"Bug Busting" & Malathion

Study flawed by use of ineffective comb

Lancet (Volume 356, August 12, 2000) publishes a questionable article comparing the use of a UK comb called the “bug busting” comb to shampooing with malathion. This report claims that malathion is the better choice of treatment when compared to combing.  Be aware:

• Not all combs are created equal.  The study offers nothing on the efficacy of the comb that they used, which is already established to be an inferior comb for removing lice and nits.
• Investigators chose malathion in spite of the fact that lice resistance to malathion in the UK is well documented.
• The Rachel Carson Council advises against malathion as a frontline treatment for kids with head lice.

A reply to this study was published in the December 9th, 2000 issue (Volume 356, Number 9246):

Sir--When publishing scientific articles it is customary to cite the pertinent literature. Contrary to what R J Roberts and colleagues state¹ there are two published reports comparing combing with no treatment, in which infection rates did not differ significantly.^2,3 They do not mention our reports on the existence of malathion-resistant head lice in the UK⁴ and changes in the prevalence of head lice after combing.⁵

Roberts and colleagues confirm the presence of malathion-resistant head lice in Wales, which corroborates our findings in the Bristol and Bath area. We have confirmed malathion resistance in six other towns and cities in the UK, and have seen the beginning of resistance to carbaryl (unpublished data). Roberts and colleagues state that they tested for resistance and found it. If there is resistance, what does the comparison of combing method with an insecticide mean in pragmatic terms?

One of the malathion preparations used contained isopropyl alcohol as a solvent, which rapidly kills head lice on contact (unpublished data). Since not all children were cured, it is important to know whether the failure rates for the treatments differed. Was malathion active on any children or were the other constituents the ingredients that killed the lice? The two treatment groups cannot be compared because of reinfestation bias. Roberts and colleagues provide no data on treatment of adults.

That all infestations were treated in children cannot be confirmed, since infestation is a fluid event and not all children met the inclusion criteria. Roberts and colleagues have not compensated for the fact that malathion has a persistent residual effect but that combing does not. Finally, factors that affect combing such as large families, long hair, curly hair, symptoms, louse load, age, hair styling products, tea-tree oil, shampoos, and conditioners were not apparently accounted for in either study group.

We also question the trial on ethical grounds. In our study on insecticide resistance in head lice, we used a small group of children to confirm that the in-vitro test results were predictive of what happens on a child's head.⁴ Once established, we judge use of pyrethroids and malathion in trials on children to be unethical when in-vitro tests show resistance. Since Roberts and colleagues state that previous workers have shown an intermediate degree of resistance to malathion, why did they treat large numbers of children with an organophosphate? We question whether products that are supposed to kill head lice and to which there is widespread resistance should continue to be sold for use on children.

*A M R Downs, K A Stafford, G C Coles

1. Roberts RJ, Casey D, Morgan DA, Petrovic M. Comparison of wet combing with malathion for treatment of head lice in the UK: a pragmatic randomised controlled trial.  Lancet 2000; 356: 540-44.
2. Sutton M. Does combing foil head lice? Prof Care Mother Child 1991; 1: 69.
3. Monheit BM, Norris MM. Is combing the answer to head lice?  J School Health  1986; 56: 158-59.
4. Downs AMR, Stafford KA, Harvey I, Coles GC. Evidence for double resistance to permethrin and malathion in head lice.  Br J Dermatol  1999; 141: 508-11.
5. Downs AMR, Stafford KA, Stewart N, Coles GC. Factors influencing the transmission of head lice in British school children.  Pediatr Dermatol  2000; 17: 72-74.

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