Food Allergy, or more correctly “adverse food reactions”, AFR, -as not all cases of food related itchy skin have an immunological allergic basis are seen from time to time and are not always associated with a recent change of diet, as owners often believe.
In dogs, age of onset can be as early as 12 weeks of age or as late as several years of age. Food allergy usually takes time to develop – it is not uncommon for an animal to become allergic to a food that it has tolerated for years. Symptoms can be almost indistinguishable form those of Atopic Dermatitis in dogs -e.g. itchy- ears, generalised and interdigital pruritus (itch) and peri-anal pruritus (which is seen with a higher frequency in adverse food reactions) and recurrent skin infections. Some individuals may also have concurrent gastrointestinal symptoms such as diarrhoea and vomiting.
AFRs in many cases, do not respond as well to steroids as atopy does. The signs associated with AFRs are non-seasonal and therefore AFRs have to be ruled out when investigating cases of suspected atopy.
In cats AFRs can present with the same range of signs seen with flea allegy and atopy – spots and scabs along the back and sides (milary dermatitis) a syndrome called eosinophilic granuloma complex which can present as ulcers and plaques on the body or lips (rodent ulcer), and often as an apparent non-inflammatory alopecia caused by excessive grooming. (Cats only scratch if their itch is from the neck upwards- from the neck downwards they tend to overgroom)
It is generally accepted amongst veterinary dermatologists that there are no reliable blood tests available to confirm a case of food allergy/AFR. There are some companies offering blood tests, but they may only be possibly of use in deciding which foods to avoid once a food allergy has been confirmed.
The only way to confirm an AFR is to feed an elimination diet for 6-8 weeks (or occasionally even longer!) This is either done by feeding a home-prepared diet of pure sources of protein and carbohydrate that have previously not been fed. This is difficult to do for most owners as it entails source exotic cuts of meat or fish that would not be used in commercial dog food manufacture and having to cook it.
Several commercial dog food manufacturers now produce diets suitable for an elimination diet – these are either based on novel proteins and carbohydrates e.g. Capelin and tapioca or are one of the new generation of hydrolysed protein diets. It has been discovered that protein molecules have to be above a certain molecular weight to be recognised by the immune system. Breaking down proteins by a process called hydrolysis renders them too small to be immunogenic. Neither of these two types of diets are absolutely perfect as an elimination diet, as recent research has shown that some dogs with AFRs which clear on a home prepared diet, relapse on a commercial novel protein diet and although this can also occur with hydrolysed diets the incidence is much lower. For this reason, I usually recommend a hydrolysed soya based diet as an exclusion diet, although on some occasions a home-prepared diet may be recommended. Hydrolysed diets also have the advantage, in theory, that an animal can not become sensitised to the diet in future - something that can happen with novel protein diets (as happened with my own cat a few years ago).
Cats present a problem when it comes to feeding eliminiation diets as they have to be kept inside for the duration to stop them supplementing their diet with the local wildlife or popping next door for a bowl of the neighbour’s cat’s food.
It is vitally important that whilst on an elimination diet, no other foods, treats, chews etc should be given, as it only takes a small amount of an offending allergen to keep the signs of food allergy going. Also beware of pet store/supermarket foods labelled as “hypoallergenic” , as it is extremely unlikely that they are truely hypoallergenic in the sense that they would be suitable as an elimination diet to test for food allergy.
