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Overdiagnosis and consequent mismanagement of head louse infestations in North America


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The Pediatric Infectious Disease Journal: August 2000 - Volume 19 - Issue 8 - p 689-694
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Human head lice (Pediculus capitis) infest people worldwide and are most prevalent in school age children. 1 Parents, school personnel and health providers in North America seem to regard such conditions as exceedingly transmissible, and infested children are perceived as threats to others. Children who seem to bear trophic lice or nits (louse eggs) in their hair consequently are often excluded from school until signs of the infestation no longer are evident. The resulting pattern of absenteeism contributes to the perception that pediculiasis is common.

Head louse infestations generally are diagnosed by nonspecialist personnel, physicians, school nurses and parent-volunteers. Few of these people have received formal instruction in identifying lice or their eggs, and diagnostic specimens seldom are referred to laboratories that employ personnel who can distinguish lice from other kinds of insects, or viable louse eggs from those that are hatched or dead, or from inert material. The accuracy of the diagnoses that are used to exclude children from school has not been critically evaluated.

It may be that louse infestations in the hair of schoolchildren frequently are overdiagnosed. To examine this suggestion we invited health professionals as well as the general public to submit to us samples they considered to represent lice or louse eggs. We attempted to verify each such diagnosis to determine how frequently noninfested children were excluded from school or were subjected to inappropriate therapeutic regimens.


We established a no-cost reference center designed to assist in the diagnoses of pediculiasis and advertised this service in a notice posted on an Internet-based site 2 that provides information on this condition. Health care providers as well as the general public were invited to submit specimens that they considered to represent lice or nits. Specimens were to be affixed to a standard form using transparent adhesive tape. The form included a questionnaire requesting information (name, address, age, sex) of the presumably infested subject as well as details relating to the infestation (diagnostician, duration of infestation, anti-louse treatments applied during previous 4 weeks and whether the school that the child attended had a “no-nits” policy). We informed these correspondents that we would verify each diagnosis and would maintain confidential any identifying information.

Samples were examined by means of a stereoscopic dissecting microscope under ×20 magnification within 2 days of receipt, and each specimen was identified as to species and life stage (adult, nymph or egg). Generally samples were sufficiently visible through the clear tape to permit identification in place; when necessary, however, the tape was cut to fully expose the specimen. We considered the subject to be infested if the sample included at least one trophic stage (adult or nymph) louse (Fig. 1A) or an egg containing an apparently viable embryo. An embryo was considered to be viable if it was intact and contained either a well-hydrated mass (Fig. 1B) or a discernibly developing embryo (Fig. 1C). Samples comprised solely of eggs that had hatched (Fig. 1D) or that contained remnants of a dead embryo (Fig. 1E) were considered to be residues of an extinct infestation. To determine the relative accuracy of identifications by category of diagnostician and appropriateness of treatment, we recorded the data supplied with each submission and correlated the responses with the identity and status of each infestation.

Fig. 1
Fig. 1:
Images of representative samples submitted to confirm identification.A, adult female head louse on nit comb;B, intact egg containing well-hydrated mass of young embryo;C, intact egg containing fully developed nymph before hatching;D, hatched egg, devoid of operculum (cap) and embryo;E, egg retaining operculum and disintegrated embryo within;F and G, pseudo-nits;H, scab tissue. B to H are at same scale.


Samples consisting of lice and their eggs, and which may have served as the basis for diagnosis, were solicited via a web site. A total of 614 submissions were harvested from 541 different subjects between April 2, 1998, and October 26, 1999. Samples contained as few as 1 and some >100 objects. Samples generally (73.8%) were accompanied by our standard form that was downloaded from our web site; others (18.7%) were mailed without the specified form; some (4.3%) were delivered in the form of conventional “clinical specimens” from hospitals or clinics; the remainder were conveyed in some other manner. Although samples were submitted mainly from the United States (from 42 states), we also received 13 submissions from Canada, 10 from Japan, two from the UK and one each from Australia and Singapore.

Identity of specimens

We first attempted to verify the identifications that accompanied each submitted specimen. Lice or their eggs were present in fewer than two-thirds of these samples (Table 1). Only about one-half (53.3%) of these 364 louse-derived specimens, however, included a louse or an egg containing an apparently viable embryo. The remaining louse-derived samples were comprised solely of eggs that had hatched or that contained remnants of a dead embryo. Specimens that included no evident louse-derived material were comprised of diverse arthropods, dandruff, scabs or plugs of epidermal cells adherent to hairs (Fig. 1, F to H). Head louse infestations are misdiagnosed more often than not.

