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Column originally published Nov 4, 2008

It Is Not Necessary To Over-Treat Head Lice

Question: Last year, our two daughters had head lice. It was a nightmare for all of us. They were sent home repeatedly by the school nurse because she found nits in their hair. They missed so many days of school that they almost didn’t pass. I am terrified that it may happen again this year. I want to know whether there are ways that we can prevent this from happening again. I cleaned everything in our house many times over, and treated everyone with lotions and shampoos recommended by our doctor and pharmacist. We even treated our dogs in desperation.


Your story is not unusual at all, unfortunately. I have heard many other parents with similar experience. Head lice infestation is not a sign of poor hygiene, but it is often misdiagnosed and over-treated.

Head lice are small insects that measure 2-4 mm in length. They have six legs and no wings. They can crawl very fast: 23 cm (about 9 inches) in one minute, although they cannot hop or fly.

Children infested with head lice usually have no more than 10 to 20 lice on the head. These lice have to stay close to the scalp for food, warmth, shelter, and moisture. Every 3-6 hours, these lice have to feed by sucking blood from the scalp and injecting their saliva at the same time. Some children can become allergic to the saliva and develop itchiness of the scalp. However, the majority of children have no symptom of infection.

Each insect (also called louse) can live for 3-4 weeks. After mating, the female louse can lay 5-6 eggs a day for several weeks. Each egg is covered in a hard shell called nit and is glued to the hair very close to the scalp. The egg hatches nine to ten days later into a juvenile insect called nymph. These nymphs also suck blood like the adults, and, after several moulting, become adult lice that are capable of reproducing again.

The eggs, nymphs, and lice can survive for up to three days away from the scalp. The eggs, however, require the higher temperature near the scalp to hatch. They don’t infect pets. Therefore, it is not useful to treat your dogs or disinfect your home.

Head lice are spread from person to person through direct head-to-head (or hair-to-hair) contact most of the time. It is possible to spread head lice by sharing brushes, combs, towels, or hats and headgear. It is certainly reasonable to disinfect these items by soaking them in hot water (over 53.5C) for 5 minutes which will kill the lice and eggs. Pillow cases and towels can be cleaned using washer (hot cycle) and dryer. Those that cannot be washed in water can be sealed in plastic bags for two weeks which is just as effective.

Diagnosis of head lice infestation depends on detection of live adult lice. Because they move fairly quickly and are hard to detect, many health professionals depend on the presence of eggs, or nits, as indication of head lice infestation. The problem is that live nits are usually less than 6 mm from the scalp and it is difficult to differentiate from dandruff and other scales that fall off the scalp. As a result, misdiagnosis and over-diagnosis are common.

A number of chemicals have been used successfully in treating head lice infestation. They are usually available in shampoos and rinses that can be applied on the hair for a period of time and then rinsed off.

Pyrethrins, extracted from chrysanthemums, were found to be toxic to the nervous system of lice but not to human. Because they were derived from plants, anyone allergic to chrysanthemums or ragweed should not use them.

Permethrin, a synthetic form of pyrethrin, works in a similar fashion and is also safe to be used in children. Although Lindane is very effective, it is potentially toxic to human nervous system and should be used with greater caution.

Recently, a totally new treatment was approved by Heath Canada for children over 4 years of age. Sold under the brand name Resultz, it contains myristate and cyclomethicone, which dissolves the protective waxy outer covering of head lice. Without this protective covering, water leaks out of the lice and they die of dehydration. Instead of poisoning the nervous system of head lice, this treatment kills them by removing their protective coat.

All of these treatments are more effective in killing nymphs and adult lice, but not the eggs. As a result, treatment needs to be repeated in 7-10 days, waiting for the eggs that have already been laid to hatch before a second treatment is used to eradicate them completely. It is reasonable to treat the child identified to have head lice, as well as close family members.

Once treatment is done properly, children are cured of the infestation. There is no need to keep children out of school because of head lice. After the first treatment, they should be allowed to return to daycare or school the following day. Children should not be kept out until they are so called nit-free.

Finally, I should mention that none of the homemade treatments have been shown to be effective. It is much better to use one of the products listed here. Although resistance to treatment was documented in some countries, it is likely uncommon in Canada. Reinfection, however, can occur because of close contact with other children who have unrecognized infestation.

I hope your children have a better school year. There is nothing that you can do to prevent them from catching head lice from other children. However, if they do get infested, they should be able to get back to school as soon as they receive their first proper treatment.