Food allergy vs. atopy (Proceedings)
The most common hypersensitivities affecting dogs and cats are: flea bite hypersensitivity, food adverse reaction (food allergy & food intolerance), and atopy.
The most common hypersensitivities affecting dogs and cats are: flea bite hypersensitivity, food adverse reaction (food allergy & food intolerance), and atopy. Typical symptoms in all three conditions are: variable degree of pruritus, erythema, lichenification, seborrhea and lesions, such as alopecia and excoriations, resulting from chronic inflammation and excessive scratching or grooming. Hypersensitivity to fleas can occasionally be differentiated from the other two condition based on the distribution of the skin problem (caudal dorsum, tail base, and hind legs). Based on historical and clinical features, food adverse reaction and atopy are usually indistinguishable, especially if they are presented with nonseasonal clinical signs. Typical features common in both conditions include: young age at onset of disease, pruritus of the ears, axillae, inguinal area and distal limbs. Recurrent otitis and secondary pyoderma and Malassezia dermatitis are not uncommon as well. It is even possible that both conditions are closely related because it is suspected that food adverse reactions may predispose a dog to develop cutaneous atopy. So, it would not be a surprise if a pet is suffering from food adverse reaction and atopy at the same time, and have been estimated to occur in up to 30% of allergic patients. Atopy and food hypersensitivity (a subset of food adverse reaction) are mainly driven by an IgE-mediated hypersensitivity, however, for the vast majority of dogs with food induced skin reactions, an immunological mechanism could not be established so far. This may be a possible explanation why allergy testing for food ingredients is not reliable and anti-inflammatory therapy (e.g. prednisone) does not work in some cases.
Work-up of allergies
The first step in working up a patient with pruritic skin and a potential underlying hypersensitivity is to rule out ectoparasites (e.g. fleas, Demodex, Sarcoptes, Cheyletiella, lice, ear mites) and bacterial (e.g. Staph. pseudointermedius) and fungal (e.g. Malassezia and dermatophytes) skin infections. In older dogs pruritus caused by recurrent skin infections underlying systemic diseases such as hypothyroidism or Hyperadrenocorticism should be considered and eventually tested. Occasionally skin biopsies may be necessary (e.g. to rule epitheliotropic lymphoma). In very pruritic patients a trial treatment for scabies should be performed because they may interfere with allergy testing causing false positive reactions to dust mite allergens. Once infections with Demodex or dermatophytes have been ruled out, a short trial with an anti-inflammatory dose of e.g. prednisone may also help to determine whether the problem is steroid responsive, as seen typically in atopic patients, but not always in pets with food adverse reaction and certain infections.
Most pets present with a non-seasonal clinical presentation of their problem might either be "food allergic" or suffer from atopy to allergens which are present in the pet's environment all year round, such as dust mites. A food trial should always be your next diagnostic step before considering allergy testing. In seasonally allergic dogs, strict flea prevention and allergy testing (intradermal skin testing preferred over serology) is strongly indicated.
Hypoallergenic diet trials
Because no reliable diagnostic tests are available a strict 8-12 week food trial with a hypoallergenic diet, to which the patient has not been exposed to before, is the only way to diagnose and manage food adverse reactions. Proteins such as beef, dairy products, fish (cats) and chicken are known to be the major sources of food adverse reactions in pets and should be avoided for elimination diet trials. Many different hypoallergenic diets in various formulations are commercially available. These diets come with either a novel protein (e.g. kangaroo, venison, and rabbit) or hydrolyzed protein (e.g. soy, chicken). As carbohydrate source most diet use potato or rice. Corn should be avoided because reports indicate that it potentially can cause food adverse reaction as well. These diets come either as dry or canned food. Canned food might be more effective, because dry formulations usually contains more by-products (e.g. preservatives and stabilizer), but the higher cost of the canned formulation usually prohibits the use in large breed dogs. The third group of elimination diets, which is still believed to be the most effective elimination diet among veterinary dermatologist, is a home-cooked diet containing one protein source (e.g. venison, pork, fish) and one carbohydrate source (e.g. regular or sweet potato).
