Chapter 43 Canine and Feline Demodicosis
DEMODEX MITES
• Every animal harbors its own host-specific species of Demodex mite. Demodex mites, in low numbers, are normal inhabitants of the stratum corneum, hair follicles, and sebaceous glands.
• Demodicosis occurs commonly in dogs and less commonly in cats. The feline mite resides more superficially on the skin than the canine mite.
• The entire life cycle (25–30 days) is completed on the host. Mites cannot survive off the host for more than a couple of hours.
• Four stages of the life cycle may be demonstrated in skin scrapings: eggs, larvae, nymphs, and adults.
CANINE DEMODICOSIS
Etiology
• Although Demodex canis is part of the normal fauna of canine skin and is the most common mite found in cases of demodicosis, two other Demodex mites have recently been identified. One is a short-tailed Demodex mite (unnamed as yet) and the other is a long-bodied Demodex mite (Demodex injai).
• The mites are present primarily in hair follicles and rarely in sebaceous glands, although the short-tailed mite resides on the skin surface.
Pathogenesis
• Affected animals probably have a mite-induced immunosuppressive disorder of T cells of varying severity in which the immunosuppression is proportional to the number of mites present.
• The tendency to develop juvenile-onset, generalized demodicosis is familial. Adult-onset and localized demodicosis are not. Although the mode of inheritance is not known with certainty, an autosomal recessive mode is suspected.
• Purebred dogs have a higher incidence of demodicosis, with the Shar-Pei, West Highland white terrier, Boston terrier, and English bulldog being among the most commonly affected breeds.
Clinical Signs
Juvenile Onset
• Onset in animals less than 2 years of age is considered juvenile onset in most breeds. Onset in animals greater than 2 years of age is considered adult onset.
Adult Onset
• Adult-onset demodicosis invariably results from immunosuppression. Approximately 50% of adult-onset demodicosis cases result from exogenous or endogenous glucocorticoids (iatrogenic or spontaneous hypercortisolism).
Localized
• Localized demodicosis is classified as less than approximately six lesions or only one body region affected. The periocular and perioral regions are most commonly affected and pruritus may be present.
• Nearly 90% of localized demodicosis cases will spontaneously resolve in young animals, while 10% will become generalized regardless of treatment.
Diagnosis
• Deep skin scraping is the preferred means to diagnose demodicosis. If performed properly, mites should be present on skin scrapings in nearly all affected animals.
• If skin scrapings cannot be performed due to location (eyelid, etc.) or discomfort, perform a trichogram by plucking hairs in the affected areas and examining them microscopically in the same manner as skin scrapings. This technique, however, can yield false-negative results in mildly affected dogs.
Skin Scraping Technique
• Apply a cover slip to the slide and examine microscopically under the low power objective (4×, 10×) (Fig. 43-1).
• Obtain scrapings from a minimum of four or five locations, including lesions, lip folds, and interdigital regions.
• Scrapes should be positive if the animal has demodicosis and the correct technique is employed. Exceptions are the Shar-Pei or dogs with chronic fibrotic lesions (especially of the interdigital skin) that may require biopsy.
• Quantify the number of eggs, juveniles, and adult mites from each location to determine the severity of the disease and for later evaluation of response to treatment.
• A single mite on skin scrapings may be normal, but perform multiple follow-up skin scrapings to rule out demodicosis.
Stay updated, free articles. Join our Telegram channel
Full access? Get Clinical Tree
