Pediculosis and Pthiriasis (Lice Infestation)

Background

Pediculosis (ie, louse infestation) dates back to prehistory. The oldest known fossils of louse eggs (ie, nits) are approximately 10,000 years old.^[1] Lice have been so ubiquitous that related terms and phrases such as "lousy," "nit-picking," and "going over things with a fine-tooth comb" are part of everyday vocabulary.

Louse infestation remains a major problem throughout the world, making the diagnosis and treatment of louse infestation a common task in general medical practice.^[2] All socioeconomic groups can be affected. Pediculosis capitis results in significant psychological stress in children and adults and missed schooldays in children, particularly in areas with a no-nit policy.^[3]

Lice are ectoparasites that live on the body. Lice feed on human blood after piercing the skin and injecting saliva, which may cause pruritus due to an allergic reaction.^[4] Lice crawl; they do not have wings and cannot fly or hop.^[4]

A mature female head louse lays 3-6 eggs, also called nits, per day. Nits are white and less than 1 mm long. Nymphs (immature lice) hatch from the nits after 8-9 days, reach maturity in 9-12 days, and live as adults for about 30 days.^[4]

Different species of lice prefer to feed on certain locations on the body of the host. Louse species include Pediculus capitis (head lice), Pediculus corporis (body lice), and Pthirus pubis (pubic lice, sometimes called “crabs”).

See the louse images below.

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The head louse, Pediculus humanus capitis, has an elongated body and narrow anterior mouthparts. Body lice look similar but lay their eggs (nits) on c....

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The pubic louse, Pthirus pubis, is identified by its wide crablike body.

See When Bugs Feast: What's Causing that Itch?, a Critical Images slideshow, to help identify various skin reactions, recognize potential comorbidities, and select treatment options.

Lice move from person to person through close physical contact. Spread through contact with fomites (eg, combs, brushes, clothes, hats, scarves, coats, linens) used by an infested person is uncommon.^[4] Overcrowding encourages the spread of lice. The body louse can be the vector of Rickettsia prowazeki, which causes typhus; Bartonella quintana, which causes trench fever; and Borrelia recurrentis, which causes relapsing fever.

Human lice have been used as a forensic tool. A mixed DNA profile of 2 hosts can be detectable in blood meals of body lice that have had close contact between an assailant and a victim.^[5]

• References

Pathophysiology

Louse infestation is prevalent throughout the animal kingdom. Mallophaga, or chewing lice, are common pests of birds and domestic animals, but humans are only rarely affected as accidental hosts.^[6]

Human lice (P humanus and P pubis) are found in all countries and climates. They belong to the phylum Arthropoda, the class Insecta, the order Phthiraptera, and the suborder Anoplura (known as the sucking lice).^[6] Mammals are the hosts for all Anoplura.

The Anoplura are wingless and have 3 pairs of legs, each with a single tarsal segment ending with a claw for grasping. The size and shape of the claws are adapted to the texture and shape of the hairs and/or clothing fibers they grasp. Their bodies are flat and covered with tough chitin.

Lice are blood-sucking insects. Human lice have small anterior mouthparts with 6 hooklets that aid their attachment to human skin during feeding. The sucking mouthparts retract into the head when the lice are not feeding. In general, lice feed approximately 5 times per day. In each species, the female louse is slightly larger than her male counterpart.

The 3 types of human lice include the head louse (Pediculus humanus capitis), the body louse (Pediculus humanus corporis), and the crab louse (Pthirus pubis). Body lice infest clothing, laying their eggs on fibers in the fabric seams. Head and pubic lice infest hair, laying their eggs at the base of hair fibers.^{[7, 8]}

Head and body lice are similarly shaped, but the head louse is smaller. Nevertheless, the 2 species can interbreed. The pubic louse, or "crab," is morphologically distinct from the other two.

