ADVERSE FOOD REACTIONS IN ANIMALS:
A DERMATOLOGIST'S PERSPECTIVE
By James Jeffers, VMD, Dipl. ACVD
Animal Allergy and Dermatology Clinic
Gaithersburg, Maryland, USA

Spring 2001, NEWS

The exact mechanisms by which adverse food reactions develop in dogs and cats are unknown. These reactions are considered acquired rather than inherited and usually develop as immediate or delayed reactions to regularly ingested proteins. Adverse reactions to food additives probably rarely occur.

No breed, age, or gender predilections to adverse food reactions have been convincingly demonstrated. Adverse food reactions usually manifest as nonseasonal pruritus. Response to corticosteroid treatment ranges from complete to none at all. Virtually every type of primary and secondary skin lesion in almost every location has been observed in dogs with adverse food reactions. Lesions are usually associated with self-trauma, seborrhea, and secondary infections. Adverse food reactions may also manifest as only nonpruritic, recurrent bacterial skin infections or otitis externa. In cats, pruritus most often occurs on the head and neck but can occur anywhere. Miliary dermatitis, self-induced alopecia, and the eosinophilic granuloma complex have been associated with adverse food reactions. Gastrointestinal abnormalities, in addition to skin problems, occur in a small percentage of cases. Inhalant, fleabite, contact, or endoparasitic allergies are seen concurrently in about 25% of cases.

Serologic or intradermal testing for adverse food reactions is inaccurate. Instead, home-prepared elimination diets are often recommended. These consist of a protein and a carbohydrate source to which the animal has never been exposed. However, home-prepared elimination diets can be expensive, time-consuming to prepare, and nutritionally unbalanced. By comparison, commercially prepared elimination diets are generally less expensive than home-prepared diets, easier to feed, and nutritionally balanced. But virtually none of the commercially manufactured elimination diets have been rigorously tested with regard to their accuracy in diagnosing adverse reactions to food.

Feeding a strict elimination diet for up to 10 weeks may be needed to diagnose an adverse food reaction (based on a reduction in clinical signs of at least 50% to 75%). Dogs and cats challenged with all components of their original food will again manifest signs of adverse food reaction (usually pruritus) within 1 hour to 14 days. Secondary seborrhea or bacterial or Malassezia species infections must be treated before ending the food elimination trial. To pinpoint all offending food proteins, one food ingredient at a time is added to the elimination diet for 7 to 14 days each. Proteins that the pet has had the most opportunity to consume over its lifetime are used. Although many different proteins have been reported to trigger adverse reactions in the skin of dogs and cats, beef, soy, chicken, milk, corn, wheat, and eggs are most commonly incriminated in dogs. Fish, dairy, beef, and eggs are most often noted in cats. About 36% of dogs react adversely to only one protein; conversely, 64% react to multiple proteins. Cross-reactivity between products derived from the same animal species or plant sources is unlikely.

Because natural desensitization is rare, avoidance is the only treatment for adverse food reactions. Animals rarely develop other adverse dietary reactions.

Re-printed with kind permission from Veterinary Medicine Publishing Group, Lenexa, Kansas