Scabies: Molecular Perspectives and Therapeutic Implications in the Face of Emerging Drug Resistance

Kate E. Mounsey; Deborah C. Holt; James McCarthy; Bart J. Currie; Shelley F. Walton

Disclosures

Future Microbiol. 2008;3(1):57-66. 

In This Article

Ivermectin: A Magic Bullet?

The discovery of ivermectin represented a major advance in parasite control for both veterinary and human medicine. More than 400 million doses of ivermectin have been administered in programs aimed at control of onchocerciasis and other filarial diseases.[13] Ivermectin is the only acaricide that can be administered orally, making it convenient for use in institutional settings and for crusted scabies, where topical application is difficult and may not adequately penetrate the thick crusts (Figure 2). The drug has been recently approved in France, the Netherlands and Mexico for the treatment of scabies. Ivermectin has also been approved on compassionate grounds for the treatment of crusted scabies, particularly in northern Australia.

Figure 2.

Crusted scabies.

It was initially envisaged that treatment with single-dose ivermectin would replace topical creams entirely, greatly simplifying treatment of scabies.[14,15] However, this has not yet occurred for a number of reasons, including uncertainty regarding the optimal number of doses, the optimal interval between doses and drug concentration. In the few randomized-controlled trials conducted using ivermectin, there has been substantial heterogeneity in study methodology, making its clinical efficacy difficult to evaluate.[16] Recently, Lawrence et al. reported the success of ivermectin mass treatment of scabies in the Solomon Islands.[17] All residents received a single dose, with children under 15 kg and pregnant women treated with permethrin. Scabies prevalence rates dropped from 25% to less than 1%, and remained low for many months after the intervention. These results suggest that ivermectin may hold promise as a tool for community-based treatment of scabies, although its higher cost when compared with topical therapies may limit its applications in some regions.

Alberici et al. compared ivermectin and benzyl benzoate in HIV-associated crusted scabies.[18] The investigators found that neither drug was effective when used in isolation and that combination therapy was the best option. Similarly, our experience with crusted scabies patients in northern Australia strongly supports combination treatment, including multiple doses of ivermectin and topical acaricides.[19,20,21] Despite such comprehensive treatment regimens, failures have been reported (see later).

Owing to the possibility that the blood-brain barrier is incompletely developed in infants, raising the potential for ivermectin neurotoxicity, the drug is currently contraindicated in children under 15 kg and in pregnant and lactating women.[22] Despite these concerns, ivermectin has been used in these groups with no adverse effects,[23] but owing to a lack of comprehensive safety data, this barrier remains. Before ivermectin can be employed as an acaricide on a widespread scale, particularly in a mass treatment strategy, its safety in young children needs to be more rigorously assessed.

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