They're back. Holidays are over, school is starting... and the nits are gathering.
After a summer of relative inactivity, head lice are about to swoop on a new generation of clean, healthy heads and scalps lining up in playgrounds and classrooms across the country.
The flightless louse may be only the size of the full stop at the end of this sentence, and it may not carry disease or cause permanent health problems, and it may take a mere 0.0001579 ml of a blood in each bite, but for parents, teachers and children, it's an embarrassing, unpleasant, itchy infestation that needs instant treatment.
More than £35m a year is now spent on creams, sprays, shampoos and other lotions, potions and devices designed to zap the head biters, and much of that will be spent during the next month.
But there are so many treatments, from pesticides and other chemicals, to herbs and olive oil, that finding one which works can be a major challenge. Scores of products are on the market that claim to kill them, but not all work as well as others, and some may not work at all. Others may have side effects. Evidence is also growing that in many areas of the UK and beyond, the bugs are developing a resistance to some of the main chemical weapons used against them.
One of the ugliest of all insects, the head louse looks like a flattened, miniature grey or brown ant with six legs, including two extremely large ones that have evolved over time to help the creature grab onto hairs as it moves around the scalp, eating three times a day and laying eggs. These eggs, "nits", are usually stuck firmly to hair shafts close to the scalp behind the ears and at the back of the neck.
Research shows that the average infected scalp has 10 nits, that most victims are aged four to 11, and that girls are more likely to be affected than boys, probably because they are more likely to put their heads together when they are playing or working. It is this coming together of heads that allows the louse to colonise new victims, although they can be caught from using infected hats, combs, brushes, and scarves.
Lice are not a major health problem - the only real symptom is they make the head itchy - but they are a significant issue largely because of reactions to them. Health authorities in some countries, including America, Canada and Australia now have strict zero tolerance strategies where any child found to be infested is sent home and not allowed back until they have been deloused.
Such strategies can create problems. Research at the Medical College of Ohio shows that in America alone, up to eight million children are treated unnecessarily for head lice every year, more than half of the total number treated. It shows too that as many as 24 million school days are lost annually, and the cost to the economy of working parents who stay home with their children is put at £2billion.
"The policy also results in serious psychological problems for children and their parents. The 'no nit' policy should be abandoned and alternative ways of examination and treatment for head lice should be found," say the researchers.
This widespread use of treatments may also have contributed to head lice developing an immunity to a number of chemicals. Some of the most common treatments are chemical insecticides like pyrethrin, but resistance has been reported from the UK and France. Resistance is also being increasing found to other chemicals, and a Cardiff University reports suggests that in some areas of the UK resistance to permethrin and malathion may be as high as 87 per cent.
The dilemma for parents is that while lice may not be a big health problem, they do not go away by themselves. Some researchers, including a team from Harvard University, argue that treatment should be tailored to meet the threat.
"Lice should not be considered a public health problem," says Harvard's Dr Richard Pollack. The greatest harm associated with head lice is from the well-intentioned but misguided use of caustic or toxic substances to eliminate the lice. A few lice on the head should not cause alarm; rather, they present an opportunity for parents to spend the needed time with their children in order to find and remove the offending insects."
The problem with such an approach, usually involving combing, is that while it can work, it is time consuming, and less suitable for large scale problems.There isn't enough research into whether combing gets rid of head lice. "It's hard to say if it works. Success seems to depend on how well you carry out this treatment. Your GP may recommend combing if head lice in your area are resistant to chemical treatments," advises the British Medical Journal's Best Treatments guide.
There's not much comfort in the guide for par ents who have switched to alternative remedies: "Herbal treatments and aromatherapy products haven't been properly tested, so we can't say whether they work."
Nor is there much positive news for users of oils: "Some people also put herbal oils, such as citronella and tea tree oil, in their children's hair to repel head lice. We don't know if they can help. The smell of the spray might keep head lice from climbing onto your hair but we're not certain."
For the vast majority of parents, the priority is to zap the insects as soon and as quickly as possible, and for years chemicals like permethrin and malathion have provided the main treatments.
But some of these treatments may no longer work. "Malathion works well against head lice. It may work better than other treatments because it kills the lice eggs too. But malathion might not work if head lice have become resistant to it in the area where you live," says the BMJ guide.
The good news for parents, and bad news for nits, is that there is a new treatment around that seems to work. Dimethicone, sold as Hedrin, is not an insecticide and works by coating the lice, stopping them from getting the supply of water they need to stay alive. Research at Insect Research and Development in Surrey shows that seven out of 10 people who used dimethicone no longer had head lice.
Because it is not an insecticide, it's unlikely the insects will evolve a resistance to it. There are hopes that it could be the best new treatment yet, but sceptics say it is unlikely to be the end of the road for the head louse. It's been around as long as man, and it's unlikely to give up its three free meals a day without some kind of fight.
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Head lice live for about three weeks.
Females lay up to six nits a day, which hatch out a week later. This is why two treatments are usually needed to take out the eggs as well as the lice.
There is no evidence that lice are a sign of bad personal hygiene. In fact there is some suggestion that they prefer eating, living and breeding in clean hair.
Researchers have calculated that the most heavily infected child they found - 2,657 lice - could be expected to lose 0.7ml of blood a day.
Don't confuse the head louse (Pediculus humanus capitis) with the body louse (Pediculus humanus corporis). Both of these unwelcome visitors keep strictly to their respective sides of the anatomical frontier: the human neck.
Clothing needs to be washed in excess of 50C to kill head lice and nits.
Research in Toronto claims that a home remedy of equal amounts white vinegar, 20 drops of tea tree oil and any edible oil works. The creamy mixture is put onto damp hair, massaged in and combed.
Home-spun failures: research at the University of California, Davis shows that mayonnaise doesn't kill lice and nor do vinegar, alcohol, olive oil, butter, petroleum jelly or six hours of water immersion.
A new shampoo based on neem is highly effective against lice, according to a study at the Federal University of Ceara in Brazil. All but one louse was dead after three hours.
Chick-chack works too, according to the Hebrew University Medical School in Israel. It contains coconut oil, anise oil and ylang ylang oil. Applied every five days,a success rate of 92.3 per cent was found in a trial.Reuse content