Head lice (pediculus capitis) are blood-feeding insects of 1–4mm in length that can look like sesame seeds. They exclusively live on a human host, which means that transmission from pets to humans (or vice versa) is impossible.
Head lice are harmless and do not spread disease, unlike fleas, but there is a sense of embarrassment associated with this condition.
Head lice can only crawl and do not jump or fly. Therefore, lice usually need direct head-to-head contact to infest a new host.
However, sharing the same bedding, headgear, combs or even earphones may lead to transmission. Therefore, it is important to consider all possible contact points within a household to prevent spread.
A typical colony of lice on one host comprises on average 30 lice. Nits are the eggs of lice, which attach to the hair shaft with a sticky substance and may look like dandruff.
After eight to 10 days, so-called nymphs hatch from these eggs and mature within another 10 days to adult lice. An adult louse can live up to 30 days while on a human body, but only survives for up to 48 hours if detached from it. Each female louse may lay about 100 to 150 eggs during her short life.
Infestations with head lice are very common and several million people in the UK, most of them school children and their families, may experience a problem with head lice each year.
There are effective treatments available OTC, so only a few patients will approach their GP for advice or help.
An infestation with head lice does not always cause physical symptoms or problems, but some patients complain of itching of the scalp. This can be a consequence of sensitisation to the saliva or faeces of the lice or due to secondary infection after scratching.
Even if there is a suggestive history for head lice in a patient, the detection of live lice is required to confirm the diagnosis. The best places to find evidence of live lice or nits during a physical examination are around the neckline and behind the ears.
The main differential diagnoses include allergic or autoimmune inflammatory skin conditions of the scalp, but there can be potential for psychosomatic or psychiatric conditions where patients believe they are infested with insects or worms.
3. Physical management
All household members who are affected should receive treatment at the same time to prevent further spread. An infestation may be self-limiting eventually, but this could take months and therefore it is better to treat actively.
There are three main management methods: wet combing with a fine-toothed comb; dimeticone lotion in silicone and insecticides as topical treatments.
Wet combing with a fine-toothed comb requires thorough sessions of at least 30 minutes at a time. The procedure needs to be repeated every three days over at least two weeks until no more adult lice are found.
Although this approach is time-consuming and needs discipline and diligence to be effective (the cure rate is estimated to be between 35 and 75% for this reason), this treatment is non-invasive, has no health implications and does not facilitate resistance to medical treatment.
Wet combing with a fine-toothed comb needs to be repeated regularly (Photograph: SPL)
4. Medical management
Dimeticone lotion in silicone is a relatively new strategy that works by suffocating head lice with a complete airtight silicone film. The first treatment needs to stay on for eight hours, best done overnight, and requires a repeat treatment after one week. The success rate is at least 70%.
This is a mechanical treatment, so there is no problem with resistance. Dimeticone is a low-risk treatment, although silicone is inflammable and should be kept away from naked flames.
Insecticides used as topical treatments (malathion, pyrethroids, such as permethrin and phenothrin and carbaryl) remain popular. Cure rates after two treatments, one week apart, are up to 80%.
Benzyl benzoate is also licensed but comparatively ineffective. Oral treatment with ivermectin is currently not recommended for general use, especially in young children, but could be useful for large outbreaks.
Pregnant or breastfeeding women should not use insecticides and should be wary of herbal head lice treatments, as these are often not tested for safety in pregnancy. In fact, some herbal preparations, such as rosemary oil, have shown a definite risk for miscarriage or premature labour. All in all, the evidence for alternative treatments, such as tea tree oil and neem seed oil, remains weak.
Alcoholic preparations can irritate any existing eczema. Patients with severe asthma or a history of multiple, severe hypersensitivity reactions to medications or chemicals need particularly careful counselling about treatment options and possible side-effects.
5. Confirming treatment success
Patients or carers should confirm successful treatment by detection combing 48 to 72 hours after treatment and again seven days later.
Treatment failures are probably due to following medication instructions incorrectly, although there seems to be a growing problem of resistance to insecticides. Referral to specialist services is usually not required for head lice.
- Dr Jacobi is a GP in York.