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Head lice deliberately removed from their hosts ceased movement in less than 55 h (mean 21.3±12.1) (29), or 35±1.7h at 18^oC or 24±1.8h at 26^oC(47). Many lice are nonviable and are unable to feed as a result of dehydration long before they stop moving or even walking (13). This means that a louse accidentally transferred to a pillow in the morning would unlikely be viable when the host goes to bed again that night. Furthermore, Chunge et al. (29) found no lice or viable eggs on brushes and combs, and eggs deliberately removed failed to hatch at room temperatures (fluctuating between 20^o and 30^oC). Experimentally, at a “high room temperature” of 26^o–27^oC, viability of louse eggs is reduced so that less than 50% of either head or body louse eggs hatched in 9–17 days at 50% relative humidity (47, 51). Consequently, the risk of transmission of infestation by displaced lice or louse eggs is epidemiologically insignificant compared with the risks of lice transferring from one person to another during physical contact, a conclusion drawn from studies conducted in the United States more than 20 years ago (45a).

Insecticide Resistance

Most insecticides employed against human lice have been used in consumer products for decades. Although it was long recognized that resistance to most insecticides would develop eventually, public health authorities and the pharmaceutical industry have been slow to respond to resistance, as now it is a reality.

CLINICAL EVALUATION
Pyrethroid insecticides are the most widely used for control of head louse infestation, with products based on natural pyrethrum, permethrin, d-phenothrin, bioallethrin, or tetramethrin. One clinical study has addressed the problem of insecticide resistance coupled with in vitro and ex vivo laboratory tests. Following reports of failed treatments in France, a comparison was made of single applications of 0.5% malathion lotion or 0.3% d-phenothrin lotion (28). In the clinical phase of the study 87 of 95 (92%) participants treated with malathion were louse-free one day after treatment, but this increased to 91 of 95 (95%) by day 7. In contrast, of those treated with d-phenothrin only 39 of 98 (40%) participants had no lice one day after treatment, decreasing to 38 of 98 (39%) by day 7. Five of the children in the malathion group and 27 in the d-phenothrin group changed their status from louse-free to infested, or vice versa, between days 1 and 7 without further  intervention from the investigators. This suggests that examinations so soon after treatment provide relatively little useful information on treatment success and also that, despite admonitions to the contrary, some parental caregivers must have engaged in some additional intervention such as nit-combing to remove lice. Coupled with the clinical evaluation, lice removed from some children were tested for sensitivity to the two preparations ex vivo by a modified form of a WHO method for evaluation of insecticide susceptibility (28, 103). All lice exposed to the malathion product were killed in 60 min, whereas 82 of 416 (19.7%) of those exposed to d-phenothrin were still alive after 24 h, 6% more than in the untreated control group, in which 44 of 320 survived.