Table 1
Table 1:
Characteristics of the presumed head louse specimens that were submitted to us for identification

Accuracy by diagnostician

The accuracy of these head louse diagnoses was then correlated with the profession of the person who performed the diagnosis. We confirmed that louse-derived material was present in nearly three-fourths of the specimens identified by parents, school nurses or teachers, but the nits they harvested frequently were nonviable (Table 2). The subjects and their physicians (including general practitioners, pediatricians and dermatologists), however, generally erred; few of the samples that they submitted actually contained louse-derived material. One person who was declared louse-free by a physician (a dermatologist) submitted a sample that included several lice and viable eggs. Although parents and school personnel seem to more accurately diagnose head louse infestations than do physicians, they each generally fail to distinguish inactive from active infestations.

Table 2
Table 2:
Accuracy and status of head louse diagnoses according to the profession of the person who performed each diagnosis

Characteristics of subjects

We then determined whether accuracy of diagnosis correlated with the age or sex of the subject. More than one-half of the subjects in our sample were younger than 11 years old, and most were female (Table 3). Although nearly 5 times as many samples in the youngest age group were derived from girls than from boys, the vast majority of samples from both sexes contained evidence of active or of extinct infestations. Viable lice or their eggs were present in samples taken from virtually all of the boys and one-half of the girls who produced louse-derived material. Samples from teenagers mainly derived from female subjects, but fewer than one-half of these contained lice or eggs, of which even fewer represented evidently active infestations. Evidence of head louse infestations most frequently derives from young girls, the group that is also most frequently overdiagnosed.

Table 3
Table 3:
Accuracy of head louse diagnoses according to the age and sex of the presumably infested subject

Appropriateness of treatment

We then determined how frequently appropriate pediculicidal therapeutic regimens had been applied in response to these diagnoses. Over-the-counter permethrin- and pyrethrin-based products were applied nearly as frequently by apparently noninfested subjects as by those who were actively infested (Table 4). Subjects without evidence of louse infestation were nearly 4 times more likely than those infested to have applied a prescription pyrethroid. Lindane was reportedly applied nearly as often by infested and by noninfested subjects, but less so by those with extinct infestations. Similarly, diverse occlusive dressings (e.g. vegetable oils, petrolatum and mayonnaise) were applied nearly as frequently whether lice were present or absent. Nearly one-third of subjects with active infestations, one-fifth of those with apparently extinct infestations and nearly one-half of those without evidence of infestation reportedly applied two or more anti-louse preparations during the month preceding the submission of their samples. We conclude that traditional as well as alternative anti-louse formulations are frequently overapplied and that noninfested subjects apply diverse pediculicidal products more frequently than do infested subjects.

Table 4
Table 4:
Therapeutic regimens applied to people who were actively infested by head lice compared with those used by others Treatments were those applied within 4 weeks of the date of specimen submission.

Duration of infestation

We attempted to determine how long subjects may have been infested by head lice. Subjects were asked to indicate the time that had elapsed since they became aware of the infestation. Of 135 subjects reporting that the infestation had been identified 1 to 6 months earlier, 47% submitted samples that included evidence of active or past infestation; even fewer (24%) had evidence of an active infestation. Of 198 subjects reporting a more chronic infestation (lasting >6 months), 52% submitted evidence of active or past infestation, with 31% being evidently active. Presumably chronic infestations are diagnosed erroneously as frequently as are relatively recent head louse infestations.

Exclusion practices

We then determined how frequently presumably louse-infested children may have been quarantined inappropriately. Of 313 respondents 82% indicated that a no-nits policy was in place at the school attended by the subject children. Although lice or their remnants were discovered in 73% of samples from children attending schools with such policies, only 39% contained lice or apparently viable eggs. Interestingly diagnoses were more frequently erroneous in the case of children who attended schools that did not quarantine infested children; 39% had evidence of infestation at some point, but only 29% were evidently active. Noninfested children are excluded from school because of presumed pediculiasis more frequently than are infested children.


