Food Allergies
Joe Bodewes, DVM
Veterinary Services Department, Drs. Foster & Smith, Inc.
Food allergies account for about 10% of all the allergies seen in cats and dogs. It is the third most common cause after flea bite allergies and atopy (inhalant allergies).
The entire process of a pet being sensitized to a particular agent in food and the complicated antibody response that occurs in the intestinal tract in pets with food allergies are not very well understood. Despite our lack of understanding of the actual disease process, there are many things that we do know including the symptoms, how to diagnose food allergies, and also how to treat them.
Food allergies affect both cats and dogs. Unlike atopy, there is no strong link between specific breeds and food allergies. Food allergies affect both males and females and neutered and intact animals equally. They can show up as early as five months and as late as 12 years of age, though the vast majority of cases occur between 2 and 6 years. Many animals with food allergies also have concurrent inhalant or contact allergies.
Food allergy or intolerance?
There is a distinction that needs to be made between food allergies and food intolerances. Food allergies are true allergies and show the characteristic symptoms of itching and skin problems associated with feline and canine allergies. Food intolerances can result in diarrhea or vomiting and do not create a typical allergic response. Food intolerances in pets would be similar to people that get diarrhea or an upset stomach from eating spicy or fried foods. Fortunately, both food intolerances and allergies can be eliminated with a diet free from offending agents.
Common food culprits
Several studies have shown that some ingredients are more likely to cause food allergies than others. In cats, the most common offenders are fish, beef, and dairy products. As you may have noticed, the most common offenders are the most common ingredients in both cat and dog foods. This correlation is not a coincidence. While some proteins might be slightly more antigenic than others, many proteins are similar in form and the incidence of allergic reactions are probably associated with the amount of exposure.
Symptoms
The symptoms of food allergies are similar to those of most other allergies seen in cats. The primary symptom is itchy skin. Symptoms may also include hair loss, excessive scratching, and miliary dermatitis.
It is difficult to distinguish an animal suffering from food allergies from an animal suffering from atopy or other allergies based on physical signs. However, there are a few signs that always make me suspect food allergies. If a cat suffers from allergies year-round or if symptoms begin in the winter, I suspect a food allergy. Cats may develop very itchy skin which does not respond to antihistamines or steroid treatment.
Diagnosis
The diagnosis for food allergies is very straightforward. But due to the fact that many other problems can cause similar symptoms and that many times animals are suffering from more problems than just food allergies, it is very important that all other problems are properly identified and treated prior to undergoing diagnosis for food allergies. Atopy, flea bite allergies, intestinal parasite hypersensitivities, notoedric mange, yeast or bacterial infections, or seborrhea can all cause similar symptoms as food allergies. Once all other causes have been ruled out or treated, then it is time to perform a food trial.
Elimination diets and provocative testing: A food trial consists of feeding a cat a novel food source of protein and carbohydrate for 12 weeks. A novel food source would be a protein and carbohydrate that the animal had never eaten before. An example would be duck and potato, or venison and potato. These diets are available commercially, or could be homemade. This must be the only thing the animal eats for 12 weeks. No treats and nothing from the table. Young growing pets have special dietary needs and a homemade diet that only contains one protein and one carbohydrate with no multivitamin or fatty acid may not be suitable even for only twelve weeks. For kittens undergoing a food trial, a balanced commercial diet like the ones listed above is recommended.
Veterinarians used to recommend that a pet only needed to be placed on a special diet for 3 weeks but new studies show that in dogs, only 26% of those with food allergies responded by day 21. However, the vast majority of the animals responded by 12 weeks. The same may be true in cats, therefore, it is very important to keep the cat on the diet for the entire 12 weeks. If the cat shows a marked reduction or elimination of the symptoms, then the animal is placed back on the original food. This is called 'provocative testing' and is essential to confirm the diagnosis. If the symptoms return after going back on the original diet, the diagnosis of a food allergy is confirmed. If there has been no change in symptoms but a food allergy is still strongly suspected, then another food trial using a different novel food source could be tried.
Blood Testing: Many owners and veterinarians attempt to look to other tests to diagnose food allergies. Blood tests such as the RAST test or the ELISA test can be performed to screen for food allergies. In addition, intradermal skin testing could also be performed. Despite the fact that these tests are routinely performed and used as a diagnostic aid, there is no evidence that blood tests are accurate for the diagnosis of food allergies. Veterinary dermatologists insist that there is no merit in these tests whatsoever in the diagnosis of food allergies. The only way to accurately diagnose food allergies is with a food trial as detailed above. While the intradermal skin testing is excellent for diagnosing atopy (inhalant allergies) it is ineffective for food allergies. While the ELISA and to a lesser extent the RAST test can be used to help in the diagnosis of atopy, they have no benefit in diagnosing food allergies. In my review of all the current books and articles on veterinary dermatology and allergies, I could not find a single dermatologist that endorsed anything other than the food trial as an effective diagnostic aid. If you want to diagnose and treat food allergies you must do a food trial.
Treatment
Once we determine a positive diagnosis, then the treatment is very straightforward. The owner of the animal has two choices. They can choose to feed the animal a special commercially prepared diet from Hill's or Purina, or they can choose to feed a homemade diet.
If the owner chooses to feed the homemade diet, then they can periodically challenge the pet with new ingredients and determine which ingredients are causing the food allergy. For example, if the animal's symptoms subsided on a diet of rabbit and potatoes, then the owner could add chicken to the diet for two weeks. If the animal showed no symptoms, then they could then add beef for two weeks. If the animal began to show symptoms, then it could be assumed that beef was one of the things the pet was allergic to. The beef could be withdrawn and after the symptoms cleared up, a different ingredient could be added and so on, until all of the offending ingredients were identified. A diet could then be formulated that was free of the offending food sources.
If homemade diets are used, it is essential that they be balanced, with correct amount of ingredients, vitamins, and minerals.
References
Bonagura, J. Kirks Current Veterinary Therapy XII. W.B. Saunders Co. Philadelphia, PA; 1995
Bonagura, J. Kirks Current Veterinary Therapy XIII. W.B. Saunders Co. Philadelphia, PA; 2000
Ettinger, S. Textbook of Veterinary Internal Medicine. W.B. Saunders Co. Philadelphia, PA; 1989
Greene, C. Infectious Disease of the Dog and Cat. W.B. Saunders Co. Philadelphia, PA; 1998
Griffin, C.; Kwochka, K.; Macdonald, J. Current Veterinary Dermatology. Mosby Publications. Linn, MO; 1993
Scott, D.; Miller, W.; Griffin, C. Muller and Kirk's Small Animal Dermatology. W.B. Saunders Co. Philadelphia, PA; 1995
