Raw food feeding is the practice of providing pets a diet consisting primarily of uncooked meat, bones and viscera (Schlesinger and Joffe, 2011; Freeman et al., 2013; Goh, 2016; Fredriksson-Ahomaa et al., 2017). In the UK, the majority of these diets are comprised of meat which has been deemed fit for human consumption (Schlesinger and Joffe, 2011; Freeman et al., 2013; Goh, 2016; Fredriksson-Ahomaa et al., 2017). Feeding pets raw meat-based diets (RMBD) has become increasingly widespread during the last decade, very much in parallel with the changes in human dietary trends for more organic products and more “natural” diets such as veganism (Laflamme et al., 2008). In the USA, sales of RMBD doubled in the five years to October 2017, whilst in the UK seven companies supplying RMBD were registered in 2007 rising to over 80 in 2018 (Wall, 2018). A study from the Netherlands found 51 percent of dog owners fed their pets either completely or partially a RMBD and a survey of Australian cat breeders by Shapiro et al. (2017) found raw meat as an integral constituent of the diet fed by 89 percent of respondents.
People who feed RMBD do so for a multitude of reasons, including but not limited to: cultural trends, the reported health benefits, the perception of “more natural” diets and allowing the pet to “stay in touch” with their pre-domesticated ancestry (Morgan et al., 2017). This short discussion highlights the key areas where there remains least veterinary consensus regarding RMBD: the risk of microbiological contamination, the purported health benefits and the risk of nutritional imbalances.
Infectious and zoonotic agents
By far the most commonly cited concern with RMBD is the risk posed by infectious agents that have not been destroyed or inactivated as they would be by conventional cooking methods (Frederiksson-Ahomaa et al., 2001; Weese et al., 2005; Finley et al., 2006; Bojanic et al., 2017; Davies, 2018; O’Halloran et al., 2018; van Bree et al., 2018; Clark, 2019; Davies et al., 2019; Jones et al., 2019; Loeb, 2019; O’Halloran and Gunn-Moore, 2019; O’Halloran et al., 2019). Instead, almost all RMBD are sold frozen to extend their expiry dates and, in the case of some organisms (eg Trichinella species), reduce or eliminate contamination (Davies, 2019; Davies et al., 2019).
Bacteria of the Enterobacteriaceae are the most frequently recovered bacteria from commercially available RMBD. One study found that 72.5 percent of samples tested did not meet the microbiological standards for Enterobacteriaceae set by EU regulations for animal by-products intended for pet food (Davies et al., 2019). Of particular concern, Escherichia coli serotype O157:H7 was isolated from 23 percent of RMBD samples in a further study, while a human disease outbreak of Shiga toxin-producing E. coli (STEC) was attributed to exposure via contaminated RMBD (Nemser et al., 2014). Whole genome sequencing has established with a high degree of certainty that bacterial strains shed in the faeces of RMBD-fed companion animals are the same as those present in their diets (Jones et al., 2019).
A number of other zoonotic bacterial species have frequently been isolated from RMBD and/or have been associated with disease in pets fed RMBD including Clostridium perfringens, Brucella suis, Listeria monocytogenes and Mycobacterium bovis whilst other studies have recovered other viable zoonotic pathogens, including Sarcocystis species and Toxoplasma gondii (Schlesinger and Joffe, 2011; Freeman et al., 2013; Nemser et al., 2014; Goh, 2016; Fredriksson-Ahomaa et al., 2017; Davies, 2018; van Bree et al., 2018; Loeb, 2019; O’Halloran et al., 2019).
Reduced inflammation, improved health
The potential risk of feeding RMBD to companion animals is often countermanded by benefits they can provide. These include better muscle condition and improved dental, periodontal, dermatological and gastrointestinal health (Free-man and Michel, 2001; Stogdale et al., 2003). The latter is particularly thought to be true in cases of chronic gastroenteropathy such as food-responsive inflammatory bowel dis-ease where many clinicians have had owners report often a dramatic and rapid improvement in clinical signs after switching from a cooked to RMBD (Freeman and Michel, 2001; Stogdale et al., 2003). However, there are currently no systematic prospective, or even retrospective, studies which have properly evaluated these observations.
In recent years there has been an increasing research focus on the intestinal microbiota and changes therein (eg dysbiosis) as both a source and consequence of disease in humans and companion animals (Kaur et al., 2011; Suchodolski, 2011; Nibali et al., 2014; Das and Nair, 2019).
