Claude Favrot,     Zurich, Switzerland

Differential Diagnosis: Exclusion of Similar Diseases

Exclusion of similar diseases is a mandatory first step, because many are readily manageable and straightforward to rule out. Practitioners should first exclude ectoparasitic infections, especially sarcoptic mange, flea infestation, and possibly cheyletiellosis and chigger bites.

Because the regional localization (face, ears, elbows, hocks) of pruritus and lesions in sarcoptic mange resembles those in canine AD, this disease is an important rule out. Numerous superficial skin scrapings should be examined. However, the sensitivity of skin scraping analysis is low, and “diagnostic treatment” (see Chapter 98) or IgG serologic testing (where available) should be performed if the clinical presentation is compatible with scabies and the superficial skin scrapings are negative for the mites.

Fleabite hypersensitivity usually affects the caudal dorsum and tail base initially; these are atypical sites for AD. However, many dogs also have lesions and pruritus of the abdomen, groin, medial thigh, and perineum, which can mimic AD. Thus fleabite hypersensitivity also should be ruled out in all dogs during the AD workup using trials of parasiticidal agents (see Chapter 97).

Bacterial and Malassezia (yeast) infections often develop secondary to canine AD, and it is critical that these disorders be identified and treated (see Chapters 100 to 102 and Web Chapter 44). Additionally, microbial hypersensitivity may play a role in some dogs with AD, in which significant clinical improvement may be anticipated with treatment of the infection.

Currently food allergy is considered a triggering factor for canine AD. Thus, in dogs with the typical clinical presentation of AD triggered by food allergens, the food allergy is regarded as a causal agent of AD, not as a separate entity. The role of diet should always be assessed in dogs with suspected AD that have a nonseasonal history of clinical signs (see Chapter 96). However, cutaneous adverse food reactions can be immune mediated or non–immune mediated and associated with a wide range of clinical signs (e.g., vomiting, diarrhea, urticaria)—signs that are dissimilar from those of typical AD.

In rare cases diseases such as epitheliotropic lymphoma or sebaceous adenitis demonstrate clinical signs with pruritus mimicking that of AD. If these diseases are suspected, skin biopsy for histopathologic analysis (a procedure rarely helpful in the diagnosis of AD) is indicated.