Topical insecticides (i.e., pyrethroids and malathion) used as a lotion, applied twice at an interval of 7 to 11 days, are typically used for treatment. Resistance of lice to insecticides, particularly pyrethroids, results in treatment failure.
For difficult-to-treat head-lice infestation, oral ivermectin, given twice at a 7-day interval, had superior efficacy as compared with topical 0.5% malathion lotion, a finding that suggests that it could be an alternative treatment.
Why does head louse not infest body and vice versa?
Possible explanation: Body lice lay their eggs on clothing fibers instead of hair fibers (http://emedicine.medscape.com/article/1108991-overview).
Oral Ivermectin versus Malathion Lotion for Difficult-to-Treat Head Lice. NEJM, 2010.