The pet food and treat manufacturing industry is highly variable. It includes large multinational corporations, small single-site manufacturers, small manufacturers that contract production to other companies and small scale (including in-home) commercial processing. Further, some pets receive food or treats prepared in the household. The foodborne pathogen challenges can vary greatly between these types, as do preventive measures. Approaches to regulation of pet food vary and the range of national regulations is beyond the scope of this review. As an example, in the U.S., The Food and Drug Association (FDA) requires that food be ‘safe to eat, produced under sanitary conditions, free of harmful substances and truthfully labelled’. Manufacturing facilities must have a food safety plan that assesses potential food safety hazards and establishes good manufacturing practices for manufacturing, processing, packing and storage.

Pathogens of Concern

Most of the attention has focused on Salmonella, but a variety of pathogens are of concern, Listeria and Shiga toxin-producing E. coli (e.g., E. coli O157), with the latter of greater health concern in humans exposed to the food. Campylobacter is an important pathogen but of somewhat lesser concern because of its relatively poor tolerance to environmental stressors such as freezing. Additional bacteria, ranging from minimally relevant spoilage organisms to serious concerns, such as Clostridium botulinum, can thrive in inadequately processed food (e.g., improper canning). Some pathogens are of broad concern, involving many different food types and production systems, while others are very narrow, such as infection by the bacterium Neorickettsia helminthoeca within the parasite Nanophyetus salmincola in raw salmonids. Parasites, most notably Toxoplasma gondii, are also of potential concern. Disease concerns include both animals and humans, since pet food associated human infections with Salmonella and E. coli O157 have been identified.^1-3 (www.fda.gov/AnimalVeterinary/NewsEvents/ucm596555.htm, www.fda.gov/animalveterinary/newsevents/ucm596071.htm, www.health.state.mn.us/news/pressrel/2019/salmonella012819.html)

Incidence and Impact of Foodborne Disease

The incidence of foodborne disease in dogs and cats is unclear. Outbreaks receive most of the attention and are undeniably important. However, they may not represent the main burden of disease, particularly associated with raw diet or boutique manufacturers, where small batch contamination is less likely to be identified because of the lower likelihood of large numbers of cases. Sporadic disease can go unidentified but represent the main disease burden. The true impact of foodborne disease on animal and human is almost impossible to accurately identify. Investigation is most often to occur when humans develop a reportable disease (e.g., salmonellosis, E. coli O157 infection).

Potential Sources of Contamination

Contamination can occur at many steps in the food preparation process. This includes raw ingredients, palatability enhancers, manufacturing equipment, post-manufacturing storage, contamination during retail storage and contamination in the household after containers are opened. A range of preventive measures at different levels may be required to identify and eliminate these risks.

Preventive Measures

Prevention of contamination and monitoring is a complex process that can involve multiple steps, each of which is designed to achieve a particular goal. Examples are discussed briefly below.

Heat

Heat is one of the most effective and reliable pathogen elimination methods, and is a key component of foodborne pathogen elimination. Canned (wet) food undergoes high temperatures during the cooking and canning process, with temperatures that are adequate to render the food sterile. Pet foods are considered to be low-acid foods, which carry an increased risk of Clostridium botulinum growth if improperly cooked and canned, so canned food must follow low-acid canned food regulations that dictate time and temperature, along with the type of monitoring and monitoring equipment that is required. Dry foods undergo cooking, with heat and desiccation eliminating many pathogens. The extrusion process also results in generation of heat and pressure that can inactivate many pathogens.

Irradiation

Irradiation is a highly effective means of eliminating pathogens, particularly from items that are not amenable to heat treatment but cost and consumer perceptions limit widespread use. Irradiation of cat (but not dog) food was banned in Australia after an outbreak of neurological disease in cats fed a specific imported diet that had been irradiated,⁴ potentially because of irradiation-induced reduction in vitamin A.⁵ While this phenomenon has not been reported elsewhere, it is an important consideration if whole diets are being irradiated, as opposed to treats.

High-pressure Pasteurization

This process involves pressurizing foods to 100–800 MPa, with only a slight (approximately 30°C/100 MPa) increase in temperature.⁶ Some raw food manufacturers use this process, largely to reduce Salmonella contamination. The bacterium, food matrix and pressurization conditions will impact efficacy of high-pressure pasteurization. Published data regarding the efficacy of methods used on pet foods are lacking and recalls of treated diets have occurred. Companies using this process should ensure that proper testing is done with their raw food, batch size and pressurization conditions, ideally with experimentally contaminated food, and make that information available publicly.

