Canine allergic dermatitis: Pathogenesis, clinical signs, and diagnosis - Veterinary Medicine
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Canine allergic dermatitis: Pathogenesis, clinical signs, and diagnosis


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Table 4. Willemse Criteria for Diagnosis of Atopic Dermatitis
Diagnostic approach. Atopic dermatitis is diagnosed based on exclusion of other diseases and characteristic clinical features. The patient must have a compatible history, the clinical signs must fit the pattern described above, and other causes of pruritus must be ruled out. Criteria for diagnosing atopic dermatitis in dogs have been proposed (Table 4);10 however, at no time should these guidelines supersede the clinician's judgment. In my practice, pruritus is the most common owner complaint, and most causes of pruritus fall into one of three categories: parasitic, infectious, and allergic. Again, some patients have more than one concurrent problem (e.g., flea infestation and secondary bacterial pyoderma).


Table 5. Diagnostic Plan for Pruritus
The keys to diagnosis are client education and a careful and methodical approach to exclusion of other diseases. Clients are more willing to participate in the process when they have a clear understanding that dermatologic problems are solved more by response to specific therapy trials than by a single diagnostic test. They also want to be confident that the clinician has a diagnostic plan. I find it helpful to give clients a checklist that describes the diagnostic process because it illustrates that the diagnosis is not obvious in most cases (Table 5). This checklist becomes part of the medical record and serves as a quick review of the patient's status in the diagnostic process. It also makes it clear that intradermal or serum IgE testing or food trials are not first line diagnostic tests for most patients.

Indications for intradermal skin testing or serum IgE testing. Intradermal testing or in vitro testing (serum IgE testing) does not answer the question, "Does this patient have atopic dermatitis?" Because these tests only reflect allergen exposure, a positive test result must be compatible with a patient's clinical signs if it is to be considered meaningful. In my opinion, intradermal or serum IgE testing is indicated only in patients with a clinical diagnosis of atopic dermatitis for which immunotherapy is being considered. These patients include:

  • dogs that have had clinical signs for a total of six months or more during the year
  • dogs with atopic dermatitis of any duration whose pruritus cannot be managed medically
  • dogs that cannot tolerate glucocorticoid therapy or dogs with conditions in which glucocorticoid therapy is contraindicated (e.g., diabetes mellitus)
  • working aid dogs (these dogs cannot be distracted from their tasks by pruritus or compromised by drugs such as antihistamines or glucocorticoids that can change behavior or have unacceptable adverse effects).

Identifying allergens to avoid is one reason for performing intradermal or serum IgE testing. Although causative allergens may be identified, in practice, few allergens can be avoided; environmental allergens are small and can travel on air currents for miles. At best, clients may be able to remove wool rugs or feathers from their home or install a dehumidifier to control molds. It is my experience that dogs with clinical signs severe enough to be considered for allergy testing almost always need polypharmacy for maximum relief of pruritus and a good quality of life.

Before performing allergy testing, be sure that owners understand that it may take up to one year to see a maximum benefit from immunotherapy, that they therefore should be prepared to commit to at least one year of trial treatment, and that if successful, immunotherapy is usually a lifelong, year-round therapy. Owners should also be informed before testing that immunotherapy alone may not relieve 100% of their pet's pruritus and that many dogs still need some type of pharmacologic support to achieve maximum clinical sign remission.


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