Pediculicide Resistance Not Well Understood - Lancet Editorial

The mechanism by which head lice become resistant to pediculicides needs further study, Howard Maibach, MD, University of California at San Francisco, et al., suggest in the Aug. 12 issue of The Lancet.

Commenting on a study published in the same issue comparing the efficacy of wet combing versus the Rx drug malathion in eliminating head lice, Maibach et al. suggest recent concern about growing resistance to pediculicide drugs such as permethrin (Pfizer/Warner-Lambert's Nix) could be misplaced.

Researchers must determine what factors have contributed to the apparent resistance of some lice to pediculicide drugs and what steps can be taken to alleviate the problem, the authors state.

If the reason for resistance is, for example, a "residual concentration of the [drug] in the hair [that] may... expose the parasites to a sublethal dose to which they may have become tolerant," a new vehicle should be developed "to enhance drug delivery and adherence to the hair and skin," the editorial proposes.

Attention must be directed to determine whether combining drugs produces synergistic effects, what the relationship is between in vitro and in vivo data on resistance, the modes of behavior and infectivity of the lice themselves and the effect of poor compliance on efficacy and resistance, Maibach et al. assert.

Knowing the causes and effects of resistance is preferable to eliminating treatment options and could lead to the end of lice infestation, the researchers say.

In the U.K., where the study was conducted, pediculicide use has been discouraged since 1996 when the Department of Health began to advocate wet combing as a first-line treatment. In the U.S., a Harvard study published last year reinforced the use of OTC permethrin to treat head lice despite concerns about resistance (¹ (Also see "OTC Permethrin For Lice Still Endorsed Despite Latest Resistance Research" - HBW Insight, 20 Sep, 1999.)).

Richard Roberts, North Wales Health Authority, et al., concludes British "policies advocating [wet combing] as first-line treatment for head lice in the general population are inappropriate" based on the findings of their randomized, controlled trial of 72 children in two Welsh counties.

Among 32 children in the wet combing testing arm, 12 (38%) were cured after two weeks compared to 31 of 40 (78%) using malathion. Parents in the non-drug arm were instructed to mechanically remove lice with a wet comb - a process referred to as "bug-busting" - every three to four days, while parents in the malathion group applied a .5% lotion twice during the two-week period.

The researchers point out malathion - introduced as Medicis' Rx pediculicide Ovide to the U.S. market last year - outperformed wet combing despite an "intermediate resistance" to the drug in the regions where testing took place. Roberts et al. also concede compliance only reached about 50% during the trial, highlighting the need for future study.

Rid marketer Bayer - which recently acquired the brand from Pfizer - has said resistance of lice to its pyrethrum product and other pediculicides can be blamed on improper usage by consumers (² (Also see "Bayer Rid Pediculicide Marketing Emphasis On Mousse Launch" - HBW Insight, 26 Jun, 2000.)).

A growing body of evidence suggesting increased resistance to various drug remedies has influenced thinking among some in the scientific community and in government agencies. Last year, FDA refused OTC status for SmithKline Beecham's Denavir (penciclovir) cold sore cream, in part out of fear of increased viral resistance.

OTC antimicrobial products also have been the subject of similar debate. The American Medical Association has urged restraint in the application of antimicrobials such as triclosan and chlorhexidine (³ (Also see "Antimicrobial Consumer Product Resistance Concerns Voiced By AMA" - HBW Insight, 19 Jun, 2000.)). In addition to expressing worries about microbial resistance, AMA says evidence does not support the efficacy of these agents.

FDA, the Centers for Disease Control & Prevention and National Institutes of Health jointly issued the "Public Health Action Plan to Combat Antimicrobial Resistance" in June (⁴ (Also see "OTC Antimicrobial Resistance Study Subject Of FDA, NIH, CDC "Action Plan"" - HBW Insight, 26 Jun, 2000.)). Under the plan, the agencies will conduct a risk/benefit analysis on the inclusion of antimicrobial ingredients in consumer products.

However, the American Society for Microbiology criticized the plan Aug. 1, maintaining it "says little about appropriate support levels or plans for funding and expanding scientific infrastructures." ASM called for "an external review of federal research agency efforts in this field."