Preservatives

A variety of preservatives may be added to pet food. Regardless of whether they are ‘natural’ or ‘artificial’ (a marketing designation rather than a microbiologically relevant factor), preservatives are not typically intended to kill contaminants. They are mainly added to prevent oxidation of the food and associated discolouration or spoilage. Bacterial inhibition by commonly used preservatives is not adequate to be relied on as a pathogen control method.

Freezing

While not a method used in commercial food preparation, freezing will eliminate some pathogens and have little effect on others. Freezing is most often used to eliminate parasites. For example, freezing (e.g., 2 days at -200°C) can effectively eliminate Toxoplasma.⁷ Campylobacter spp. are also relatively intolerant of freezing, although freezing is not a tool that can be used with confidence to eliminate the risk of campylobacteriosis.

Freeze-drying

Freeze-drying does not eliminate most pathogens. In fact, freeze-drying is a common and effective tool for long-term preservation of bacteria. The freezing component of the process may eliminate pathogens such as parasites or Campylobacter that are susceptible to freezing.

Cleaning and Disinfection

Persistence of Salmonella in processing facilities has been implicated in large Salmonella outbreaks involving commercial dry food, from combination of poor equipment, poor maintenance and inadequate cleaning and disinfection. Detailed protocols and practices are used in well-run manufacturing facilities, but deficiencies can be present and can lead to contamination. Key aspects of cleaning and disinfection are the use of proper cleaners and disinfectants, the presence of surfaces amenable to cleaning and disinfection, provision of adequate time, and, perhaps most importantly, actually performing the task thoroughly.

Pathogen Testing

Testing can allow for detection of contamination before food reaches the consumer, as well as provide an indication that something may have gone wrong in the food preparation process (with subsequent investigation).

However, testing does not cover all potential pathogens. Sporadic testing may not be adequate to determine uncommon (but still relevant) contamination. The quality of results also depends strongly on the methods (i.e., if you don’t really want to find Salmonella, you can use a protocol that is poorly sensitive). Specific pathogen testing practices and thresholds are typically proprietary. However, the FDA has a zero tolerance policy for Salmonella in pet food,⁸ so any positive Salmonella test would result in a recall.

Hazard Analysis and Response

This is a standard food safety procedure that identifies and evaluates potential (foreseeable) hazards encountered during the manufacture, processing, packing and storage of food. The concept of hazard and critical control point (HACCP) analysis is well established and an expectation of any manufacturer. This approach is designed to identify key control points in the production system and to employ preventive and/or monitoring measures at those points to prevent and detect contamination. Thorough description of HACCP and individual control points is beyond the scope of these proceedings.

References

1.  CDC. Update: recall of dry dog and cat food products associated with human Salmonella Schwarzengrund infections--United States, 2008. MMWR Morb Mortal Wkly Rep. 2008;57(44):1200–2.

2.  Imanishi M, Rotstein DS, Reimschuessel R, Schwensohn CA, Woody DH, Davis SW, et al. Outbreak of Salmonella enterica serotype Infantis infection in humans linked to dry dog food in the United States and Canada, 2012. J Am Vet Med Assoc. 2014;244(5):545–53.

3.  Public Health England. Investigation into an outbreak of Shiga toxin producing Escherichia coli O157 PT21/28 Stx2 in England, August 2017. London, UK; 2018.

4.  Child G, Foster D, Fougere B, Milan J, Rozmanec M. Ataxia and paralysis in cats in Australia associated with exposure to an imported gamma-irradiated commercial dry pet food. Australian Vet J. 2009;87(9):349–51.

5.  Caulfield CD, Kelly JP, Jones BR, Worrall S, Conlon L, Palmer AC, et al. The experimental induction of leukoencephalomyelopathy in cats. Vet Pathol. 2009;46(6):1258–69.

6.  U.S. Food and Drug Association. Kinetics of microbial inactivation for alternative food processing technologies -high pressure processing: U.S. Food and Drug Association; 2011 [Available from: www.fda.gov/Food/FoodScienceResearch/SafePracticesforFoodProcesses/ucm101456.htm.

7.  Kijlstra A, Jongert E. Control of the risk of human toxoplasmosis transmitted by meat. Int J Parasitol. 2008;38(12):1359–70.

8.  U.S. Food and Drug Association. Compliance Policy Guide Sec. 690.800, Salmonella in Food for Animals. 2013.