Pediculus humanus capitis

The head louse (see the image below) is the most common of the 3 species. The average length of the head louse is 1-2 mm. Female head lice are generally larger than males.^[4] The louse is wingless and white to gray and has a long, dorsoventrally flattened, segmented abdomen. It has 3 pairs of clawed legs. Its average life span is 30 days.^[4]

View Image

The head louse, Pediculus humanus capitis, has an elongated body and narrow anterior mouthparts. Body lice look similar but lay their eggs (nits) on c....

The adult female louse lays eggs, called nits, and glues them at the base of the hair shaft. Nits are placed within 1-2 mm of the scalp, where the temperature is optimal for incubation. The female head louse lays as many as 10 eggs per 24 hours, usually at night. Egg and glue extrusion onto the hair shaft takes 16 seconds. Nits are typically located at the posterior hairline and postauricular areas.^[4]

Nits hatch in about 8-9 days if they are kept near body temperature and mature in another 9-12 days.^[4] Nits can survive for up to 10 days away from the human host. Cooler temperatures retard both hatching and maturation. The nymph molts three times before reaching its adult form. The adult head louse survives only 1-2 days away from its host.

Head louse infestation is spread by close physical contact and occasionally by shared fomites (eg, combs, brushes, hats, scarves, bedding).^[4] Lice can be dislodged by combs, towels, and air movement (including hair dryers in either low or high setting).^[9] Hair combing and sweater removal may eject adult lice more than 1 meter from infested scalps. Head lice can travel up to 23 cm/min.^[6] The head louse has difficulty attaching firmly to smooth surfaces (eg, glass, metal, plastic, synthetic leathers).^[4]

Pediculus humanus corporis

The body louse is larger than the head louse. Body lice range in size from 2-4 mm. Female lice are larger than male lice. Like the head louse, the body louse is flat and white to gray with a segmented abdomen.

Unlike the head louse and the pubic louse, the body louse does not live on the human body. P humanus corporis prefers cooler temperatures; it lives in human clothing, crawling onto the body only to feed, predominantly at night. Females lay 10-15 eggs per day on the fibers of clothing, mainly close to the seams. Adult body lice can live up to 30 days but die within 1-2 days when away from the host and without blood meals.^[10] On average, no more than 10 adult female lice can be found on a person with an infestation, although a thousand have been removed from the clothes on a single infested individual.^[11]

Body lice are spread through contact with clothing, bedding, or towels that have been in contact with an infested individual, or through direct physical contact with a person who is infested with body lice.

Pthirus pubis

The pubic louse gets the nickname of "crab" from its short, broad body (0.8-1.2 mm) and large front claws, which give it a crab-like appearance. The pubic louse is white to gray and oval and has a smaller abdomen than both P humanus capitis and P humanus corporis. Pubic lice live for approximately 2 weeks, during which time the females lay 1-2 eggs per day.^[6] Nymphs emerge from the eggs after 1 week and then mature into adults over the subsequent 2 weeks.^[6]

Their large claws enable pubic lice to grasp the coarser pubic hairs in the groin, perianal, and axillary areas. Heavy infestation with P pubis can also involve the eyelashes, eyebrows, facial hair, axillary hair, and, occasionally, the periphery of the scalp.

Pubic lice are less mobile than P humanus and P corporis, mainly resting while attached to human hairs. They can crawl up to 10 cm/day.^[6] They cannot survive off the human host for more than 1 day.

Nits

The average nit (ie, ovum) of the 3 types of lice is 0.8 mm long. The nit (see the images below) attaches to the base of the hair shaft (in the case of head or pubic lice) or to fibers of clothing (in the case of body lice) with a strong, highly insoluble cement; thus, nits are difficult to remove. The nit is topped with a tough but porous cap known as the operculum. This porous operculum allows for gas exchange while the nymph develops in the casing.

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Nit on a hair. Note the thin, translucent cement surrounding the hair shaft. Photo courtesy of David Shum, MD Western University, London Ontario.

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Two empty nits from Pediculus humanus capitis. Note the open shells still attached to the hairs and the porous operculi through which the lice have ha....