The main benefit of a home cooked diet is that you know exactly what is added to the diet and additives and by-products are eliminated. Home-cooked diets are usually more expensive and time-consuming to prepare and so as well rather limited to smaller dogs and cats. They are also not recommended in young fast-growing dogs, due to incorrect calcium-phosphorus ratios, unless you consult with a nutritionist and balance out the home-cooked diet with minerals and vitamins as needed.
Food trials in cats are often difficult to perform due to their tendency to be picky eater. In such situation the owner should be given samples of different hypoallergenic diets to try at home, and to use the one the cat likes the most. It is very important not to force the owner into the diet of your choice. My strategy is to explain the pros and cons of each diet group and let the owner decide which diet they want to try. Involving the pet owner in the decision making will help to increase owner compliance. Owner compliance is crucial, especially in food allergic patients, because small amount of food containing the offending allergen can cause a lack of response or a relapse of clinical signs. That compliance is a major issue we have to work on, has been shown in studies where compliance rates for prescription diets in general has been as low as 30%. It is extremely important to make sure that during the diet trial no other food, flavored drugs (e.g. beef flavored heartworm preventatives; gelatin capsule of drugs or essential fatty acid capsules), treats, flavored toys, bones, raw hides are given to the patient, and that the patient is fed separately from other pets in the household getting regular diets.
Symptomatic therapy is needed for most patients during the first couple of weeks, but must be discontinued 2 weeks prior to the completion of the diet trial. This will allow assessing the response of the new diet without the interference of other drugs. In case the patient should relapse or not show any improvement on the hypoallergenic diet, you may consider either another elimination diet trial (e.g. switch from novel protein to hydrolyzed, dry to canned food, or hydrolyzed to home-cooked) or perform allergy skin/serum testing to rule hypersensitivities to environmental allergens. Because none of the diets available are 100% effective another food trial is justified if the pet owner is cooperative and willing to try. It is very important to rule out food adverse reaction because it is usually easier to manage than atopy with desensitization.
Available prescriptions diets are:
- Hydrolyzed protein diets: Hill's z/d ultra (chicken); Purina HA; Royal Canin DR
#160; - Novel protein diets: Hill's d/d (venison, duck, rabbit, salmon), Royal Canin (dogs: duck, venison, whitefish, egg; cats: duck, rabbit, venison), Eukanuba (fish, kangaroo)
General recipe recommendations for home cooked diets should only be used very carefully, because they can lead to nutritional imbalances if used longterm. Crucial ingredients in a home cooked diet for the longterm use are: a protein source, linoleic acid rich vegetable oil, calcium and phosphorous supplement or source like bone, human adult multivitamin/multimineral or liver, salt, and taurine (cats). Specific recipes for a home-cooked diet can be obtained online through BalanceIT.com. Professional consultation is provided by board-certified veterinary nutritionists. Nutritional consultation is also available through petdiets.com.
Diagnosing canine atopic dermatitis
If the skin problem persists after completion of 1-2 different hypoallergenic diet trials a tentative diagnosis of atopic dermatitis is justified. At this point it would be reasonable to perform allergy skin testing to confirm your preliminary diagnosis and identify the offending allergen(s).
For allergy testing the pet should be off anti-inflammatory therapy (e.g. 10 days off of antihistamines, 3-4 weeks off of oral steroids). For allergy skin testing sedation is required and the need of clipping the coat in a large area on the left-lateral chest side should be mentioned to the owner. Allergy skin testing should be performed by a dermatologist. All test allergens are injected within 15 minutes and reactions at the injection sites are assessed 15 and 30 minutes after injection. A skin test reaction indicates that the skin is reactive, but it does not automatically means that the pet is allergic that to specific allergen. For this reason it is important that the findings from the allergy test are compared with the pet's history to determine which of them are potentially present in the pet's environment. Various allergy serum tests are also available, but should be used cautiously. Serum allergy is measuring the amount of allergen-specific IgE. However, more false-positive reactions can be seen with the serum test compared with the skin test, which is probably due to some degree of cross-reaction of non allergen-specific IgE.
Based on allergy test results desensitization ("allergy shots") can be compounded and used to increase the pet's immune-tolerance to the offending allergen over time. A response to desensitization may take 6-9 months. For this reason we strongly recommend to desensitize the pet for at least one year. In the meantime the animal will require symptomatic therapy to treat pruritus and recurring infections.
Keep in mind: We rarely cure allergies, but can often control them successfully!