Sarcoptic Mange

Sarcoptes scabiei is a parasitic mite that burrows into the skin of animals and man causing a disease condition known as scabies or sarcoptic mange. The mites of this species, that may be found on different hosts, are usually regarded as being a variety of the species S. scabiei that are physiologically adapted to one or more usual hosts, but can live, temporarily at least, on other unusual hosts. Adult mites penetrate initially the skin of a new host by attaching to the skin with the suckers of the two first pair of legs and then cutting the skin with the helicera and the cutting-hooks of the last segment of the first two pairs of legs. The mite usually burrows below the horny layer of the skin (Stratum corneum).

The burrows contain male and female mites, their feces, their eggs, hatched larvae and nymphs. Eggs laid in the burrow hatch in 3-5 days. Six days later the six-legged larva molt and become an eight-legged nymph and the latter molt to adult female or male mites. Two stages of females are known, a first pubescent female that moult two days later to a mature (ovigerous) female. The whole life history from egg to mature female takes about two weeks. All developmental stages of S. scabiei, except the egg, create burrows in the skin.

Red papules and vesicles appear on the skin that are followed by appearance of crusts formed of dry lymph. The the skin becomes thickened and wrinkled. The hair that are deprived of blood supply fall out creating bald patches. Secondary bacterial infection makes the condition worse and when large areas of skin are affected progressive emaciation and eventually death occur.

Treatment of Sarcoptic Mange

S. scabiei is considered to be a frequently transmitted zoonotic parasite. In one study, forty two dogs with naturally acquired infestation of S. scabiei obtained from commercial dog kennels where an outbreak followed an inadvertent introduction of infested dogs were treated on day 0 and day 30 with selamectin at the recommended dose and or with an inert vehicle mixture.

Counts of S.scabiei from skin scrapings were performed every 14-15 days up to 60 days after treatment. S.scabiei mites were reduced 93.5 % on the first count after treatment and by 100% for all remaining counts. There was a clear reduction of the severity of the clinical signs for the selamectin treated dogs compared with those treated with the vehicle mixture. When a dog is diagnosed with S. scabiei infestation it is recommended that all dogs in contact also be treated irrespective of whether they are showing clinical signs.

Selamectin is a semi-synthetic modification of doramectin produced from a bio-engineered new strain of Streptomyces avermitilis. Selamectin is absorbed rapidly after topical application and achieves a peak plasma concentration three days after treatment. Following topical application, selamectin is absorbed into the bloodstream and some of the compound is excreted into the intestinal tract. Substantial amounts of circulating selamectin are deposited in the sebaceous glands, which then act as reservoirs to provide persistent activity against various ectoparasitic infections.

References:
1. Recent development in the Control of Ectoparasites and Endoparasites of Dogs and cats with Selamectin, E. Pipano.