Resistance and cross-resistance to insecticides in human head lice from Florida and California

Head lice from Florida (SF-HL) and California (SC-HL) were resistant to permethrin compared with colonized susceptible lice from Panama (PA-HL) (5.5- and 3.4-fold, respectively) and Ecuador (EC-HL) (8.5- and 5.3-fold, respectively). Permethrin-resistant lice were cross-resistant to pyrethrum and DDT. DNA sequencing validated presence of kdr-type mutations (T929I and L932F). Permethrin resistance was synergized by piperonyl butoxide (PBO) in SC-HL. Resistance to malathion in SF-HL (1.4-2.2-fold) and SC-HL (2.1-3.6-fold) was detected. Malathion resistance in SF-HL was synergized by S,S,S-tributylphosphorotrithioate (DEF) and by PBO in SC-HL. Malathion/permethrin-resistant lice from the UK (BR-HL) were synergized by DEF but not synerziged by PBO. PBO protected BR-HL from malathion, indicating suppressed desulfuration. Abamectin resistance in SF-HL (1.7-2.5-fold) and SC-HL (1.8-2.3-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. (C) 2004 Elsevier Inc. All rights reserved.