Salmonellosis and food contamination (Proceedings)

Multiple recalls of commercial pet food, treats, and supplements have occurred in the United States in the past several years, on account of potential contamination with Salmonella. These incidents raise concern of salmonellosis in both pets and the owners who care for them. Clients who are concerned may turn to their local veterinarians for nutritional advice to keep their pets safe and healthy. By maintaining up-to-date knowledge regarding recalls and risks of salmonellosis, veterinarians can help educate clients and provide practical pointers for minimizing this risk in their households.

Salmonella prevalence and classification

Salmonella are Gram-negative bacilli in the family Enterobacteriaceae that are capable of colonizing the intestinal tracts of most vertebrates. The prevalence of Salmonella spp. in the feces of clinically healthy or hospitalized dogs ranges from 1-63% and from cats 1-18%; however, a true prevalence is most likely <5%. Salmonella spp. can be classified into two distinct species, Salmonella enterica and Salmonella bongori. Salmonella enterica has over 2400 serotypes, classified by agglutination reactions to their somatic (O) and flagellar (H) antigens, while Salmonella bongori has 20 serotypes. The majority of cases of human and companion animal salmonellosis are caused by Salmonella enterica serotypes Enteritidis or Typhimurium.

Contaminated food products

Consumption of food or water contaminated with Salmonella is the most common route of transmission for this infection. In dogs and cats, consuming raw or undercooked infected meat is a risk factor and likely responsible for the majority of cases. Some pet owners choose to feed raw meat diets for a variety of reasons, and these diets can be either commercially packaged or homemade. Either type of raw diet may be contaminated during processing, handling, and storage, if safety precautions are not taken. Studies have shown that feeding raw diets (commercial or homemade) can increase a pet dog's risk of shedding Salmonella spp. Raw canine diets containing chicken have been shown to be 5x more likely to be contaminated with Salmonella spp. than raw diets not containing chicken. A second study found that 45% of commercial raw diets used to feed Greyhounds were contaminated with Salmonella spp., with a high rate of antimicrobial resistance.

Commercial dog kibble, treats and supplements can also become contaminated with Salmonella spp. due to cross-contamination during processing. Salmonella are killed by cooking, but some products are not cooked and other products may become contaminated during processing but after the cooking stage. Canned food has not been associated with Salmonella contamination, because cans are vacuum-sealed and sterilized during processing, prohibiting cross-contamination while the can remains sealed. Cross-contamination of any pet food product, including canned food, can occur in the consumer's home once that product's packaging is opened and the product is handled, if precautions are not taken.

Several reports have documented pig ear treats contaminated with Salmonella spp. During processing, pig ear treats are frozen, cleaned, and flavored, but they are not cooked, allowing opportunity for contamination to survive processing. Many pig ear treats are packaged in bulk, rather than individually, allowing opportunity for cross-contamination in stores. In 1999, a human outbreak of Salmonella enterica serotype Infantis in Canada was traced back to contact with pig ear treats; illness in dogs was not reported. Testing of pig ear treats found in stores at that time found 51% were contaminated with Salmonella spp. In the U.S., 41% of dog treats derived from pig ears and other animal parts purchased from retail stores were contaminated with Salmonella spp., including isolates indistinguishable from the Canadian outbreak. Recommendations that may help minimize cross-contamination of pig ear treats include irradiation of pig ears and individual packaging rather than storing pig ears in bulk bins.1

An outbreak of human disease in 2002 was traced back to pet treats containing dried beef that were contaminated with Salmonella enterica serotype Newport. Testing of fecal samples from pets owned by patients in the outbreaks did not isolate Salmonella from these pets, and there were no reports of illness in pets. While it is suspected these owners acquired Salmonella from handling these treats, it remains unclear what role (if any) pets play in amplifying and shedding Salmonella during human outbreaks from contaminated pet products. In 2004-2005, a third outbreak of human illness occurred, infecting 11 people, that was associated with contact with salmon and beef pet treats contaminated with Salmonella enterica serotype Thompson. Although no canine feces were tested in this outbreak, one dog owned by an infected person had diarrhea, and the remaining pets owned by other infected people remained healthy. Sharing of Salmonella between pets, owners, and veterinary staff has been documented previously in 3 outbreaks (unrelated to contaminated food or treats), presumably due to fecal-oral contact with diarrheic cats. In future outbreaks, fecal samples from pets owned by human beings with salmonellosis should be tested to further investigate the zoonotic potential and role of fecal-oral transmission. However, even DNA fingerprinting evidence of sharing the same strain of Salmonella spp. does not prove transmission between species, as these individuals may have acquired the bacteria from a similar source (such as a pet food product).

