An improved in vitro rearing system, based on a silicone-reinforced Parafilm® M membrane, human hair tufts and reconstituted human blood, enabled the large-scale rearing of pediculicide-susceptible (EC-HL) and resistant (FL-HL and BR-HL) strains of human head lice. Developmental time differed for early instars but differences became synchronized as lice matured. Mean survivorship amongst the three strains reared in vitro were not significantly different when compared to EC-HL and FL-HL reared in vivo. FL-HL and CA-HL were resistant to permethrin compared with susceptible PAHL (5.5- and 3.4-fold, respectively) and EC-HL (8.5- and 5.3-fold, respectively). Permethrin-resistant lice were cross-resistant to pyrethrum and DDT. Permethrin resistance was synergized by piperonyl butoxide (PBO) in CA-HL. Resistance to malathion in FL-HL (1.4- to 2.2-fold) and CA-HL (2.1- to 3.6-fold) was detected. Malathion resistance in FL-HL was synergized by S.S.S-tributylphosphorotrithioate (DEF) and by PBO in CA-HL. Malathion/permethrin-resistant BR-HL lice were synergized by DEF but not synergized by PBO. PBO protected BR-HL from malathion, indicating suppressed desulfuration. Abamectin resistance in FL-HL (1.7- to 2.5-fold) and CA-HL (1.8- to 2.3-fold) was detected. No resistance to lindane was found. Thus, multiple resistance mechanisms against commonly available and widely used pediculicides and insecticides are apparently occurring. Results from linkage analysis substantiate that permethrin resistance in human head louse populations in the U.S. is widespread but occurs at varying frequencies. The log survival time versus logit mortality regression lines of susceptible-homozygotes, resistant-homozygotes and heterozygotes determined that the resistance trait was complete recessive. Permethrin resistance is highly correlated with the presence of the T917I and L920F point mutations, which are suitable for detection by a variety of DNA-based diagnostic techniques. Electrophysiological assessments determined that the three mutations (M815I, T917I and L920F) associated permethrin- and DDT-resistance in head louse caused depolarizing shift of ∼3.2 and ∼8.8 mV in the voltage dependent activation and steadystate inactivation, respectively, and completely abolished permethrin sensitivity when correspondingly inserted into house fly para-orthologous, voltage sensitive sodium channel and expressed in Xenopus oocytes (M827I, T929I and L932F). Amongst these kdr mutations, T929I appears to be the major resistance-causing mutation
