Studies in vitro on the relative efficacy of current acaricides for Sarcoptes scabiei var. hominis

Resistance of Sarcoptes scabiei to various topical therapies has been descr ibed, but clinical assessment of treatment failure is problematic and in-vi tro assays are generally not available. We describe a simple in-vitro analy sis used to evaluate the relative efficacy of a range of topical, oral, and herbal treatments available in Australia for the treatment of scabies. S. scabiei var, hominis mites were collected from skin scrapings obtained from 7 crusted scabies patients over a period of 2 years (1997 and 1998). Larva e, nymphal instars, and adult mites were tested within 3 h of collection an d continuously exposed to selected commercially available treatment product s until death, with the elapsed time recorded. Neem was the only product to show little acaricidal activity. Survival curves indicated that, of the ot her agents, 5% permethrin (Lyclear) had the slowest killing time, with 35% of mites still alive after 3 h, and 4% still alive after 18-22 h of constan t exposure. In contrast, no mites were alive after 3 h exposure to 25% benz yl benzoate (Ascabiol), 1% lindane (Quellada), 5% tea tree oil and 100-8000 ng/g of ivermectin (Equimec). Despite the slower killing time with 5% perm ethrin, there was no evidence of any mite tolerance in vivo or treatment fa ilure in any patients or contact cases.