DEMODEX DIAGNOSIS AND TREATMENT

Several species of Demodex occur on dogs and cats. The classical demodectic mange of dogs is caused by the follicular mite Demodex canis. The features, diagnosis, and treatment of conditions caused by D. canis and other Demodex species are described in Table 1.

INTERDIGITAL FOLLICULAR CYSTS

Interdigital follicular cysts develop in traumatized hair follicles, causing swelling of the follicular ostia. The most common cause of follicular cysts is excessive weight on the feet and conformational changes. The affected follicles occur on the ventral surface of the feet in the palmar (most common) and plantar regions. Trauma causes the follicles to dilate and fill with keratinocytes. The follicles rupture and drain dorsally, most often between P4 and P5, releasing their contents into the surrounding dermis and provoking an inflammatory reaction.

Close examination of the ventral foot reveals lichenification and comedones, from which keratinaceous debris can be expressed. The presence of follicular cysts can be confirmed by skin biopsy and histopathologic examination. Cytologic examination may help identify active infection or ongoing furunculosis, such as eosinophilic to pyogranulomatous inflammation. Even though staphylococci are usually found, bacterial culture should be performed to guide antibiotic selection. The lesion should be packed with mupirocin and antibiotics given for 3 to 4 weeks. Two negative cultures may indicate presence of a sterile pyogranuloma.

Laser ablation is the treatment of choice for follicular cysts, but it is a complex procedure that requires training to perform properly. The cyst may also be treated with surgical debridement. Other therapies include immunomodulating agents, shampoo therapy and residual topical rinses, and topical benzoyl peroxide shampoo or gels for early or mild lesions. Glucocorticoids are indicated for some allergic conditions and sterile pyogranulomas. Fusion podoplasty is successful in some cases. Finally, counseling the owners about weight control is critical for ensuring that the problem does not recur.

SKIN BIOPSY AND BACTERIAL CULTURE

The key to obtaining an adequate biopsy is to take a sample of any abnormal skin areas. Although histopathologic examination may not always provide a definitive diagnosis, it can help with the differential diagnosis. According to Dr. Noxon, local anesthesia is adequate for most skin biopsies, “depending on the demeanor of the patient.” General anesthesia is recommended for biopsy of the pinnae, foot pads or distal extremities, nasal planum or facial region, and periocular skin.

Indications for culture include recurring pyoderma, failure to respond to standard therapy, and unexpected cytology results. The sample should be taken from a pustule, intact lesion, under the crust of an epidermal collarette, or tissue biopsy. The lesion should not be scrubbed before sampling. Draining tracts should not be directly sampled. Sterile technique should be used, avoiding contamination from skin or hair near the lesion. The collected material should be placed in a sterile culture container and sealed for shipping to the laboratory.