella Copenhagen isolate previously obtained from a commercial raw food product. They inoculated multiple bowls and left them at room temperature for 7 days. The average concentration of Salmonella spp. was 5.4 × 10⁵ colony-forming units per gram, and Salmonella was isolated from all bowls on days 1, 2, 4 and 7. They then took six stainless steel and six plastic food bowls and inoculated them with the same Salmonella strain. After assigning the bowls to various disinfectant treatment groups, the researchers evaluated them for Salmonella persistence. No significant difference was noted among disinfection methods for the plastic bowls. Scrubbing the stainless steel bowl with soap and then soaking it in bleach was significantly more effective than no cleaning, warm water rinse, and warm water rinse with scrubbing. None of the cleaning techniques used was able to eradicate all traces of bacteria, including use of a dishwasher. Some of this may have been due to the amount of organic debris used to inoculate the dishes, but the risk of food bowl contamination is an important concept to discuss with owners participating in raw feeding.

Other Pathogens

In addition to Salmonella, other bacterial species common in raw meat include Campylobacter spp., Escherichia coli, Yersinia enterocolitica, Listeria monocytogenes, Clostridium perfringens, Staphylococcus aureus, and Bacillus cereus.²⁴ In a study of raw food diets purchased in Ontario, Canada, coliforms were present in all 25 raw diets sampled, with a mean value of 8.9 × 10⁵ colony-forming units per gram.⁵¹ Table 19-1 illustrates the total number and types of bacteria isolated from these raw food diets, which contain enteropathogens of both animals and humans.

TABLE 19-1 Bacteria Types Recovered from 25 Commercial Raw Food Diets Purchased in Ontario, Canada

Another study evaluated bacterial and protozoal contamination of commercially available raw, dry, and canned dog food diets. Of 233 samples 153 (53%) were positive for non–type-specific E. coli, including dry and canned food varieties. Salmonella enterica was found in 17 of 233 (5.9%) samples from raw products, and 3 of 144 (2.1%) samples contained Cryptosporidium spp. in raw and canned products. Unfortunately, the design of this study did not allow for direct comparison of the degree of bacterial contamination in raw versus dry and canned diets.⁴²

A third study compared two commercial raw diets and three homemade raw diets. E. coli O157:H7 was found in one of the homemade diets, whereas all three of the homemade diets were negative for Salmonella spp.¹⁷ These data are especially troubling because E. coli O157:H7 is an important human pathogen causing hemorrhagic colitis in all ages and hemolytic–uremic syndrome in children and the elderly, with mortality rates between 3% and 5%. There is also no current recommended antimicrobial therapy in humans for this specific bacterial agent.¹²

Animal

When nutritional inadequacies are present in a diet, clinical disease may be a consequence depending on the affected nutrients, the duration of feeding the deficient diet, and the life stage of the animal. One of the most commonly reported nutritionally related clinical diseases is osteodystrophy attributed to derangements of calcium, phosphorus, or vitamin D. Even though the incidence of such musculoskeletal diseases is reportedly low in cats,⁴⁹ it is not unheard of, and all owners of affected animals should be asked about the specific diet being fed so that the likelihood of this etiology can be assessed. There are multiple case reports involving both suspected and definitively diagnosed cases of nutritional secondary hyperparathyroidism^19,45,46,52 and vitamin D–dependent rickets^18,38,43 manifested as bone abnormalities in domestic and wild cats. Although not all of these cases were specifically linked to raw diets, most of them involve animals fed unbalanced diets. Therefore, as previously explained, it is important to remember that any raw food diet (even those commercially obtained) should be considered unbalanced until evaluated by a veterinary nutritionist.

Arguably the most severe consequence to feeding a raw-prepared diet involves the risk of septicemic salmonellosis. A case report involving diagnosed septicemic salmonellosis in two cats originating in the same household recounted the diagnostics and results obtained. Both were fed a homemade raw beef diet. The first, a 14-year-old Exotic Shorthair, was presented deceased with a 1 week history of weight loss, soft stools, and anorexia. Histopathologic evidence of necrotizing hepatitis, chronic enteritis, and interstitial pneumonia was noted on necropsy, with S. typhimurium isolated from the lung, liver, spleen, and kidney. Nine months later, a 10-week-old Exotic Shorthair kitten from the same household was presented moribund with a history of possible respiratory obstruction. After euthanasia, gross evidence of reduced fat and muscle stores, serous nasal discharge, corneal opacity, and consolidated lungs was noted along with histopathologic signs of suppurative pneumonia and enteritis with villi erosion and blunting. Bordetella bronchiseptica was isolated from the lungs, and Salmonella newport was identified in the lung and small intestine. Samples of the raw beef fed to the kitten yielded S. newport and Salmonella bardo and E. coli among many other bacterial isolates.⁴⁰

Human

The public health concerns associated with raw diets deal with possible human exposure to Salmonella spp. and E. coli, among other pathogens. Some raw food proponents believe that the risk of human bacteriosis such as salmonellosis is inconsequential because there are so few documented cases of direct transfer from raw-fed pets to their owners. For the average healthy adult who is aware of foodborne pathogen risk and prevention, the chance of salmonellosis may be low. It is important to remember, however, that unless the gastrointestinal signs are severe, this same average healthy adult is unlikely to submit fecal culture samples from themselves and their pets, and many veterinarians reserve use of fecal cultures to those animals with refractory diarrhea. In many of the case reports referenced in the preceding sections, regulatory health officials actively obtained diagnostic fecal cultures only after the diagnostic laboratory noticed an outbreak pattern between a small subset of human or animal samples.^11,36,53