History and exam
Key diagnostic factors
common
scalp pruritus
An intensely itchy scalp is often the first symptom of head lice infestation, although it can take an individual with a first-time infestation up to 1 to 3 months to become sensitized to the louse saliva and experience pruritus. With subsequent infestations, itching will develop sooner.[2][3][4][14]
live nymphal or adult louse
Finding live lice is pathognomonic to the diagnosis.
Lice can be difficult to find because they tend to crawl quickly away from any disturbance; however, they are more common near the nape of the neck.
Wet combing with a fine-toothed comb is the best way to detect an infestation, but is not necessary if a louse is seen on casual observation.[33][34]
Inspect teeth of comb for lice after each stroke or two. If unsure of whether the insect removed is indeed a louse, examination with magnifying lens or microscope will reveal the classic shape of a head louse.[33][34]
eggs visible on hair shaft within 1 cm of scalp
Live eggs within 1 cm of the scalp are diagnostic of an infestation, but can be difficult to see.
Empty egg casings (nits) are more easily visible, but suggest past infestation.
Live eggs develop an eye spot several days after being laid, and the empty egg casings of hatched lice take on a whitish appearance.[1][3][4]
Because not all eggs will hatch, finding eggs alone without documenting the presence of live lice is not an accurate diagnosis.[14]
Examination with a magnifying lens or microscope can distinguish nits from other hair debris.
Other diagnostic factors
common
small red papules under hairline at nape of neck
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.[14]
uncommon
lymphadenopathy
Pea-sized, mobile, nontender posterior occipital lymph nodes when palpated.
Infestation rarely causes lymphadenopathy, unless there is a secondary infection because of disrupted skin integrity secondary to scratching. If enlarged lymph nodes are found with an infestation, look carefully for a secondary bacterial infection.[2][3][4][14]
erythema with honey-colored crust on scalp
Impetigo can accompany head lice infestations. It is usually rare but may be common in settings where children are not cared for properly and diagnosis is delayed. Impetigo is caused by skin surface bacteria, such as streptococci and Staphylococcus aureus, entering through a break in the skin secondary to scratching, and not from the lice themselves. Head lice should be carefully looked for if impetigo is found.[39]
Risk factors
strong
3 to 12 years of age
Most studies of head lice, both in the US and in other industrialized countries, report the highest incidence in young school-aged children. This may be because children of this age tend to maintain a closer "personal space" than adolescents and adults.[12][13]
Lice are associated with kindergarten and school attendance only insofar as these are places where children come in close contact with other children. Kindergartens and schools do not harbor lice. School nurses often detect an infestation before parents do, thus reinforcing in parents' minds the association with school.[8][9][20]
In nonindustrialized countries, infestations are common in adults.[6][15][21]
Very few studies have been done involving younger children and infants in childcare settings.
female sex
Most studies that report incidence of head lice infestation by sex in school-aged children indicate about a 2 to 3:1 female-to-male ratio.[12][13][22]
This may be due to the fact that girls spend more time in close play. In addition, girls with longer hair could have nits (i.e., hatched eggs) from past infestations in their hair for longer periods of time, whereas boys would have those nits eliminated with more frequent, shorter haircuts.[12][13][22]
Finally, it may be more difficult to eliminate an infestation in a girl with longer, thicker hair.[12][13][22]
close contact with an infested individual
overcrowding or close living conditions
weak
low socioeconomic status
In the US, head lice have been shown to affect children of all socioeconomic levels.[13]
The link of head lice to low socioeconomic status happens only when there is concomitant overcrowding, such as that occurring when several people share sleeping quarters, or in homeless shelters.
One study in Belgium did show a reduced prevalence of head lice in higher socioeconomic status children, presumably due to easier access to and better compliance with treatment.[23]
poor hygiene
In the US and worldwide, it has not been shown that head lice prefer unclean hair. However, it is thought that in cultures where daily brushing and combing is the norm, it is possible that the inevitable killing of lice that would happen with this practice could keep serious infestations to a minimum.[24]
contact with contaminated clothes, hair care items, bedding
Most investigators agree that fomite (i.e., an inanimate object that can carry infectious organisms) transmission of head lice is not the major route of transmission, although it is possible.[12][13][15][24][25][26][27]
Most head lice will cling firmly to the hair closest to the scalp, and are reluctant to leave except under extenuating circumstances.[13]
Lice recovered from combs and brushes are usually dead or injured and no longer capable of reproducing. The claws of lice are not adapted to crawl on smooth surfaces such as plastic, glass, and wood.[13]
In temperate climates, lice rarely live more than 24 hours off the scalp of a host. Even after close contact with an infested individual, pillowcases were found to harbor remarkably few lice.[28] However, it is prudent to avoid head contact with potentially contaminated items, and to do some routine cleaning of such items belonging to an individual with a known infestation.[15][24][25][26][27][28]
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