Produced by Dr Andrew Carter, Specialist Veterinary Dermatologist.
What is Food Hypersensitivity?
Food hypersensitivity (allergy) is an uncommon cause of pruritus (itch) in dogs and cats. It is triggered by your pet’s immune system becoming sensitised to certain components of the diet (allergens). These allergens are most commonly proteins present in meat, dairy or cereal components of the diet but in some cases they will be artificial additives in commercial pet foods. Food hypersensitivity resulting in skin disease can develop at any age and usually involves foods that have been in the diet for some time.
In most cases pets with skin disease resulting from food hypersensitivity will be itchy. They may scratch, lick themselves or rub against things frequently. The skin disease may affect only certain areas or may be generalised. In some cases it can be the primary cause of otitis externa or claw disease without other signs. In most cases the skin appears normal or a little red to start but later it may develop red sores, pimples, dandruff, dark areas or thickened skin as the skin becomes traumatised, chronically inflamed or develops secondary infections with yeast (Malassezia) or bacteria (Staphylococcus intermedius). They may also have a history of frequent defaecation, easily upset stomachs or colitis.
How is Food Hypersensitivity Diagnosed?
Food hypersensitivity can only be diagnosed by completing an elimination diet trial and challenge. Although blood tests are available they are not very specific and are therefore only useful in selecting components for an elimination diet trial.
How Do I Perform an Elimination Diet Trial?
An elimination diet trial is conducted by feeding a diet containing a very limited range of potential allergens. This may either be a home cooked diet or a commercial diet where the proteins have been broken down to the point where they can no longer produce allergic reactions (Hills Z/D ultra).
If a home cooked diet is chosen it should contain a single protein source (usually meat) from one species of animal and a carbohydrate source (usually a grain or starchy vegetable).
This option should only be used for a period of under 2 months and not be used for growing or pregnant animals as it not nutritionally complete.
The alternative is to use one of the commercially available limited protein diets or Hill’s Z/D ultra. Ideally the primary components of these foods should not have previously been a part of your pet’s diet.
The diet should be introduced slowly over 1 week to reduce the risk of digestive upsets. The diet should then be continued for at least 6 weeks. During this time flavoured medications should be avoided as they may contain enough allergens to trigger food allergies even if fed only once each month. For ongoing medications such as heartworm prevention or arthritis treatments, the vet will discuss non-flavoured option, as missing out could be harmful to your pet. It is also important that any parasite infestations or infections are controlled before the diet trial is complete as pruritus resulting from these conditions can mask any response to the food trial.
Most pets that have skin disease resulting from food hypersensitivity show a noticeable reduction in pruritus by 6 weeks but if the owner is unsure the diet can be continued for a further 2 weeks.
NB. It may take up to 12 weeks for the pruritus caused by food hypersensitivity to resolve completely.
The most important phase of the elimination diet trial is the dietary challenge. During the challenge phase no treatment should be altered otherwise it is difficult to be certain that any change is diet related. To challenge a diet as many different components of your pet’s previous diet are fed daily for at least 2 weeks. In the majority of cases the pruritus increases within 2 days of reintroducing the offending food although delays of up to 2 weeks are reported. If the pruritus recurs between 3 and 14 days the elimination diet should be reintroduced and challenged again to confirm the diagnosis of food hypersensitivity. The time taken to cause the pruritus to recur should be noted.
If the pruritus does not return within 2 weeks we can rule out food hypersensitivity.
Identifying the Allergen
The next phase is to identify the specific dietary components that cause the food hypersensitivity. To do this the elimination diet is reintroduced until the pruritus settles down again and then a nutritionally balanced, long term diet is introduced. If this does not cause the pruritus to recur it can be fed long term. Individual food items are then sequentially introduced to the diet at intervals longer than the time taken for the pruritus to recur when the diet was initially challenged. I usually start with meats (beef, lamb, chicken, pork, turkey) followed by dairy (cheese), soy (tofu or soy milk), then wheat (flour), corn (cornflour) and any other components identified on the ingredients list of the pets previous foods. If a component causes the problem to recur it should be noted and removed from the diet. Once the pruritus settles down again the remaining potential foods can be tested until all foods that cause problems are identified. Normal commercial diets can then be checked and those that do not contain the offending food items can be tested to determine which are suitable as long term diets.
From VetDerm SA, by Dr Andrew Carter BVSc (hons), CertVD, MRCVS.