WHY ARE WE FAILING TO CONTROL HEAD LICE WITH INSECTICIDES?

Abstract:
Infestation with head lice, Pediculosis capitis, is a widespread, persistent and recurring problem amongst pre-school and school aged children. The problem poses no direct threat to health which may explain why such a physiologically distressing condition affecting millions of young children every year receives little attention from researchers. Infestation continues to be associated with poor hygiene and poverty in the minds of the general public, but there is only limited evidence that the environment or social standing plays any part in transmission. In some areas, children found to have head lice are physically excluded from school, which leads to humiliation and needless loss of education. Head lice also place an increasing burden on an already over-stretched primary healthcare system in terms of both cost and manpower. In the UK, community nurses and Consultants in Community Disease Control (CCDC) expend up to 25 % of their total workload counselling families and teachers on head louse treatment issues, without any likelihood of reducing prevalence. In 1997, the UK bill for head lice treatments topped £ 26 million and more than 6 million bottles of insecticide are used on young children each year. To make matters worse, it is becoming increasingly difficult to control these pests. It is widely agreed that none of the existing pesticide treatments available are fully effective due to rapidly developing insecticide resistance. Treatment failure has led to an alarming increase in reports of pesticide abuse involving multiple applications over many weeks or months by frantic parents and carers who are faced with no realistic alternative. The treatments are designed for infrequent use and the health risks surrounding prolonged use of organophosphates and other chemicals these products contain remains unknown. The reasons for resistance developing so rapidly is not clear, although several theories are put forward. The misuse of products by those applying treatments is one likely cause (too little applied or washed off too soon), as is the advent of shampoo formulations (insufficient contact time) and failure to kill eggs (hatching nymphs contacting decaying residues). The mechanisms of resistance are still poorly understood. Our studies on head lice collected from the field across the UK have identified a range of biochemical mechanisms of resistance well documented in other insect species, including increased general esterases and altered-acetylcholineesterase. In addition, cross resistance to a range of pyrethroids in the absence of specific metabolic enzymes makes a kdr type mechanism likely.