Solutions for challenging canine demodicosis cases

Solutions for challenging canine demodicosis cases

May 01, 2009

Photo 1. An adult Demodex canis mite (40X). (PHOTOS: COURTESY OF KATHY TATER, DVM)
Demodicosis is a common skin condition in dogs. Dogs have three recognized species of Demodex mites. Clinically, the most common mite is Demodex canis (photo 1). This mite is limited to the hair follicle and sometimes the sebaceous gland. Dogs can also develop demodicosis due to the large-bodied Demodex mite or D. injai (photo 2). This mite usually causes a dorsal, seborrheic dermatitis. The third species of canine Demodex is the short-bodied D. cornei. Its clinical signs are similar to those of D. canis.

Photo 2. An adult Demodex injai mite (40X).
Treating dogs with demodicosis can be challenging for both the veterinarian and the client. This article discusses some of the common reasons why demodicosis can be so challenging and offers solutions for how to meet the challenges head-on.

CHALLENGE: If demodicosis isn't on your radar, you may forget to check for demodicosis and miss the diagnosis.

SOLUTION: Know the common clinical signs of canine demodicosis so you never miss a case!

Canine demodicosis can be diagnosed with simple equipment. Most cases of demodicosis can be diagnosed with hair plucks or skin scrapes using mineral oil, a scalpel blade, a hemostat, glass slides, and a light microscope. Skin biopsies and histopathology are sometimes necessary to diagnose demodicosis, but this occurs more commonly in dogs with thickened or scarred skin.

Photo 3. Follicular casting on the muzzle of a dog with demodicosis.
Since everyone has the tools to diagnose demodicosis, why do veterinarians sometimes miss demodicosis cases? On rare occasions, the veterinarian's scraping or plucking technique is at fault. It is important to sample multiple lesion sites, scrape until capillary bleeding is evident, and obtain hair plucks in addition to skin scrapes, especially when the sites are already hemorrhagic and difficult to scrape. More commonly, I find that a veterinarian misses demodicosis cases because he or she didn't have demodicosis on the differential list.

Photo 4. Follicular casts after the hairs have been plucked from the skin.
Remember that canine demodicosis can present with a variety of clinical signs. The primary lesions of canine demodicosis include spontaneous alopecia, scaling, follicular casts [keratosebaceous material adhered to the hair shaft (photo 3 and photo 4)], papules, and comedones. Comedones are sometimes misidentified by veterinarians as non-specific, post-inflammatory hyperpigmentation unless a thorough dermatologic examination under good lighting is performed (photo 5). Other lesions of demodicosis include crusts, erythema, hyperpigmentation, and lichenification. Demodicosis can occur with or without pruritus. It is usually more pruritic if concurrent secondary infections are present. Demodicosis due to the short-bodied form of the mite (D. cornei) is often pruritic.

Photo 5. Comedones on a dog with demodicosis.
If you think demodicosis is a differential diagnosis for a particular patient, check for the mite. It is rarely wrong to perform skin scrapes and hair plucks in a patient presenting with dermatologic lesions because these tests are inexpensive and not invasive, and missing demodicosis could result in other, more expensive diagnostic tests.