Diagnosing canine atopic dermatitis

Photo: Yulia Usikova/Adobe Stock

Julia E. Miller, DVM, DACVD, an assistant clinical professor at the Cornell University College of Veterinary Medicine in Ithaca, New York, discussed the proper approach when presented with a pruritic dog in the clinic, during her 2-part session, “The Itchy and Scratchy Show: How to Confidently Diagnose Canine Allergies,” presented at the Fetch Coastal dvm360 conference in Atlantic City, New Jersey. In her lecture, Miller focused on cases of canine atopic dermatitis (cAD), including likely causes, and proper diagnostic procedure.

cAD is multifactorial, and, as emphasized by Miller, an especially complex diagnosis. Types 1 and 4 of the condition show hypersensitivity. Type 1 indicates that the condition is immediate and involves the lgE cell, and Type 4 indicates a delay, involving T-cells (CD4+, CD8+). There are various triggers, and numerous mediators released, including histamine, leukotrienes, prostaglandins, proteases, cytokines, among others.

In the inflammatory syndrome, skin is the main route of exposure. Part of its complexity, the condition is the result of an inherited predisposition to develop hypersensitivities to environmental allergens that do not induce a problem in nonallergic individuals. Common allergens include dust mites, pollens, dander, insect particles, and mold.

Before a dog should be called atopic, they should undergo a thorough history-taking, a detailed dermatological physical exam (PE), and all other differentials, or “itchy conditions,” should be ruled out, according to Miller. These alternative conditions include ectoparasites, flea allergy dermatitis (FAD), neoplasia, and secondary microbial infections—including Staphylococcal folliculitis and Malassezia dermatitis.

"When you're diagnosing allergic dermatitis, [it's] always a diagnosis of exclusion," Miller explained. "That means you must do a workup before you get to the point that you call a dog atopic."

Miller emphasized the importance of history-taking in making a proper diagnosis. Although, questions need to be specific to dermatologic factors that may be in play. “Your traditional history-taking questions that you ask of any other patient do not apply to derm patients,” Miller said during her presentation. “[Dermatologic-specific] history-taking makes all the difference.” A comprehensive background should include breed, age at onset, seasonality, pruritic/pruritis level, whether or not other pets or humans are also itchy, whether the itch or rash presented first, response to treatments, what flea preventative the dog is on, gastrointestinal signs, diet, and whether it is endocrinopathy-associated.

Age can be a significant indicator in the diagnoses of cAD. Miller shared that, if a patient began itching between the ages of 1 year and 4 years, that should trigger a red flag for cAD. Exceptions, however, include breeds with predispositions, like French bulldogs, bully breeds, GSD, wheatens, and more.

"I will tell you this, in the world of doodles... in the world of pit bulls... in the world of [French bulldogs], some of these atopic dogs are starting at about 6 months of age," Miller said. "They've kind of lowered the threshold."

When diagnosing cAD, seasonality is trivial factor. cAD can be seasonal, nonseasonal, or nonseasonal with seasonal flares. In dogs with cAD, paws, axillae, abdomen, perianal skin, and face, are common areas where pruritus and secondary conditions are developed. Nearly all dogs with pruritus have atopic conditions.

According to Miller, most dogs with uncomplicated atopic dermatitis show a satisfactory response to 1 mg/kg/day of prednisone. If a dog is not responding to prednisone, ectoparasites, secondary Malassezia dermatitis, food allergy, epitheliotropic lymphoma, or a secondary bacterial infection should be considered. Alternatively, a new steroid may be required.

Reference

Miller J. The Itchy and Scratchy Show: How to Confidently Diagnose Canine Allergies. Presented at: Fetch dvm360 Conference; October 14-16, 2024; Atlantic City, NJ.