Approach

The diagnosis of pediculosis capitis (head lice infestation) should rest on the discovery of a live nymphal or adult louse by personnel with appropriate expertise.[33][34] Nits that are within 6 mm from the scalp are usually viable and are highly suggestive of an active infestation.[3][4] A viable nit has a dark eyespot on microscopic examination.

History

Most studies of head lice, both in the US and in other industrialized countries, report the highest incidence in elementary school-aged children (3-12 years of age).[12][6][15] Lice do not hop or jump, they can only crawl, so adults with head lice are most likely to be parents of children with lice, or those living in overcrowded conditions.[5] Indirect transmission through contact with personal belongings is much less likely to occur. Most studies that report incidence of head lice infestation by gender in school-aged children indicate about a 2 to 3:1 female-to-male ratio.[12][15][22]

Other key risk factors include race other than black, overcrowding or close living conditions (e.g., sleepovers or camp in the preceding month, boarding school), and close contact with an infected individual.[12][6][15] Lice spread most readily through head-to-head contact. Any situation that brings infested children in close contact with others is very likely to cause spread of that infestation.[24][27][31] Therefore, having a family member or bedmate with head lice greatly increases the risk.[24]

Generally, a parent or teacher will observe excessive scalp scratching in the child, or a child may complain of a pruritic scalp.

Physical examination

People who are suspected of having head lice, including those who have been in close contact with an individual with head lice, should have a careful scalp examination, even in the absence of symptoms. Also, anyone who is noted to have a scalp infection (e.g., impetigo or pyoderma) or otherwise unexplained lymphadenopathy in the head or neck region should be carefully checked for the presence of head lice infestation.[12][3][4]

The definitive standard for diagnosis is finding a live louse or lice on the head.[1][5][35] This can be difficult to do, because lice tend to crawl quickly. The child should be positioned with his or her head tilted to the chest and the hair closest to the nape of the neck and behind the ears examined under good light, separating the hair into segments with fingers, sticks/throat culture swabs, or tongue depressors.

Combing is more accurate than visual inspection at diagnosing infestation. Visual inspection has been reported to underestimate the true prevalence of active infestation by a factor of 3.5, although visual inspection has a higher sensitivity for the diagnosis of historic infestation.[36]

The hair is moistened with water (to make it easier for lice to stick to the comb) or conditioner (to slow the lice and make combing easier). The moistened hair should be combed with a fine-toothed nit comb, especially near the nape of the neck and behind the ears, checking for nymphs or adult lice. Dry combing can produce false-negative results because the lice are apt to crawl away quickly from the site being combed. In addition, dry combing should be undertaken with caution, as vigorous combing with a plastic comb can generate enough static electricity to eject lice, which can then set up an infestation elsewhere, depending on where they land.[29][31][32][33][34][37]

Some will accept the presence of live eggs within 1 cm of the scalp as a diagnosis of head lice infestation, but several investigators have shown that many patients with eggs alone never "convert" to an active infestation.[29][35][38] Finding empty egg cases (nits) >1 cm from the scalp should not be accepted as a diagnosis of a lice infestation.

Although not always seen, tiny papules at the nape of the neck just below the hairline are not uncommon, and represent louse bites. Surrounding inflammation is secondary to the body's reaction to the louse saliva.[13]

Lice very rarely leave a head, but can occasionally be seen on the collar area of clothing in an unusually severe infestation. Lice discovered on the collar area of clothing could either be head lice (most likely diagnosis for a child) or body lice (if the person is homeless or destitute).[35]

Examination with magnifying lens, trichoscope or microscope

In cases where the diagnosis is in question, examination with a magnifying lens, trichoscope, or microscope can distinguish between a nymph or adult louse and other insects or hair debris. An egg with an eye spot can be distinguished from an empty egg case, or nit. A nit can be distinguished from dandruff or other hair debris, which can sometimes be difficult to distinguish on casual observation with the naked eye. Nits are firmly attached to the hair shaft, and can be removed using fingernails or a fine-toothed lice comb. An alternative is plucking the hair with the nit, and examining that under the microscope.

Because school nurses rarely have these devices available, a referral to the health care provider's office may be required.[35][Figure caption and citation for the preceding image starts]: Adult louse seen under a microscopeFrom the collection of Dr Richard Pollack; used with permission [Citation ends].com.bmj.content.model.Caption@4469d645

Use of this content is subject to our disclaimer