Food allergy in the cat may be associated with concurrent flea bite, inhalant or flea-collar hypersensitivity (White 8c Sequoia, 1989). Food allergy as a disease is a well-recognised cause of feline dermatological problems (Carlotti et al, 1990; Rosser, 1993), the prevalence of which varies from rare to common according to different authors. The pathophysiology of this syndrome is not completely understood. Adverse reactions to food may be either toxic or nontoxic; if they are non-toxic they may be either due to intolerance or to immunological (allergic) reactions.
Food allergy may be defined as an immunologically mediated excessive reaction to food substances, to be distinguished from food intolerance where enzyme deficiencies, pharmacological factors and toxic reactions, among others, may be implicated.
Allergens implicated include milk, beef, mutton, pork, chicken, rabbit, horse, eggs and fish. In principle there is no hypoallergenic protein source for a cat: it is possible for any protein source to be an allergen. Adverse reactions to artificial flavours, dyes and preservatives are considered rare. Additives are usually too small to be antigenic and they are more likely to cause a reaction via intolerance mechanisms.
Clinical signs include non-seasonal pruritus often affecting the head and neck, including otitis externa (Plate 6.9). Miliary dermatitis, symmetrical alopecia, eosinophilic plaque and ulcerative dermatitis secondary to severe self-trauma may also be seen. Gastrointestinal signs of vomiting or diarrhoea occur occasionally.
The diagnosis of food allergy is achieved with a diet trial using a source of protein and carbohydrate to which the cat has had no or minimal exposure. This may be home cooked or, more commonly, a commercially prepared diet. There is debate about the value of home-cooked versus commercial diets. The home-cooked diet may not be balanced and can be a considerable practical challenge for the owner to prepare; however, according to some authors this is preferable to commercial diets that may contain the same dietary constituents but through food processing the antigenicity of the food is altered.
There is controversy over the optimum period for which the diet is fed. This may range from 3 to 9 weeks. The author recommends a period of 8 weeks with a commercial diet. The commercial diets currendy available include lamb and barley, capelin and tapioca, lamb and rice, turkey and rice, and chicken and rice. The choice of diet will depend on the previous food items fed, although it may not be possible to know about all of the minor food ingredients in feline diets. The compliance with the diet trial will depend on the willingness of the cat to eat the food and not being able to access other sources of food; for example, by hunting or being outdoors for long periods and eating food at another home, or even just eating cat food for other cats in the house. Consequently, it may prove difficult to evaluate a suspected food allergy problem for a cat allowed outdoors. The ideal candidate would be a single cat living exclusively indoors and willing to eat any diet provided.
Amelioration of signs following an elimination diet can only be confirmed by the return of pruritus after rechallenge with the previous diet; this may take up to 7 days and even 14 days in some cases. Provocative exposure may be tried sequentially with a new protein item added to the hypoallergenic diet; this is not very often pursued.
It is possible for the food-allergic cat to relapse while eating the hypoallergenic diet that initially cured the problem. That is, the cat could become allergic to the new protein source and require a second diet trial. In some cases, if the first trial fails and there is no evidence for other causes a second diet trial may be required to establish a diagnosis.
Intradermal testing and in vitro tests are not useful in the diagnosis of this syndrome.
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