Canine and feline demodicosis - Veterinary Medicine
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Canine and feline demodicosis
You know demodectic mange frequently occurs in dogs. Now it appears to be more common in cats than previously thought, and new species have been discovered. Find out the latest on diagnosing and treating this frustrating skin disease.


VETERINARY MEDICINE


Diagnosis

Canine demodicosis is usually diagnosed by identifying mites in skin scrapings and hair plucks; however, other techniques for obtaining mites may be used.


Figure 7. Using a scalpel blade to obtain skin scrape samples to diagnose demodicosis.
Skin scrapings. Skin scrapings can be performed with a No. 10 scalpel blade (Figure 7) or with the blunt end of a metal spatula (Figure 8). To minimize the risk of a patient being injured during sampling, especially when performed by inexperienced team members, dull the scalpel blade first on a hard surface or use a metal spatula instead of a blade.


Figure 8. Using a metal spatula to obtain skin scrape samples to diagnose demodicosis.
Obtain samples from lesional skin. The skin can be squeezed before or during the scrapings to promote extrusion of Demodex mites from the hair follicles. The skin or the scraping instrument can be wetted with mineral oil to better collect the sample. In a long- or medium-haired dog, gently clipping the area to be scraped will minimize the loss of the scraped material into the surrounding hair. Skin scrapings to identify follicular Demodex species mites should be deep enough to result in capillary bleeding.

Transfer the scraped material to a slide, and place a coverslip to enhance sample examination and avoid contamination of the microscope objective with mineral oil. To make the mites easier to find, lower the microscope condenser and decrease the light on the microscope to increase the contrast in the microscope field. Evaluating the scraped material with the 4X or 10X microscope objectives is sufficient.

Evaluate skin scrapings for the approximate number of mites. Look for the presence and proportion of immature vs. adult mites. With an effective treatment plan, a decreasing proportion of immature mite forms may be the first sign of improvement. The presence of any mites—even dead mites or mite fragments—is considered a positive result.


Figure 9. Using the hair pluck technique to obtain samples to diagnose demodicosis.
Hair plucks. Hair plucks can be performed with mosquito hemostat forceps to grasp and pull hairs (Figure 9). If alopecic lesions are being sampled, collect hairs from the lesion's margin. You may squeeze the skin before or during the hair plucks to promote mite extrusion from the follicles. Place the plucked hairs on a glass slide with mineral oil, and add a coverslip. Sample multiple sites in each patient.


Figure 10. A dog with demodectic pododermatitis. The diagnosis of demodicosis was missed on deep skin scrapes by the referring veterinarian because of the difficulty in scraping the ulcerated paws. Mites were found through hair plucks upon referral.
Hair plucks may be less sensitive in diagnosing canine demodicosis than skin scrapings are,7 but they are useful for areas of the skin that are difficult to scrape, such as the face and paws. Hair plucks can also be helpful if the sample area is already showing marked hemorrhage from ulceration or draining tracts. In these situations, it may be difficult to determine how deep to scrape, and it may be uncomfortable for the patient to have skin scrapings performed while awake (Figure 10).

Exudative samples. In cases of canine demodicosis with concurrent deep pyoderma, direct examination of the exudate from pustules or fistulous draining tracts may also reveal mites. Collect samples by squeezing the exudate onto a glass slide, and add mineral oil and a coverslip.

Acetate tape preparations. Acetate tape can be used to detect the superficial canine mite D. cornei. Repeatedly press a piece of acetate tape to the skin and fur, and then lay the tape flat on a glass slide for microscopic examination. No mineral oil or staining is needed, but staining will not interfere with visualization of the mite. Lower the condenser on the microscope and decrease the light to increase the contrast in the microscope field. Evaluate the entire tape sample by using 4X or 10X microscope objectives.

Otic swabs. Demodectic otitis externa is best detected by collecting samples from the ears with cotton swabs. Roll swabs from each ear onto a glass slide, add mineral oil, and evaluate the slide after placing a coverslip.

General considerations. Although Demodex mites are part of the normal cutaneous flora in dogs, it is rare to find mites unless there is an overgrowth. If low numbers of Demodex mites are found on a skin scraping or hair pluck, sample additional sites. Finding large numbers of mites or immature forms of the mite at other sites supports a diagnosis of demodicosis.


Figure 11. Canine demodicosis on histologic examination. Multiple mites are present in the hair follicle (hematoxylin-eosin; 20X).
Occasionally, an overgrowth of Demodex mites may be missed even with properly performed skin scrapings and hair plucks, especially in dogs with thickened or fibrotic skin due to chronic dermatitis or because of their breed (i.e. Shar-Peis, bulldogs). If demodicosis is strongly suspected but no mites are found on skin scrapings and hair plucks, skin biopsy is recommended to evaluate for the presence of mites histologically (Figure 11).


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