THEY are the unwanted visitors that every mother dreads. As a nation, we spend £30 million every year on lotions and potions to rid our children of head lice.

But the little critters must be mocking our efforts for they have defeated most of what the chemist can throw at them.

Public health doctors in Wales, who had the unenviable task of combing 4,000 lice from the hair of primary schoolchildren, discovered that 80 per cent were resistant to the insecticides commonly used to eradicate them.

Tomorrow is National Bug Busting Day, an event organised by the charity Community Hygiene Concern. But anyone wanting to do some bug busting of their own may have little success with common over-the-counter treatments based on the chemicals permethrin and phenothrin.

A team led by Daniel Thomas, of the National Public Health Service for Wales Communicable Disease Surveillance Centre, found that four out of five lice were resistant to these chemicals, found in well-known brands such as Lyclear and Full Marks.

Eliminating them has become big business. Yet the number of diagnosed cases has continued to rise, largely because the creatures have become increasingly immune to common treatments.

Eight per cent of the Welsh children tested had lice, and the researchers said this was probably typical of Britain.

The traditional comfort, that lice like clean hair, may not even be true. Some experts suggest that this is simply something that nurses have told parents to comfort them, and that the bugs are as likely to set up home in a dirty mop.

There is no escaping the strength of the new enemy. Where insects are exposed to poisons, as lice have been for many years, resistance follows. Insects either evolve methods for detoxifying the poison before it reaches its target in their bodies, or they become less sensitive to the poison even if it does hit the target.

The process is natural selection in action. Where random variation in the insect genes confers resistance on a single creature, that creature increases its chance of survival and of having offspring sharing the same gene.

Pretty soon, a resistant population emerges, as seems to be the case in Dr Thomas’s study. He and his team report in Archives of Disease in Childhood that relatively few lice, only about 5 per cent, had a double copy of the resistance gene for the pyrethroids. Each gene comes in two copies, but having both copies — though it makes the insect impervious to the poison — seems to disadvantage it in other ways. So relatively few insects carry both copies.

This hardly matters, since a single copy makes them to all intents and purposes resistant, and fully 77 per cent of the lice had one copy. This means that more than 80 per cent of the lice tested were resistant to pyrethroid insecticides.

All this has Dr Thomas and his team scratching their heads. Permethrin and phenothrin are the most popular chemical treatments for lice, primarily because they require relatively little contact time with the hair and smell less unpleasant than the alternatives.

Other solutions include the organophosphate malathion, found in the brand Prioderm, or a new product, dimeticone lotion, which has recently been licensed and has proved as effective and as safe as phenothrin.

Where resistance rates are as high as in Wales, one or other of these may be a suitable alternative, the researchers say. There are no data yet to show which works best. But a study published in the British Medical Journal last year suggests that the traditional approach might be more effective than all-out chemical warfare.

Scientists at the London School of Hygiene and Tropical Medicine studied 126 young people, and found that Community Hygiene Concern’s “bug buster kit”, containing nothing more high-tech than four specially designed combs, was more effective than chemical treatments.

HOW TO KEEP A CLEAR HEAD

Neem oil Products using neem oil are increasingly popular in beating headlice. Many parts of the neem tree have antimicrobial properties, which provide ingredients for traditional and modern toothpastes, medicines, cosmetics and insect repellents in South Asia

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