In a recent study published in the journal Eurosurveillance, researchers investigated the potential role of the increasingly popular ‘raw meat-based diets (RMBDs)’ as reservoirs and human and pet infection risks for bacteria such as Salmonella sp. and pathogenic Escherichia coli. Unlike conventionally processed pet food, the majority of which undergoes various stages of heat or mechanical processing (cooking, extrusion, grinding, and dehydration), RMBDs are sold and often consumed in their raw or minimally cooked state.

Study: Raw meat-based diet for pets: a neglected source of human exposure to Salmonella and pathogenic Escherichia coli clones carrying mcr, Portugal, September 2019 to January 2020. Image Credit: Amy Rene / Shutterstock

The present study highlights the alarming fact that despite stringent, legally enforceable European Union (EU) regulations concerning the use and microbiological safety of animal by-products, RMBDs, even from reputed European brands (sampled from Portugal), may serve as a vehicle for multidrug-resistant (MDR) Salmonella and E. coli, some of which were found resistant to Colistin, a last-line antibiotic.

Conventionally processed foods are thus shown to be the safer food alternative for the pet dogs who consume them and their owners who come into close physical proximity (e.g., handling) their food. This merits promoting awareness of the dangers of RMBDs, the health advantages of processing pet food, and the need for constant microbiological monitoring of pet food ingredients, lest we witness the rise of a new bacterial pandemic untreatable by current last-line antibiotics.

Isn’t raw food healthier than processed food?

Increasing global sensitivity to the health and fitness demerits of extensively processing food items for humans and their pets has resulted in some pet owners choosing non- or minimally processed raw meat-based diets (RMBDs) as allegedly healthier alternatives to conventionally processed pet foods. Growing human-pet bonds, particularly following the Coronavirus disease of 2019 (COVID-19) pandemic and its associated ‘lock-down,’ have further escalated the demand for high-quality RMBDs. Still, a growing body of literature hypothesizes that these ‘healthy’ foods may not be as safe as popularly assumed.

Unlike their substantially more processed conventional pet food alternatives, almost all of whom have been validated as safe for consumption and free from microbiological contamination, RMBDs are often devoid of heat or mechanical treatment, with freezing, a processing methodology with almost no immunological benefit, representing the bulk of RMBDs manufacturing process. This makes these novel pet foods likely reservoirs and vehicles of pathogenic bacteria, a concern that more and more studies are discovering to be valid, given the recall of over 20 batches of RMBD in the European Union (EU) since 2020 alone.

Alarmingly, some of these contaminated food sources have been found to contain traces of multidrug resistance (MDR) Salmonella sp. and Escherichia coli, most of which are human pathogens, and some of which have been discovered resistant to Colistin, one of the most critical last-line human antibiotics currently available. Unfortunately, given the relative novelty of these reports, the pet food industry and pet owners alike remain oblivious to the potential dangers that RMBDs present, with most concerned individuals assuming that EU regulations are sufficient in preventing these adverse public health risks.

Phylogenetic trees of Salmonella Typhimurium and Escherichia coli isolates from raw pet food samples, Portugal, 2020 (n = 3) and related available genomes on EnteroBase up to 8 June 2023 (n = 22 Salmonella Typhimurium, n = 20 E. coli). The core genome minimum spanning tree was created within the EnteroBase pipeline using the MSTreeV2 algorithm and GrapeTree tool. The cgST is indicated in each node. The yellow circle highlights the pet food isolate of each Grape tree. For the geographical analysis, the cgST was annotated using the country data (the number of genomes by country is indicated within parentheses). The scale bar corresponds to the number of cgMLST allelic differences.

About the study

The present study aims to identify and characterize any Salmonella sp. or Enterobacteriaceae populations present in RMBDs from popular European pet food brands, particularly those having genetically confirmed MDR bacteria with human infection potential. Food samples were physically obtained from popular pet stores and supermarkets in Portugal’s Porto metropolitan area. They comprised 55 food samples representing 50 unique dog food items from 25 manufacturers, 21 of whom are global and distribute their products across more than just the EU.

Standard International Standard Organisation (ISO) detection methodologies were used to identify non-typhoidal Salmonella sp. Similarly, Simmons citrate agar + inositol (SCAi) and Tryptone Bile X-glucuronide agar plates (TBX) with and without Colistin were used to identify and validate mcr-carrying Enterobacteriaceae. To confirm the phenotypic manifestation of MDR genotypes, isolated bacteria were subjected to disk diffusion assays alongside popular current antibiotics such as amoxicillin, cefepime, meropenem, streptomycin, and others. HiSeq Illumina next-generation sequencing techniques were used to elucidate representatives of bacterial colonies displaying substantial antibiotic resistance genetically.

Core genome multilocus sequence typing (cgMLST), a phylogenetic technique to elucidate the presence and potential phenotypic purpose of a set of genes (herein, pathogenic and antibiotic resistance genes) common to distantly related phylogenetic clades (Salmonella and Enterobacteriaceae), was used for comparative genomic analysis of strains obtained in the present study versus those previously uploaded to various online databases including Enterobase.

Study findings and conclusions

Study findings highlight the presence of Enterococcus spp. across all 55 investigated RMBDs, all 22 wet items of which contained EU-limited-exceeding quantities of MDR Salmonella and Enterobacteriaceae populations. Alarmingly, the Salmonella sp. detected in this study belonged to the ST34 genotype, a predominant human-pandemic-causing genotype for which animals are asymptomatic carriers.

“Our results strongly suggest that conventionally processed pet food is a safer option, emphasising the critical role of heat treatment in pet food production for effectively mitigating microbiological hazards.”

Despite the growing belief that non- to minimally-processed foods are the healthiest and most nutritious food types for both humans and their pets, lack of proper pre-serving cooking and direct human contact with infected meat (at the slaughterhouse or during serving) may serve as easy points of zoonotic transfer, which given the MDR phenotypes of identified bacteria, may rapidly snowball into a global pandemic.

“Resistance to commonly used veterinary antibiotics such as β-lactams, fluoroquinolones, tetracycline, and sulphonamides was especially pronounced, mirroring trends seen in other European studies on pet food samples of diverse origins.”

Additional research, and more importantly, public awareness, is required to safeguard pets and their owners from misinformation readily available on the internet, particularly on social media.