Head lice

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Head lice are parasitic insects called Pediculus humanus var. capitis. They only live on the heads of people. They reproduce asexually, laying eggs which are cemented to the hair.

They cannot survive off the body for any length of time without being damaged by dehydration. After 10 minutes, a louse will no longer be able to lay eggs, so it is unable to reproduce, and therefore no longer a problem.

Clinical and epidemiological

Head lice are not a serious health problem in this country. They rarely, if ever, cause physical health problems other than itching of the scalp. Adverse health effects mainly derive not from the lice themselves, but from the human perception of them:

  • Excessive public and professional reactions lead to an inflated perception of prevalence, to unnecessary, inappropriate, or ineffective action, and to a great deal of unwarranted anxiety and distress.
  • These actions and reactions in themselves cause problems, especially from the misuse and overuse of treatments.

This does not mean that they are not common; but we should not over-react to them.

Role as a vector

It has been speculated that head lice could be a vector for other microbial diseases, but there is little or no evidence to suggest that they have a significant role as a vector.

Management of head lice

Early diagnosis and treatment

Head lice are common; particularly in primary school aged children, who tend to put their heads together with other children frequently. Early identification and treatment of head lice will minimise their spread. Children of primary school aged should probably have their heads checked for head lice at least weekly, especially if their friends are known to be infected. Information on how to do this is included in the Stafford Group document.[1]


A single head louse.
A single head louse and a separate ovum attached to a strand of hair. Scale in millimetres.

Wet combing - a living, moving louse must be seen for the diagnosis to be confirmed. The presence of nits (the empty egg cases cemented onto hair shafts) or dirt on the pillow does not confirm the diagnosis.


Head lice ovum can survive up to 12 days and take a minimum of 8.5 days to mature from newly hatched nymph to egg-laying adult.[2]. This is important in treatment. Pediculicides that are not ovicidal such as pyrethroids and lindane require 2 to 3 treatment cycles. Malathion which is ovicidal requires 1 to 2 treatments.

You need to know local resistant patterns as it is pointless using an agent local lice are resistant to. Nit combing is recommended in some countries as adjunctively and others as therapy.

It is essential that head lice are correctly diagnosed. This can only be done by identifying a living, moving louse. No treatment should be instituted unless this has been done.

Until relatively recently the only treatments that were proven to be effective were lotions containing chemicals from the following three main groups: pyrethroids; malathion; and carbaryl. Shampoos should not be used. The lotions do not reliably kill eggs, so they must be reapplied 7 days after the first treatment to kill newly hatched lice before they are old enough to lay eggs.

Treatment 'failure' is usually due to re-infestation, misdiagnosis; or to failure to apply the product correctly, or to reapply the lotion 7 days after the first application. Rarely treatment will fail despite being used correctly. In this situation it may be appropriate to use a different chemical lotion – but ideally only after treatment failure has been confirmed by somebody who has been trained to identify head lice.

Recent and current research suggests that "bug-busting" may be almost as effective as the treatments described above, but it requires a great deal of motivation on the part of the family to do it properly. Even if it doesn't eradicate head lice very effectively, it might remove the larger, more mature lice which are the ones that can spread to others, and thereby prevent infection.

In late 2005 or early 2006 a new treatment, dimeticone (brand name Hedrin, not to be confused with sanhedrin), has been licensed, following a report of its efficacy in the BMJ[3] and considered in the DTB.[4]

Dimeticone is a silicone polymer, often an ingredient in shampoo conditioner and anti-foaming agents. This works by coating the head lice, blocking their spiracles, and impairing their fluid balance. The absence of a chemical insecticide means that resistance is not an issue. It has largely superceded insecticide-based treatments as the firstline treatment, replacing malathion, permethrin or phenothrin.

In 2009, phase three trials on the use of spinosad were reported to show that its efficacy compares with that of permethrin.[5]


Most Welsh headlice are resistant to pyrethroids by 2006. Some areas have historically tried to make a rotation through several groups of insecitides, in order to avoid a build-up of resistance.


There is little evidence that keeping hair short is much help - transmission requires close proximitiy of scalps.

Role of school nursing services

Routine scalp checks by school nurses ("the nit nurse") have been shown to be ineffective, and withdrawn. It would not be worthwhile re-introducing them.

The Stafford Group did recommend that school nurses may have a part to play in the management of head lice. But this may be a relatively low priority for this hard-pressed service.

External links

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  • PRODIGY 2007 on Head Lice
  • Head lice - offer choice of treatments explaining the advantages and disadvantages of each
  • Stafford Group report of the PHMEG (follow "PHMEG documents (guidance, policy, consultation)" and "Head lice guidance (Stafford document)" links for the original version and updates). This was first published in 1998. Since then more evidence has been published on the efficacy of wet-combing and other treatments, and dimeticone has been licensed. An updated version was published in March 2008.[6] Key points from the Stafford Report are here.
  • Summary information from the HPA