Although head lice neither generally cause harm nor transmit pathogens, they are culturally abhorrent. The revulsion thus engendered often motivates people to quarantine children presumed to be infested and to require unnecessary treatment. Identifying head lice or their eggs on a patient would intuitively seem a fairly simple matter, so much so that parent volunteers are often empowered to conduct mass screenings within schools. Our findings, however, suggest that pediculiasis is often misdiagnosed by medical and lay personnel alike. Numerous “infestations” seem to present without convincing entomologic evidence, and of those based entirely on the presence of eggs, many represent apparently extinct infestations. The case definition for pediculiasis should be based on the observation of live lice rather than on eggs.

Head louse infestations frequently are misdiagnosed. Our observations on the relative diagnostic skills of health professionals and nonspecialist personnel, however, must be viewed with caution. Our analysis is limited to samples submitted for evaluation; such samples consisted of the actual specimens on which the initial diagnoses were based or specimens representative of what was later detected on the hair. Similarly the associated identifications may not be representative of the diagnostic prowess of each category of diagnostician. Furthermore it may be that samples sent to us were derived from “difficult cases” associated with chronic reinfestation, insecticide resistance or delusory parasitoses. Such samples may more likely be taken from subjects who search actively for additional information and treat more often and with alternative products. Nonetheless diverse arthropods are often misidentified, 3 and mislabeled images 4, 5 perpetuate such errors.

The no-nit policies variously drafted and adopted by schools and other institutions aim to reduce the transmission of head lice by excluding infested children from school. Although meritorious in principle, such policies are often counterproductive when applied and appear not to affect prevalence. School nurses appear to be particularly effective in their ability to spot lice or eggs but generally lack the expertise, equipment and time to distinguish active from inactive infestations. Many schools lack a nurse and engage other personnel and parent volunteers to perform such tasks. Presumably infested children are then excluded from school until after treatment or until they have been rendered free of all nit-like objects. Repeated failure to eliminate completely all offending material (regardless of type and viability), and extended absences due to school-enforced exclusion occasionally cause schools to institute legal action charging parents with neglect or abuse. The no-nits policies that have been adopted by many schools should be reviewed and their merit considered critically.

Louse eggs are generally not eliminated by pediculicidal treatments, and remnants of dead and hatched eggs may remain firmly affixed to hair shafts for several months or even years. Because health care workers, school personnel and parents may lack the ability, time or inclination to distinguish louse eggs that contain a developing embryo from those that have hatched or contain a dead embryo, the no-nits policies effectively regard any louse egg a “bad” egg. Elimination of all eggs, therefore, has long been proclaimed necessary to ensure that the patient poses no risk to others. 6, 7 Such policies, however, continue despite early acknowledgement of their failure to interrupt transmission or lower prevalence 8 and their effect on depopulating schools. 9 Documentation of resistance of some head lice in the US to pyrethroid-based pediculicides 10, 11 may further encourage overzealous efforts to enforce exclusion policies. Such over-the-counter preparations, however, continue to serve as the primary insecticidal option, with prescribed preparations of malathion and lindane available as secondary options. Manual grooming may supplement or replace insecticidal treatment, particularly when dealing with young children (<2 years old).

Diagnosis of pediculiasis should be based on microscopic examination of specimens to confirm the identity of the offending organism(s) as well as to distinguish active from extinct infestations. Ideally the diagnostician should have relevant expertise in parasitology or entomology. Health care providers and laboratory workers who examine such specimens should have demonstrated proficiency in identifying ectoparasites and in determining the viability of the specimens. Nonspecialists (non-medically trained school staff or the lay public) generally should not perform such diagnostic tests. The assertion that pharmacists be permitted to perform certain diagnostic functions 12 requires scrutiny, but the accuracy of their diagnoses of pediculiasis capitis may be comparable with that provided by other personnel. Treatment is generally warranted solely when live trophic forms or if apparently viable eggs are present. Discovery of live head lice or their eggs does not seem to warrant exclusion of a child from school.

We thank Deane Eastwood, Assistant Director, Academic Communications Internet Applications (Harvard School of Public Health) for assistance in hosting and managing our web site.


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Head lice; Pediculus; pediculicide; diagnosis; identification; no-nit; exclusion policies

© 2000 Lippincott Williams & Wilkins, Inc.