Pediculosis humanus var capitis is a bloodsucking parasitic insect specific to humans. It is 2.5-3 mm long and flattish. It can’t jump or fly or even walk efficiently, but is easily transferred, usually by head-to-head contact with an infected person or less often with an infected person’s headgear, comb, towel, or other object. Infestation is not a sign of poor hygiene. Lice bite and suck blood 4-5 times daily, injecting an anti-coagulant in their saliva. Mommy lice live for up to 3 months and lay up to 300 eggs at a rate of 3-4 a day. They glue the eggs individually to a hair shaft, usually close to the scalp but in warm climates as far as 6 inches from the scalp. They hatch in 6-10 days, after which the empty egg cases move further and further from the scalp as the hair grows out. The diagnosis can be made by seeing live, moving lice and finding nits (the egg or young lice) on the hair. The best place to look for them is behind the ear and at the nape of the neck. Nits can be confused with dandruff and debris, but these can usually be brushed away while nits remain firmly stuck to the hair shaft. Nits alone are not enough to make the diagnosis of active infestation. They may be either alive or dead: empty or nonviable egg cases may still be present long after the infestation has resolved.
Combing . Removal by wet combing with a fine-toothed lice comb every 3 days for at least 2 weeks produces reported cure rates of 47-75%. Downside: time-consuming, especially if hair is long and thick. Upside: no risks, quality time with kids, satisfies primate grooming urges (chimps pick lice for social bonding and they eat what they find). Shaving the head also reliably removes lice, but it is a rather drastic solution that few people would choose.
Desiccation. Hot air kills both lice and eggs. A study of hot air devices found that the LouseBuster was the most effective: a single 30 minute session was well-tolerated and essentially 100% effective. Only one treatment is needed, and the lice aren’t likely to develop resistance. The device is expensive and requires special training; using a regular blow dryer is not recommended because it can propel live lice airborne to infest other people.
Suffocation. If you can block its air intake, the critter dies. Nuvo lotion was 95% effective in a studythat was questioned because it was non-randomized, uncontrolled, unblinded, and had other design flaws. It turns out the doctor who headed the research was re-packaging Cetaphil Skin Cleanser and selling a $10 product under a new name (Nuvo) for $285. That means he’s sneaky, but it doesn’t mean it doesn’t work. More studies are needed.
Home remedy suffocants have included Vaseline, mayonnaise, olive oil, and almond oil. Vaseline and mayonnaise are no longer recommended because they are messy and hard to wash out. Essential oils have also been used, including tea tree oil, lavender oil, neem oil, clove oil, eucalyptus oil, aniseed oil, cinnamon leaf oil, red thyme oil, peppermint oils, and nutmeg oil. There have been a few promising pilot studies of some of these treatments, but the studies have not been replicated; the research is far from conclusive and success rates have been as low as 3.3%.
Most effective drugs are neurotoxic, designed to paralyze and kill the lice. They do not reliably destroy the eggs, so repeat treatment is typically needed after 7-10 days. Available neurotoxic options include ivermectin, lindane, malathion, permethrin, and pyrethrins. Since 2006 a prescription suffocant, 5% benzyl alcohol lotion, has also been available. The drug of choice today is permethrin; lindane is discouraged because of neurotoxicity concerns but is a good second-line choice in areas where lice have become resistant to permethrin and pyrethrins.
Permethrin (Nix). Generally recommended as a first-line treatment, permethrin is a synthetic version of a natural insecticide that is more effective and less allergenic than natural pyrethrins. Advantages: the most studied pediculicide and the least toxic, reported side effects are mild (itching, redness, and swelling), low cost: $8-9. Disadvantage: resistance is increasing and treatment failures are becoming more common. For treatment failures with 1% permethrin, using a 5% solution and leaving it on the hair overnight under a shower cap may be effective.
