Food Allergy Dermatitis
Food allergies are a common problem in cats. Such allergies present as skin problems or gastrointestinal problems (see Inflammatory Bowel Disease). The skin sores are usually thought to be due to fleas and the owner often becomes frustrated that the problem persists in spite of regular worm and flea treatment.
Food allergy is about as common as atopy in cats. The history is that of a non-seasonal pruritus, with little variation in the intensity of pruritus from one season to another in most cases. The distribution of pruritus and lesions varies markedly between animals. Ear canal disease that manifests as pruritus and secondary infection with bacteria (usually Staphylococcus intermedius , Pseudomonas spp , Proteus spp , or Escherichia coli ) or yeast (Malassezia pachydermatis ) are common and may be the only presenting complaint. Other patterns seen include blepharitis, generalized pruritus, generalized seborrhoea, a papular eruption, or a distribution pattern that may mimic that of atopy (feet, face, and ventrum) or flea allergy dermatitis (dorsal lumbosacrum and hindlegs). The most common areas of involvement include the ears, feet, inguinal region, axillary area, proximal anterior forelegs, periorbital region, and muzzle. The degree of pruritus is usually moderate to severe. Response to glucocorticoids varies from poor to excellent.
There is no reliable diagnostic test other than a strict food elimination diet. Serologic testing and intradermal testing for food allergens have proved unreliable. The ideal food elimination diet should be balanced and nutritionally complete and not contain any ingredients that have been fed previously to the animal. Many diets contain novel protein or carbohydrate sources (eg, lamb and rice). However, it is often misunderstood that if any previously fed ingredient is present in the elimination diet, the animal may be allergic to the novel ingredient and the diet trial will be a failure. The key point in any food elimination diet trial is that only novel food ingredients can be fed.
The trial diet should be fed for up to 3 mo. If marked or complete resolution in the pruritus and clinical signs occurs during the elimination diet trial, food allergy can be suspected. To confirm that a food allergy exists and that the clinical improvement was not just coincidental, the animal must be challenged with the previously fed food ingredients and a relapse of clinical signs must occur. The return of clinical signs after challenge is usually between 1 hr and 14 days, although it is sometimes within 3 days. Once a food allergy is confirmed, the elimination diet should be reinstituted until clinical signs resolve, which usually takes <14 days. At this point, previously fed individual ingredients should be added to the elimination diet for a period of up to 14 days. If pruritus recurs, the individual ingredient is considered positive for having a causative role in the food allergy. If pruritus does not recur the individual ingredient is not considered important in causing the clinical signs.
The number of offending food allergens varies from 1-5 ingredients. The most frequently identified causative allergens in canine food allergy include beef, chicken, eggs, corn, wheat, soy, and milk. Once the offending allergens are identified, control of the food allergy is by strict avoidance. Concurrent diseases (such as atopy or flea allergy) may complicate the identification of underlying food allergies. Infrequently, a dog will react to new food allergens as it ages.
Clinical presentations of food allergy in cats include miliary dermatitis, feline symmetric alopecia, eosinophilic granuloma complex ( primarily the eosinophilic plaque), and severe head and neck pruritus. No breed, sex, or age predilection is seen. Age of onset varies from 3 mo to 11 yr. In one study, however, 46% of affected cats became symptomatic at ≤2 yr of age, and Siamese cats represented 30% of the cases.
Response to steroids is variable, but about two-thirds of cats show excellent response initially. Many cats develop a poor response to steroids with repeated treatments. As with canine food allergy, an elimination diet should be fed for up to 3 mo. The elimination diet should not contain any previously fed ingredients. Food elimination diets can be difficult in cats because many cats are reluctant to change diets. Cats should not be starved or forced into eating a new elimination diet due to the serious nature of hepatic lipidosis that may be induced by prolonged anorexia.
Response time to the elimination diets varies from 1-9 wk. Time until relapse of pruritus after challenge with the offending food varies from 15 min to 10 days. The most frequently identified food allergens in cats include fish, beef, and chicken. Avoidance of the offending allergens will control the clinical signs associated with the food allergy.