In 2006, contaminated dog and cat food produced by a single manufacturer was responsible for a multistate outbreak of Salmonella enterica serotype Schwarzengrund. The manufacturing process at this plant had appropriate time and temperature conditions to kill Salmonella; however, it is believed that contamination occurred after the cooking stage, while the food was being sprayed to enhance palatability. This was the first reported documented outbreak of human illness due to dry dog or cat food. This multistate outbreak affected 79 people, mostly children, in 21 states, and all recovered from their illnesses. While the same strain of Salmonella was isolated from fecal samples of five dogs consuming food manufactured at this plant, there were no reported illnesses in these or other pets related to this outbreak.

Vitamins and nutritional supplements can also become contaminated with Salmonella, most likely due to cross-contamination during production. A recall occurred in April 2010 for a canine joint supplement due to possible Salmonella. Several months later, in July 2010, 56 veterinary nutritional supplements were recalled by a single manufacturer due to Salmonella concerns. These supplements were distributed throughout the U.S., by various retailers and under numerous brand names and included: multivitamins, cranberry tablets, ear powder, glucosamine, coprophagia treatment, fresh breath treatment, calcium supplement, gas prevention tablets, and calming tablets. No cases of veterinary or human illness were reported relating to these supplement recalls.

The Food and Drug Administration (FDA) is responsible for regulating pet food, treats, and supplements in the United States. When contamination is detected in a food product, the manufacturer first initiates a recall of the product, to minimize risk of exposure and infection for consumers. Two helpful up-to-date websites are maintained by the FDA and are helpful in providing information during such recalls. It is common for manufacturers to voluntarily recall more products than what tested positive for Salmonella, in attempt to ensure the safety of consumers. This type of recall may include all products made at the facility involved. In other cases, manufacturers may just recall the item that tested positive for Salmonella spp.

Clinical salmonellosis

Many adult animals exposed to Salmonella become carriers without developing clinical illness. Clinical salmonellosis is seen most often in puppies, kittens, stressed or immunosuppressed animals, those with concurrent illness, those receiving antibiotic therapy, or those who ingest a high dose of organisms. For those who become ill, signs begin 3-5 days after exposure and can include fever, malaise, anorexia, vomiting, abdominal pain, and diarrhea, which may contain fresh blood. Songbird fever is a manifestation in cats who prey on wild birds infected with Salmonella Typhimurium, and is characterized by fever, anorexia, vomiting, hemorrhagic diarrhea, and lethargy. In very young or immunosuppressed animals, bacteremia and endotoxemia can occur, with or without concurrent signs of gastroenteritis, and may lead to infection of distant organs (lungs, liver, brain).2 In utero infections can cause reproductive failures or weak infected offspring.

In the U.S., there are a reported 1.4 million human cases of nontyphoidal salmonellosis resulting in 400 deaths each year. Clinical signs in humans include diarrhea, fever, and abdominal cramping that develop 12-72 hours after exposure and typically lasts for 4-7 days. While most people recover without treatment, salmonellosis is most severe and can spread systemically and can be fatal in infants, elderly, and immunosuppressed people.

Public health recommendations

While the FDA and manufacturers continue surveillance to ensure the safety of pet food and treats, pet owners can take certain precautions to minimize risk of infection within their homes. Veterinarians play an important role in educating clients about the risks of Salmonella and can provide practical steps that clients can take to keep their pets and family healthy (Table 1).

Table 1. Safety recommendations for pet owners to minimize the risk of salmonella infection within the home.

Conclusion

Pet owners turned to veterinarians as a trusted source of nutritional information. It is important to continue to reassure pet owners of the safety of pet foods and treats produced in the United States, but also remain up-to-date on recalls. While the risk of salmonellosis from contaminated pet food and treats is low, following the above guidelines may help to minimize the risk of Salmonella for pets and family members.

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