Pyrethrins with piperonyl butoxide shampoo (Rid). A natural extract from chrysanthemum flowers, pyrethrins are less effective and more allergenic than permethrin. Advantages: sold over-the-counter, no prescription required, low cost ($5-6). Disadvantage: treatment failures are common.
Benzoyl alcohol 5% lotion (Ulesfia). This is a suffocant that is applied to dry hair, left on for 10 minutes, then rinsed; the treatment must be repeated after a week. Advantages: approved for children 6 months and older, OK for pregnant or lactating women, two double-blind studies showed it to be 75% effective, resistance is unlikely to develop. Disadvantages: not as effective as some other treatments, and higher cost than permethrin ($60).
Malathion (Ovide). An irreversible cholinesterase inhibitor, malathion is the fastest-killing, most effective head lice treatment ever marketed. Advantages: safe and effective, kills eggs, resistance has been reported in other countries but not yet in the US. Disadvantages: not recommended for children under 6 or for women who are pregnant or nursing, long application time (8-12 hours), objectionable odor, flammability, high cost ($150). An earlier version was removed from the US market by the manufacturer but a new formulation was reintroduced and approved by the FDA in 1999.
Lindane (Kwell). An organochlorine insecticide similar to DDT, lindane is stored in fat and nerve tissue. There are concerns about contamination of drinking water, rivers, and lakes, with hazards to fish and wildlife. The FDA issued a public health advisory about the risk of neurotoxicity and seizures, and lindane is banned in California. It is a shampoo applied to dry hair for 4 minutes as a single application; it costs $136. The Medical Letter cautions against its use because there are safer alternatives. It is usually safe when used as directed, but seizures and deaths have occurred with overuse, misuse, or accidental ingestion, and in rare cases death has reportedly followed a single application according to directions. It should be used with caution by those who might be more susceptible to side effects, including infants, children, women who are pregnant or breast-feeding, people who weigh less than 110 pounds, HIV-positive patients, the elderly, and patients with skin conditions like atopic dermatitis or psoriasis. It is contraindicated in premature infants and patients with seizure disorders.
Ivermectin lotion. Two double-blind studies showed this was 76% and 71% effective. It is not recommended for children under 33 lbs. or for women who are pregnant or nursing. Side effects include dizziness and itching. It costs $260.
Ivermectin pills. The pill form of ivermectin is not FDA approved for head lice, although it has been reported to be 95% effective and has been used successfully when all other therapies have failed. The tablets are taken as a single dose repeated after a week. The cost is around $10.
Spinosad (Natroba). A new neurotoxic drug approved by FDA in 2011, spinosad provokes hyperexcitation and eventually paralyzes the lice and kills them. Two comparative trials showed it was twice as effective as permethrin, and it is safe. Side effects are mild and infrequent: irritation at the application site, eye redness, dry skin, etc. It is approved for age 4 and up. It is expensive ($240) but it usually requires only one application and does not require nit combing.
It’s hard to draw any firm conclusions about the relative effectiveness of different treatments. Comparing results from different studies is problematic. Different endpoints are used. The incidence of treatment-resistant lice varies with geographic location. Home treatments may not be administered uniformly. Did they follow instructions precisely? Did they leave the product on for the recommended time? Did they comb out all the nits? Did the study measure clinical cure or death of lice?
Back to school
Some schools have had no-nit policies where a child couldn’t return to school until all nits were removed. International guidelines say these policies are unjust and should be discontinued because they are based on misinformation rather than on objective science. Dead nits remain after effective treatment, so finding nits in the hair doesn’t mean the child can infect others.
Head lice are more of an annoyance than a health problem. There is a wide choice of treatments to fit every budget, philosophy, and personal preference. Permethrin is a safe, effective first choice; other remedies can be used if treatment with permethrin fails. Even the most potentially toxic product is seldom hazardous when used as directed. The hot air treatment is a new but very promising non-drug option; I’m staying tuned.
This article was originally published in the Science- Based Medicine Blog.