To determine whether or not a food allergy or intolerance is causing the skin problem, a hypoallergenic diet is fed for a set period of time. If the pet recovers, the original diet is fed for up to two weeks to see if itching resumes. If the result is recovery on the test diet and a return of the symptoms with the original diet, then a food allergy is diagnosed and the pet needs to be returned to either the test diet or another appropriate food. Looking for a “good hypoallergenic diet” can be approached in two ways:
Obviously, the test diet must be of a food source that the pet could not possibly be allergic to. The traditional method is the use of a novel protein and carbohydrate source, i.e., something the pet has never eaten before. In the past, lamb has been the protein source of choice as pet food companies had traditionally not offered any lamb based pet foods. Unfortunately, recent production of lamb and rice based foods have removed lamb from the acceptable hypoallergenic diet list due to the high number of such products now available on the market.
Many pet food companies have discerned the need for diets using unusual protein and carbohydrate sources with a minimum of additives. Foods can be obtained based on venison and potato, fish and potato, egg and rice, duck and pea, and even kangaroo, brush tail, ostrich, llama, herbs and more.
It is important that during the diet trial no unnecessary medications be given. No edible chew toys like for example rawhides or bones should be given. Treats must be based on the same food sources as the test diet. Be careful though, like for example stay away from rice cakes since wheat is commonly used as a filler. Replace chewable heartworm preventives with tablets.
Home cooking was originally the only option felt to be appropriately free of allergens but for most animals these special commercial foods are adequate. Occasionally home cooking ends up being necessary after all.
In the past, 4 weeks was thought to represent a complete trial period. More recent work has shown that some food allergic animals require 8 to 10 weeks to respond. This may be an extremely inconvenient period of time for home cooking. My current recommendation is to reevaluate after four weeks of diet trial and then again after eight weeks of trial. Eighty percent of food allergic dogs will have responded to diet trial at least partially by six weeks. Labradors, Retrievers and Cocker Spaniels appear to require up 10 weeks of trial diet before showing a response. Some dogs may even require a longer period.
To confirm that your pet indeed has a food allergy, return to the original food; itching resumes within 14 days generally if food allergy was truly the reason for the itchy skin. Many pet owners do not want to take a chance of returning to itching if the patient is doing well; it is not unreasonable to simply stay with the test diet if the pet remains free of symptoms.
It is possible to more specifically determine the identity of the offending foods after the pet is well. To do this, a pure protein source, like for example chicken or any other single food is added to the test diet with each feeding. If the pet begins to itch within 2 weeks, then that protein source represents one of the pet's allergens. Return to the test diet until the itching stops and try another pure protein source. If no itching results after two weeks of feeding a test protein, the pet is not allergic to this protein.
And if the trial is unsuccessful? Generally, this strongly suggests that an inhalant allergy is the primary problem but there are a few considerations that should at least be mentioned: Are you certain that the dog received no other food or substances orally during the trial? Your pet may require a longer diet trial. Are you certain regarding the factor that pointed us toward the food allergy?