Five Questions for a Dermatologist

From atopy to food allergy, we’ve compiled the answers to the top 5 questions you’ve asked about dermatology.

1. What test do you use to diagnose atopy: blood analysis or intradermal skin testing?

Atopic dermatitis is a clinical diagnosis based on ruling out parasites (Demodex species, fleas, ticks, lice, and mites), pruritic infections (bacterial and yeast overgrowth), flea-allergy dermatitis, and food allergy. Intradermal skin testing and in vitro blood allergy tests reflect exposure to allergens but do not answer the question, “Is my patient atopic?” In my practice, these tests are reserved for patients in which atopic dermatitis has been diagnosed and the owners want to pursue immunotherapy.

I generally start by performing a second-generation E-Screen (heska.com), a mini-screening serum allergy test. In addition to positive and negative controls, the test has 3 wells, 1 each for trees, weeds, and indoor allergens. If there is a positive reaction on this screening test, serum is submitted for a full screening. If the test result is negative, an intradermal skin test is performed.

A study in dogs showed that if the result of the E-Screen test was positive, the probability of obtaining a positive result from an in vitro allergy test or intradermal test was > 90%.¹ In a recent study in cats, there was strong agreement between a positive E-screen result and a positive finding on in vitro allergy  testing (88%).²

2. How long is the steroid withdrawal period before intradermal skin testing, and how do you manage a patient’s pruritus during this time?

The exact time required varies depending on how long the patient has been receiving glucocorticoid medication and the type of medication (oral or repositol). In general, most dermatologists recommend no less than 4 to 6 weeks, and possibly longer. This recommendation can be very frustrating to clients, but the time can be used productively to ensure that the clinical diagnosis is correct. During this period, it is important to make sure that the patient is receiving flea control.

The medical record should be reviewed to ensure that scabies has been ruled out as a cause of pruritus; most scabies treatment protocols span 6 weeks. It is also very important to make sure that the pet does not have bacterial and yeast overgrowth. This condition may not be immediately obvious when steroid therapy is stopped because the antiinflammatory effects of steroids mask clinical signs. If the pruritus markedly increases, the pet should be examined; more often than not, bacterial and yeast overgrowth are present. Four-week treatment is recommended. If active and widespread pyoderma are present, it may not be possible to perform an intradermal skin test due to the risk for false-positive reactions.

Cyclosporine modified, 5 mg/kg orally Q 24 H, can be used to manage a pet’s pruritus before intradermal skin testing. A recent study has shown that cyclo­sporine modified does not interfere with intradermal skin test reactions and can be continued until the day of testing.³ Maximum benefit from this drug may take up to 30 days, and owners should be warned that the most common side effects are vomiting and diarrhea. Antihistamines can be used until 7 days before the test, but I have not found that these provide adequate relief.

Regular grooming and bathing are important and can be very soothing to the skin. I like to use Douxo shampoo and spray products (sogevalus.com).

3. When should a food trial be done?

Food trials are indicated in pets with nonseasonal pruritus. In addition, food trials are not first-line diagnostic tests. They are indicated only after parasitic and infectious causes of pruritus have been ruled out:

• Repeated skin scrapings consistently negative for Demodex mites

• Flea control measures in use

• Mite infestations ruled out by a response-to-treatment trial

• No bacterial and yeast overgrowth.

If a food trial is indicated, it is always best to make sure the client is able and willing. The client must commit to a strict trial that will last at least 4 to 8 weeks, and possibly longer. “Strict” means no table food, dog treats, raw hides, flavored chew toys, or vitamins.

• Visits to “doggie day care” or dog parks may need to be suspended to ensure strict adherence to the diet.

• If there are multiple dogs in the house, it may be best and easiest to feed all dogs the same diet. If cats are in the house, determine whether the owners can keep the dog out of the cat food.

• Determine whether the owners can ensure that small children, grandparents, or guests will not give the dog treats. If major holidays are in the near future, the diet trial may need to be suspended until after this time.

If there is a well-defined winter in the practice area, I prefer to do food trials during this time so that changing pollen seasons can be avoided.

4. What type of food do you recommend for food trials?

With regard to the diet itself, this is a difficult question; opinions in the dermatology community vary. Some clinicians strongly favor home-cooked diets because they provide control over the ingredients. Other clinicians prefer commercial diets because adherence is directly related to ease of preparation.

An easy answer to provide is that just changing from one diet to another is not a diet trial, and that no conclusive evidence supports the use of blood analysis or intradermal skin testing to identify food allergies. My approach is to ensure that the test is appropriate for the patient and that the client is able and willing; then I offer the client the option of a home-cooked or commercial diet.

There are 2 major types of commercial diets: hydrolyzed and novel protein:

• Hydrolyzed diets consist of protein sources subjected to chemical or enzymatic digestion, resulting in small protein fragments that are believed to be too small to trigger an immunologic reaction. Evidence suggests that hydrolysate diets are best used in dogs not suspected of being hypersensitive to their ingredients.⁴

• Novel protein diets consist of ingredients the pet does not eat in its normal diet. Novel protein diets are “novel” only if the pet has not been fed the protein previously.

The bottom line: there is no one best diet, and diet trials need to be individualized.

5. What is the diagnostic approach to a cat with symmetric alopecia?

Symmetric alopecia of cats is a clinical syndrome and not a specific disease. The major diagnostic challenge is to determine whether the condition is due to an underlying inflammatory cause or is psycho-genic. In my experience, psychogenic alopecia is uncommon.

History

Clinical history is important; unfortunately, in many cases the problem has been long-standing and clients do not remember a clear timeline of events. I find it very helpful to give clients a preprinted questionnaire and have them complete it at home with input from all family members. Often, someone remembers some key information. Try to determine from the history whether the alopecia responds to corticosteroids; if so, this supports the presence of an underlying allergy.

Physical Examination

In addition, a careful physical examination should be performed to make sure that overgrooming is not related to pain. Cats with bladder stones or fractures will overgroom as a response to pain.

Parasites

In general, I first rule out parasites as a cause of the pruritus. Skin scrapings and flea combing are part of the core diagnostics, along with fungal culture. Flea control is instituted for all animals for at least 3 months.

Demodicosis

If there is no response to flea control, empirical treatment for feline demodicosis (D gatoi) should be pursued because symmetric alopecia is increasingly being noted as a manifestation of this disease. I prefer lime sulfur rinses once weekly because this treatment also kills other mites in the hair. Unlike those in dogs, feline Demodex mites are not easy to find.

Bacteria & Yeast Overgrowth

If there is no response to treatment for demodicosis, I assess response to antimicrobial therapy with a combined regimen of antibiotics and itraconazole. Bacterial and yeast overgrowth in cats are underdiagnosed in my experience; these are invariably the result of some underlying trigger.

Allergies

Next, I pursue food and environmental allergies. Symmetric alopecia may be a manifestation of food allergy in cats. If no cause can be found, I perform trial therapy with steroids or cyclosporine. If the cat responds to one or both of these drugs, then the cause is allergic.