Veterinarians have a responsibility to provide pet owners with information about zoonotic disease that gives a realistic appraisal of any risks pets could present to the household’s human inhabitants and how to minimize this risk. This is especially true if the household contains small children or immuno-compromised individuals. While there are myriad potential patho-gens, this article will focus on one major concern: Salmonellosis.
Why It’s Important
Salmonellosis was in the headlines continuously last summer (2008) during a large outbreak associated with contaminated food. In this outbreak, at least 1,438 people were proved to be infected and 282 were hospitalized.1 Food-associated outbreaks are not unusual because contaminated food is the primary way that humans become infected with Salmonella. The infection can be acquired from meats and eggs, but many outbreaks have been linked to other sources such as sprouts, peanut butter, tomatoes or, recently, chili peppers.
The majority of humans who become ill are young. The rate of diagnosed illness in children under 5 years old is five times higher than any other age group. This probably relates to the disease’s affecting children more severely than otherwise healthy adults. It is estimated that Salmonellosis causes 1.4 million illnesses and 600 deaths annually in the U.S.2
The true frequency of Salmonellosis is unknown since many of the less severe infections will not be diagnosed and fecal cultures are not routinely obtained from adults with gastroenteritis.
Pet ownership can increase the risk of acquiring Salmonellosis. Exotic pets are a known potential source of Salmonella. Reptiles especially can play a role as a source of human infections. It has been estimated that 90 percent of all reptiles carry and shed Salmonella.3 It is thought that 7 percent of Salmonella infections in humans are the result of contact with reptiles.4
Dog and cat ownership is associated with a potential risk of acquiring Salmonellosis. Rawhide and pig ears can harbor Salmonella. In one outbreak it was shown that contact with the treats or pets that consumed them was responsible for human Salmonellosis.5 Of 94 pig ear samples obtained from retail outlets, 51 percent harbored Salmonella. Salmonella was found in other treats as well, including beef hoof, braided chews and similar products.
It certainly is not surprising that dogs can also harbor Salmonella species. Most recent studies have shown a prevalence of about 1 to 2 percent in normal dogs and cats.6,7 The percentages may be higher in animals with diarrhea. Much higher prevalences were found in racing sled dogs, where 69 percent of them without diarrhea were shedding Salmonella.8
In greyhounds with diarrhea, 61 percent were positive for Salmonella; in non-diarrheic dogs, the percentage was 11 percent.9
The increased proportion of Salmonella-positive tests in these dogs may relate to the stress of athletic performance or to their diets. Both racing greyhounds and sled dogs are fed a significant amount of raw meat.
In cats, group housing is associated with a much higher percentage of Salmonella shedders. In one study, 51 percent of group-housed cats, 9 percent of sick cats and only 0.36 percent of healthy house cats shed Salmonella.10
Of great concern is the recent interest in raw diets for pets. The Internet is replete with sites that popularize raw food and its supposed health benefits. These diets significantly increase the risk that humans will acquire Salmonella. In a study of a small number of dogs, 30 percent of them fed a biologically appropriate raw food diet were shedding Salmonella, and 80 percent of the food samples were positive.11
Feeding these diets results in routine contamination of owners’ homes through potentially infectious materials such as raw chicken. Dogs are not known to be especially clean eaters and it is highly likely that infectious organisms are disseminated throughout the home.
Even if owners try to limit contamination by thorough cleaning, they may not be successful. In an experiment, both plastic and steel bowls were inoculated with 2 grams of meat containing Salmonella. The bowls were either warm-water rinsed, rinsed and scrubbed, scrubbed with soap, soaked in 10 percent bleach, put in a dishwasher at 85 degrees Celsius, or rinsed and washed and then soaked in bleach for 5 minutes.
Salmonella was cultured in more than two-thirds of bowls with the exception of the scrubbed and bleached ones, where still over 40 percent were positive.12
The best evidence for the real risk associated with raw food diets in pets comes from a recent outbreak of Salmonella. In this outbreak, regular dry dog food was contaminated with Salmonella. Seventy cases were identified in humans. The median age was 3 years, with 39 percent being less than a year old.15
It would make sense that most of the infants were infected indirectly, since it would seem unlikely that children less than 1 year old would be feeding the dogs, though some may have had some inadvertent contact with food while crawling around. It is common sense to consider the likelihood of similar infections occurring with raw diets given the high prevalence of Salmonella in these diets.
Direct and indirect transmission of Salmonella in association with pets has been commonly documented. Though rawhides, pig ears and even dry dog food can be sources of Salmonella, it seems to me that raw-meat diets represent the greatest threat.
Commercial raw meat diets are often contaminated with Salmonella, something that is not surprising given the prevalence of Salmonella, especially in poultry.
A healthy adult can most likely acquire Salmonella and deal with it easily, but the real risk is to children and other immunocompromised individuals.
To me the risk is unacceptable and I counsel all clients to avoid raw food diets. Of course it also makes sense to discuss limiting predation—another source of raw meat—in pets and to use good hand-washing procedures whenever pet foods or treats are handled.
1. CDC website. www.cdc.gov/salmonella/saintpaul/archive/082208.html
3. Mermin J., Hoar B., Angulo F.J. “Iguanas and Salmonella marina infection in children: a reflecti on of the increasing incidence of reptile-associated Salmonellosis in the United States.” Pediatrics. 1997; 99:399-402.
9. Stone G.G., Chengappa M.M., et al. “Application of polymerase chain reaction for the correlation of Salmonella serovars recovered from greyhound feces with their diet.” J Vet Diagn Invest. 1993; 5:378-385.
12. Weese J.S., Rousseau J. “Survival of Salmonella Copenhagen in food bowls following contamination with experimentally inoculated raw meat: effects of time, cleaning and disinfection.” Can Vet J. 2006; 47:887-889.