Slowly Poisoning Fido, But With All the Best Intentions

Because a pet is now as much a member of the family as any other household inhabitant, “pet parents” (self-described pet owners), are now examining pet food labels expecting to see familiar human food ingredients such as potatoes, peas, etc. Feeding the family pet human food is increasingly common, especially when the pet presents with seemingly intractable allergic reactions to commercially produced pet food. However, the U.S. Center for Veterinary Medicine (CVM), which oversees the safety of animal feed, including pet food, has repeatedly stated that foods commonly consumed by people may not be safe for consumption by pets. The oft-used example of this point is that while chocolate is safe for humans, the presence of methylxanthine derivatives (including caffeine and theobromine), may be fatal to dogs because of the inability of dogs to detoxify methylxanthines through the same metabolic processes present in humans that we all take for granted.  Likewise, onions are poisonous to cats (probably due to sulfides, which provide the characteristic “bite” and odor of an onion) and garlic is poisonous to equine species (N-propyl sulfide is likely the proximate toxin), but both vegetables are harmless to humans because of a facility for sulfide metabolism, detoxification and excretion. Likewise, raisins and grapes are known to be toxic to dogs (resulting in acute renal failure) and although the specific toxin has not been identified, a mycotoxin to which dogs are acutely sensitive may be to blame, but to which humans are resistant, at least at levels of exposure not tolerated by dogs. With few exceptions, most of the species-substance incompatibilities are relatively intuitive – dogs and cats had little opportunity to evolve protective mechanisms to chocolate, onions or garlic, all three of which are relatively new additions to man’s diet  and are largely based on hedonistic desires, rather than nutritional requirements. Further, food technology and knowledge of nutrition has pushed the boundaries of traditional nutrient sources, but not always without unexpected effects – for example, excess addition of soy protein to zoo cat food as a protein supplement to traditional animal carcasses, had the unexpected effect of curtailing reproduction by males because of the estrogenic effects of some of the phytosterols present in the soy extract – an issue since having been corrected.

The knowledge that “one [species’] sustenance may be another [species’] poison” is not new. When the original generally recognized as safe list (or GRAS list, for humans) was adopted and placed into the CFR (now in sections 21 CFR 182 and 184), CVM performed a triage of the human GRAS list, selecting only those substances for inclusion in 21 CFR §582 for which data supported safety in a wider range of species. Since the performance of that triage (~1960), even more information has been uncovered illustrating more species differences and resulting in an increased emphasis at CVM for “target” species testing. In the words of CVM, “a mouse is not a cow,” and despite a high degree of genetic conservatism between mammalian species, there are some profound differences.

Traditional testing utilizing the American Association of Feed Control Officials (AAFCO) basic protocols designed to evaluate the nutritional adequacy of dog and cat feed may no longer be adequate. CVM, a long-time proponent of recognizing species differences, has indicated that while these basic studies may evaluate nutrition, they do not sufficiently evaluate biological endpoints necessary to determine the safety of a feed ingredient. But, what could these endpoints be and how would species differences be detected without using a large number of animals (loath to pet food manufacturers and pet owners alike) and long term (read: cost-prohibitive) testing? The answer probably lies in part in understanding the metabolism of pet species (including the different breeds) and a determination of safety based on the rate and extent of metabolism and excretion of the substance. Single-minded, check-list toxicology is not going to ensure safety – well documented and logical research characterized by creativity, thinking outside the box and pragmatism is the only practical response to the idiosyncratic responses of the various species.