Demodicosis (Proceedings)

Demodicosis is one of the most common skin diseases of dogs. It is rare in cats. Among dogs examined at North American veterinary teaching hospitals, prevalence rates of 0.38% and 0.58% have been reported. Among free-roaming dogs in Korea and Mexico, prevalence rates of 5% and 23% are described.

Demodex canis is by far the most common of the canine species. A long-bodied species, D. injai, and a short-bodied species, D. cornei (unofficial name) may also be identified. In cats, the two clinical variants correspond with two mite species. Immunosuppressed cats may develop alopecia associated with D. cati infestation. D. gatoi is associated with overgrooming and pruritus.

The suspected risk factors for canine demodicosis vary with the age of onset and the extent of lesions. Though uniform criteria are lacking, demodicosis is generally considered localized when five or fewer lesions are present. This form occurs in young dogs and is usually self-limiting. Dogs with numerous lesions, two or more feet involved, or large regions of affected skin are classified as having either juvenile-onset, generalized demodicosis (JOGD) (developing before 18 months of age), or adult-onset, generalized demodicosis (usually occurring in dogs older than 4 years). Classification into one of these three groups aids in providing a prognosis and making recommendations for patient management. Multiple risk factors have been suspected to contribute to the occurrence of JOGD, including internal parasitism, estrus status, poor nutrition, debilitating disease, recent anesthetic procedures, geographic location, and short hair length. Few studies have critically evaluated these observations. "Bad" and "regular" body conditions were found to be risk factors in one study, and regional differences were reported in another.

Genetic risk factors have received more attention. Although many breeds are reportedly predisposed to demodicosis, reports are often anecdotal, based on a relatively small number of patients, conflicting, and/or reported without a confidence interval which would allow the reader to judge the strength of the conclusion. Old English Sheepdogs were found to have a relative risk of 28.9, but this was derived from only three dogs with demodicosis. Lemarie and others found the Shar Pei, Lhasa Apso, and Rottweiler breeds to have an increased relative risk of developing JOGD in a retrospective study of demodicosis over a six year period at a veterinary teaching hospital.

The diagnosis of demodicosis is generally straightforward. Skin scrapings, exudate cytology, or trichograms are usually sufficient to make a diagnosis. In Shar pei dogs, or in scarring pododermatitis, a punch biopsy may be required to identify mites. D. gatoi is more difficult to demonstrate, thus a therapeutic trial may be necessary.

Treatment of localized demodicosis is not recommended, beyond topical antibacterial therapy. Numerous studies have evaluated various treatment protocols for generalized demodicosis. Many studies fail to report long-term follow up, making comparison difficult. The response rate to topical amitraz (dips or collar) ranges from 0-100% in these studies. When a 12-month follow-up period is reported, the combined relapse rate after presumptive cure is 11%.

Ivermectin and milbemycin have been widely used off label for the treatment of canine demodicosis. Combining results of multiple studies yields a response rate of approximately 70%, similar to that seen for amitraz therapy. Oral ivermectin therapy is much less expensive, but must be used with caution. Collies and Australian shepherds breeds are among the most common ivermectin-sensitive breeds. At the doses commonly used for demodicosis, milbemycin is a safer choice.

Two relatively recent additions to the list of possible treatments for generalized canine demodicosis are spot-on products: Promeris® and Advantage Multi®. Results have been mixed, but they may prove useful for the management and prevention of mild to moderate cases of generalized demodicosis. Advantage Multi® is labeled for the treatment of demodicosis in some foreign countries, while Promeris® received approval for the treatment of canine demodicosis in the United States in 2009. For each of these products, long-term follow up was not reported. Approximately 50-60% of dogs were mite free at the end of monthly treatment (3-4 doses). Advantage Multi® was found to be "non-inferior" to milbemycin in a European trial. In practice, these products are often administered every 2 weeks.