This article was originally published in the
NPA's Newsletter Progress, Vol. 6 #1, 1990. History has shown us
that lice developing a resistance to permethrin was predictable. Resistance
(particularly when manufacturers continue to falsely state the effectiveness of
their products) wastes money, prolongs infestation duration and puts
children's health at risk with unnecessary over-treatment.
THE PROPHYLACTIC USE OF PEDICULICIDES
A Formula for Resistance Development
by Dr. John D. Edman, Medical Entomologist and
Dr. John M. Clark, Insecticide Toxicologist, University of Massachusetts at Amherst
The introduction of several new synthetic pyrethroid insecticides in the last decade brought to the marketplace, for the first time, safer and more effective treatments for head lice. These products (such as NIX) are also more ovicidal than their predecessors but still leave some eggs viable and thus require follow-up treatment to assure complete control. This procedure coupled with the physical removal of all nits (i.e., a NO NIT POLICY) provides a sound strategy for the management of head lice in schools.
These new products are so accessible to the general public that some public health care providers are now encouraging the treatment of all school children in the community - whether they have lice or not - as a strategy for preventing infestations before they occur. However
mesmerizing such a suggestion may sound, overuse should be avoided if this new class of pediculicides is to remain as an effective treatment for very long into the future.
The post WWII history of the development of drug resistant parasites and pathogens, pesticide resistant insects, nematodes, fungi and weeds, and anticoagulant-resistant rats, clearly warns against any unnecessary and widespread use of these chemicals (NAS 1986). Yet chemo-prevention strategies do just that! Such prophylactic use of chemicals (as opposed to their more restricted use in the treatment of existing infections or infestations) has been responsible for penicillin-resistant gonorrhea, chloroquine-resistant malaria,
DDVP-resistant fleas, anticoagulant-resistant rats, etc. Permanent neck collars and ear tags (for pets and livestock that are impregnated with slow release formulations of insecticides to provide long-term protection from fleas, ticks, flies, etc.) have provided classic examples of how prophylactic use strategies lead to the development of wide-spread resistance. Chemical and pharmaceutical companies do not have a particularly good track record of warning against this misuse and overuse of their products. Their policies tend to be driven by immediate sales. Thus, any policy that may limit or reduce the current years' sales (even if it would extend sales in the long run) is unlikely to be very popular with company executives who may not be around next year if sales start to lag.
The number of new pesticides entering the market has been steadily declining since 1970. At the same time the number of resistant species rose from < 10 in 1950 to >600 by the mid-80s (NAS 1986). It is therefore critical that we preserve the best chemicals in our arsenal for as long as possible. The pyrethroids are now widely used and over 32 species of insects were already resistant by 1984, and the list will no doubt keep growing. Resistance to pyrethroids has often evolved rapidly in the footsteps of DDT resistance because the semi-recessive gene, kdr, one of the components of DDT resistance, also provides protection against pyrethroid insecticides. Since DDT has been used much more in developing countries than in the U.S. (including use in louse control), the effective life span of the pyrethroids due to cross-resistance problems may be much shorter in many of these countries (NAS 1986).
There are few current field reports available on head lice. Nevertheless, minimal resistance to date would undoubtedly be related to the previous non-prophylactic use pattern of DDT, BHC (as
Lindane in Kwell), and now the pyrethroids, rather than to lice being any less prone to developing resistance than other insects. Body lice have developed resistance and, in laboratory selection tests, head lice developed resistance to DDT in just 12 generations (Eddy et al., in Brown & Pal 1971). If head lice populations are widely and repeatedly exposed to pyrethroids through chemo-prophylactic use strategies, there is little doubt that resistance will develop in a relatively short time and these valuable tools will be lost.
Some may argue that head lice do not have a sufficiently short life cycle and high reproductive rate to easily develop resistance. Current knowledge of the genetics and dynamics of resistance as well as the past record of resistance developing among organisms with greater dispersal characteristics and far lower reproductive potential than head lice, does not support this naive view. We can ill afford the consequences when such a poorly conceived experiment fails, so lets not conduct it!
Brown, A.W.A. and R. Pal. Insecticide Resistance in Arthropods. World Health Organization, Geneva (1971)
Tactics for Management, Nation Academy Press, Washington, ac. (1986)
© National Pediculosis Association's Newsletter Progress, Vol. 6 #1, 1990