Raw diets have been shown to lead to a wider diversity and complexity of organisms in the faeces of RMBD-fed dogs (Bermingham et al., 2017; Kim et al., 2017; Sandri et al., 2017; Shmidt et al., 2018). However, based on the species of bacteria present and their relative abundance, these differences most likely reflect the higher fibre and carbohydrate content of most commercial cooked diets compared to RMBD that tend to contain more protein and fat (Kim et al., 2017; Sandri et al., 2017; Shmidt et al., 2018; Sandri et al., 2019).
A study published in the Veterinary Journal analysed the effect of cooked kibble and RMBD on peripheral blood mononuclear cell gene expression in dogs (Anderson et al., 2018). The study found that diet significantly influenced canine immune cell gene expression, with RMBD generally causing a decrease
in the expression of pro-inflammatory cytokine genes supporting the frequently highlighted anecdotal observation that RMBD can be beneficial in the management of chronic inflammatory conditions (Anderson et al., 2018). However, these changes, seen at the three- and six-week time points, had been lost by the time dogs had been on their respective diets for nine weeks. Additionally, quantifying the mRNA of a cytokine is not the same as measuring the concentration of cytokine secreted as there are a number post-translational modifications which could prevent protein synthesis and secretion, even in the presence of increased mRNA copy numbers. The dogs in this study were clinically normal so it was not possible for the authors to assess if the RMBD had an active anti-inflammatory effect.
Nutrient balance: are diets complete?
Concerns regarding nutritional imbalances are a frequent feature of debates about feeding companion animals RMBD. It has occasionally been proposed that raw diets are not complete according to the legal definition: that a complete pet food must contain every nutrient required by an animal in sufficient amounts to keep it healthy so that the diet in question should not be detrimental to an animal’s health if fed as a sole ration for an extended period of time (Food Standards Agency, 2020). It is widely accepted that home-prepared RMBD are the most likely to be incomplete due to the inherent difficulties in providing sufficient macro-nutrients, minerals and vitamins in combination so as to allow adequate bioavailability, and as such these diets are best avoided without the input of an experienced specially qualified veterinary nutritionist (Weeth, 2013; van Zelst et al., 2015; Oba et al., 2019). Reported problems with such diets have included thiamine deficiency causing neurological syndromes, taurine deficiency resulting in feline dilated cardiomyopathy, hypervitaminosis A reportedly caused by diets with a high liver content, joint dysplasia due to inappropriate calcium to phosphorus ratios and hyperthyroidism resulting from the ingestion of raw thyroid glands (Deka, 2009; Kritikos et al., 2017; Bischoff and Rumbeiha, 2018; Mansilla et al., 2019; Stogdale, 2019).
In recent years, many food companies have produced RMBD products and brought them to market, offering similar reassurance to consumers with regards to complete-ness as the “traditional” cooked food products. Importantly, cooked diets are not necessarily always safe alternatives to RMBD with respect to nutritional imbalances. There has recently been good evidence published which links (cooked) grain-free diets with dilated cardiomyopathy in dogs, whilst Hill’s voluntarily recalled a dog food due to excessive vitamin D levels after a dog was presented with clinical signs of hypervitaminosis D (Hill’s, 2019; Adin et al., 2019).
RMBD have long been used for environmental enrichment, particularly for zoo animals where raw carcasses provide carnivores with significant opportunities to display appropriate natural behaviours as well as the obvious olfactory and gustatory enrichment. The same principle applies to domesticated carnivores, whose wild ancestors evolved many behavioural traits for the acquisition of food. Providing (supervised) RMBD enrichment can help owners maintain body weight and condition, whilst chewing raw bones can be beneficial for oral hygiene and chewing is also a self-soothing behaviour of dogs, so may help prevent or alleviate stress-related conditions (Lawson et al., 2020).
Overall, there are clearly both advantages and disadvantages to feeding RMBD to our domestic carnivores. The main risk comes from the potential for microbiological contamination and therefore owners with particularly vulnerable health statuses, such as long-term immunodeficiencies, should think carefully before opting to feed their pets RMBD. Conversely, there are a large number of advocated benefits to raw feeding when the diets are complete but as yet there is only weak evidence published in support of them. As such, many veterinary associations such as the AVMA, BVA and CVMA have warned against the practice of raw feeding and currently the WSAVA Global Nutrition Committee recommends that RMBD not be fed to dogs and cats, but these position statements may well change as stronger evidence accumulates in the scientific literature (Cima, 2012; Veterinary Record, 2015).