BOSTON - Although louse susceptibility to pyrethroid pediculicides remains uncertain in the United States, these non-toxic, over-the-counter treatments should remain the treatment of choice for most lice infestations, researchers said. This recommendation resulted from a recent study which found that lice from chronically infected American children previously treated with a pediculicide were not affected by permethrin.
In cases of lice survival after pyrethroid treatment (topical medications containing synthetic permethrin-containing products or synergized pyrethrum extracts), the researchers discouraged repeated or more concentrated applications, since resistant lice will remain resistant with the second treatment regardless of dose. Furthermore, if pyrethroid treatments are not effective in eliminating an infestation, prescription treatments containing lindane or malathion should be considered. However, these treatments should be applied conservatively, due to their potential toxicity. In the last decade, pyrethroid treatments replaced lindane as the treatment of choice against head lice, due to their efficacy and lack of toxicity to the patient.
In the study published recently in the Archives of Pediatric and Adolescent Medicine permethrin susceptibility of head lice taken from children in the United States, where pediculicides are readily available, and Sabah, Malaysia, where pediculicides are not available. The data were compared by researchers from the Harvard School of Public Health here, Idaho Division of Health, Boise, and the Sabah Department of Health, Kota Kinabalu, Sabah, Malaysia. Lice survival was tested after exposure to varying amounts of permethrin-containing pediculicides in an in vitro bioassay. The permethrim-containing treatments immobilized lice sampled from Malaysia.
The researchers pointed out that the results of this study should be considered cautiously, since the results could have been biased toward unsusceptible lice. Many of the infested U.S. children sampled were repeatedly or chronically infested and practically all of them had been previously treated for head lice. Few susceptible lice would remain in the infestations that comprised the sample used in the study.
Questionnaire's distributed to the care givers of the infested children in the study revealed that head lice is perceived as a heavy social burden in the United States and is treated aggressively with pyrethroid-containing pediculicides. A synthetic permethrin-containing product (Nix, Warner-Lambert Co.) was the pediculicide used most frequently, comprising a third of all treatments used. Pyrethrin-based alternatives comprised 17% RID (Pfizer), 4% A-200 (Hogil Pharmaceutical Corp.) and 4% Pronto (Del Laboratories).
Researchers investigated three pharmacies in Sabah and found that none of them displayed shampoos containing pyrethroid products. The investigators also found that none of the pharmacies sold lindane or malathion, the main pediculicidal alternatives to pyrethroid treatment. Also, two traditional Sabahan herbalists said their customers had no need for pediculicides. The researchers concluded that the Sabahan people associated no burden with lice infestation.
School nurses collected lice from 75 American school children 5 to 8 years of age in Massachusetts (48 lice) and Idaho (215 lice). Nurses from the regional department of health collected lice from 59 Sabahan boys age 6 to 13 years, in a residential community and elementary schools in Kota Kinabalu, Sabah, and Telupid, Sabah (239 lice).
The researchers used a graded assortment of permethrin-impregnated filter paper disks for evaluating the susceptibility of head lice to permethrin residues. Lice that appeared undamaged from collection were transferred to filter papers within minutes of gently combing them from children's hair and were randomly distributed onto filter disks. Some lice were deposited onto non-treated disks. The lice were monitored after specified intervals. Mobile lice were considered alive, while non-moving lice were considered dead.
The questionnaires completed by care givers of the infested children included information regarding each child's prior experience with lice infestations; lice infestations of other members of the household; and pediculicidal treatments that may have been previously applied.
Virtually all of the U.S. school children had previously been treated with pyrethroid products as opposed to the Sabahan children.
For more information:
- Pollack RJ, Kiszewski A, Armstrong P, et al. Differential permethrin susceptibility of head lice sampled in the United States and Borneo. Arch Pediatr Adolesc Med 1999;153:969-73.
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