Adverse reactions to food encompass food poisoning, food aversion, and non-immunological food intolerances (including dietary indiscretion) as well as true food allergy, but the frequent misuse of the expression 'food allergy' to describe any such reaction has confused our understanding. Yet the prevention of any adverse reaction to food is simple: feed a diet that excludes the offending foodstuff and the patient will be free from clinical signs. Client and patient compliance is obviously essential, but the key to successful management is making the diagnosis and identifying the foodstuff that must be excluded. It is here that difficulties arise, although the relatively recent advent of hydrolysed diets has broadened the choice of suitable exclusion diets.
True food allergy is a reproducible adverse reaction to a specific food or food additive with a proven immunological basis, and which typically causes dermatological and/or gastrointestinal (GI) signs. The diagnosis is in vogue with pet owners, and the growth in special veterinary exclusion and hydrolysed diets would suggest that the condition is both common and readily diagnosed. Neither is necessarily true: other causes of pruritus and gastrointestinal signs are more common and, as some of them respond to dietary manipulation, and these should be ruled out before an exclusion diet trial is performed.
The cornerstone of the diagnosis of food allergy remains the response to dietary manipulation. The principle of an exclusion (elimination) diet trial is to feed dietary components to which the animal has not previously (or recently) been exposed. This diet should be the sole source of nutrition for the duration of the trial. Complete owner compliance is essential, and it must be stressed that all treats and supplements must be withheld. Clinical signs should resolve on exclusion of the offending dietary component and return with rechallenge.
Objective criteria by which to judge a response to a diet trial are generally lacking, especially as the signs of other diseases may also respond to dietary manipulation. Walker et al. (2013) studied 30 dogs with food-responsive chronic enteropathy and showed that, compared with controls, duodenal biopsies showed a higher overall WSAVA score, with villus stunting, increased lamina propria eosinophils and ultrastructural changes. There was some improvement in eosinophils and ultrastructural changes after an exclusion diet trial. Yet these microscopic changes are not pathognomonic and studies demonstrating complete histopathological resolution after a diet trial are rarely reported.
Options for exclusion diets include both home-prepared and commercial single-source protein diets, and hydrolysed protein diets. Traditionally, exclusion diets were composed of single protein and carbohydrate sources. The literature suggests that in dogs the most frequent foods incriminated are beef, dairy products, and gluten (wheat protein), whilst in cats, beef, dairy products, and fish are reportedly significant causes. However, the prevalence of allergy to different food substances may simply reflect geographical variation in the nature of the foods most commonly fed. Lamb or chicken or rabbit with rice or potato have been historical choices for home-prepared diets but, given the increased diversity of pet diets, these components are often not appropriate. Thus, more esoteric food sources may be required.
Home-cooked diets have been preferred to commercial diets because of reports of relapse when patients are switched to an equivalent commercial diet. However, many clinicians recommend the use of a commercial diet initially, because it is easier for the owner and thus compliance is enhanced. A wide range is now available, containing different sources of protein (e.g., chicken, soy, fish [capelin, catfish and salmon], venison, or duck) and carbohydrate (e.g., rice, corn [maize], tapioca, or potato). Thus, it should be possible to find an appropriate diet in most cases, although manufacturers cannot always guarantee a constant supply of diets with more exotic components.
Hydrolysed protein diets, usually based on either chicken or soy protein, are an alternative exclusion diet. Such diets are significantly more expensive to produce than standard exclusion diets, and both palatability and owner compliance may be problematic. In principle, the hydrolysis process splits proteins into components of a molecular weight below that which would be expected to elicit an immune response. Most veterinary hydrolysed diets (e.g., Hill's z/d ULTRA Allergen-freeTM and z/d Low AllergenTM, Purina HA®, Royal Canin Hypoallergenic, Affinity Advance® Hypoallergenic) contain peptides with an average molecular size in the range of 5 to 15 kilodaltons. Yet to completely abolish all antigenicity, peptides need to be less than 1 kD, but at that size they usually have a bitter taste. Very recently, a diet containing free amino acids and much smaller (< 1 kD) oligopeptides (Royal Canin Anallergenic®) has been marketed; it is too soon to know whether this offers any further advantages. But even at the molecular size achieved in most hydrolysed diets (i.e., 5 to 15 kD), peptides/proteins are too small to be able to cross-link IgE molecules on mast cells, and type I reactions should be abolished.
Hydrolysed diets are currently the easiest way to guarantee the feeding of novel "protein," and this approach has become the preferred method for many clinicians. Studies have suggested a beneficial effect of such diets, although it has not been proven that such diets have an advantage over traditional single-source protein exclusion diets. However, some clinicians now claim a positive response to a hydrolysed diet alone in a majority of cases that typically would have been treated with steroids in the past. Again histological evidence of resolution is usually lacking, but hydrolysed diets may be a useful treatment of inflammatory bowel disease (IBD), even if diet is not the actual cause. Hydrolysed diets may contain anti-inflammatory nutrients, such as omega-3 fatty acids, or even simply provide an inflamed intestine with a more digestible diet that the diseased gut can assimilate. Thus, animals with what would formerly have been termed idiopathic IBD, now called chronic enteropathy, may benefit clinically from a hydrolysed diet and food trial is recommended before commencing steroid therapy.
Once remission is achieved with an exclusion diet trial, the patient should be challenged with the original diet to demonstrate relapse and confirm the diagnosis. However, some clients refuse challenge, particularly if relapse is likely to bring recurrence of diarrhoea. After relapse during the challenge phase, for clients who are willing, a series of provocation tests can be performed to identify the offending food or foods. Remission is first regained by "rescue" with the exclusion diet, and single foodstuffs are then introduced sequentially as provocation tests. If there is no relapse, the food is identified as "safe," and the next food is tested; if there is a relapse, rescue is repeated and the food is identified as "unsafe." When all the offending foods have been identified, or at least sufficient safe ones have been recognized to permit the choice of a regular diet, the animal is maintained on a safe diet.
Given the difficulties of diet trials, indirect allergy blood tests have been attempted; unfortunately, none are reliable in veterinary medicine for the diagnosis of a food allergy.
References
1. Cave NJ. Hydrolyzed protein diets for dogs and cats. Veterinary Clinics of North America - Small Animal Practice. 2006;36:1251.
2. Foster AP, et al. Serum IgE and IgG responses to food antigens in normal and atopic dogs, and dogs with gastrointestinal disease. Veterinary Immunology and Immunopathology. 2003;92:113.
3. Hannah SS, Laflamme DP, Marks SL. Hydrolysed proteins in diets for the management of food hypersensitivity or inflammatory bowel disease. In: Proceedings of the WSAVA/FECAVA World Congress; 2000: 517.
4. Jackson HA, Jackson MW, et al. Evaluation of the clinical and allergen specific serum immunoglobulin E responses to oral challenge with cornstarch, corn, soy and a soy hydrolysate diet in dogs with spontaneous food allergy. Veterinary Dermatology. 2003;14:181.
5. Osborn DA, Sinn J. Formulas containing hydrolysed protein for prevention of allergy and food intolerance in infants. Cochrane Database Systematic Review. 2003;(4):CD003664.
6. Ricci R, et al. A comparison of the clinical manifestations of feeding whole and hydrolysed chicken to dogs with hypersensitivity to the native protein. Veterinary Dermatology. 2010;21:358.
7. Walker D, et al. A comprehensive pathological survey of duodenal biopsies from dogs with diet-responsive chronic enteropathy. Journal of Veterinary Internal Medicine. 2013;27:862.