The ova require optimum conditions of 30°C and 70% humidity to hatch within the average time frame of 8-10 days; the incubation period is longer at lower temperatures. Ova do not hatch at temperatures lower than 22°C but can remain alive for as long as 1 month away from the body (ie, on fomites, clothing, brushes).

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Etiology

Causative organisms include P humanus capitis (head louse), P humanus corporis (body louse), and P pubis (pubic louse)

P humanus capitis

Pediculosis capitis is spread by direct contact with an infested person. Head-to-head contact with an infested individual at school, at home, and while playing may result in head lice infestation; personal hygiene and environmental cleanliness are not risk factors.^[4] Fomites, such as clothing, headgear, hats, combs, hairbrushes, hair barrettes, may occasionally play a role in the spread of head lice.^[4] Factors that predispose to head louse infestation include young age; close, crowded living conditions; female sex; white or Asian race; and perhaps warm weather.^[12] The risk of nosocomial transmission is low, unless close patient-to-patient contact (eg, playrooms, institutions) is present.

P humanus corporis

Risk factors for body lice infestation include close, crowded living situations (eg, crowded buses and trains, prison camps)^[11] and infrequent washing and/or changing of clothing. P corporis can be acquired via bedding, towels, or clothing recently used by an individual infested with lice; thus, individuals who are homeless, who are impoverished, or who are living in refugee camps are at high risk for infestation.^[10]

P pubis

Intimate or sexual contact with an individual who is infested with pubic lice is a common risk factor for pubic lice infestation. Risk factors for infestation of the pubic louse include sexual promiscuity and crowded living conditions. Contact with clothing, bedding, and towels used by an infested individual may occasionally be the cause of infestation.^[13] It is a myth that pubic lice are spread by sitting on a toilet seat; pubic lice’s feet are not designed to walk on smooth surfaces such a toilet seats, and the lice cannot live for long away from a warm human body.^[13]

Because these organisms are most often spread through close or intimate contact, P pubis infestation is classified as an STD. Condom use does not prevent transmission of P pubis. Upon diagnosis of pubic lice, concern should be raised about the possibility of concomitant STDs.

In children, infestation of pubic lice is usually contracted from a parent who is infested. Sexual transmission to children is rare. In most cases of infestations in children, transmission results from shared bed linens and close nonsexual contact.

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Epidemiology

Since pediculosis is not a reportable disease, exact numbers concerning incidence are unknown. Pediculosis may be underreported because of the social stigma attached—namely, the preconceived notion that lice of any kind are related to dirt and poor personal hygiene. In fact, personal cleanliness is not a factor in head lice infestation rates. On the other hand, false-positive nit diagnosis is common.^[4]

United States statistics

Pediculosis is very common; a report from 2000 estimates that 6-12 million Americans aged 3-11 years are infested each year.^[14] No reliable data provide the exact number of annual cases among adults. Head louse infestation is more common in the warmer months, while pubic louse infestation is more common in the cooler months.^[12]

Head louse infestation is most common in urban areas and may occur in all socioeconomic groups. Head louse infestations occur most commonly in school-aged children, typically in late summer and autumn. The reported prevalence ranges from 10%-40% in US schools. One study estimates that 12-24 million days of school are lost because of "no-nit" school policies.^[15]

Body louse infestation in the United States mainly affects homeless persons. Pubic lice generally are spread as an STD. Pubic louse infestation serves as a marker for other STDs, which may have been acquired simultaneously.^[6]

International statistics

Pediculosis has a worldwide distribution and is endemic in both developing and developed countries. The prevalence of pediculosis capitis is usually higher in girls and women and varies from 0.7%-59% in Turkey, 0.48-22.4% in Europe, 37.4% in England, 13% in Australia, up to 58.9% in Africa, and 3.6%-61.4% in the Americas.^[3]

In a study of 6,169 Belgian school children aged 2.5-12 years, the prevalence of head lice was 8.9%.^[16] The prevalence in 1,569 school children in Izmir, Turkey, was 16.6%.^[17] In 2005, the incidence of pediculosis doubled in the Czech Republic.^[18] Live lice were detected in 14.1% and dead nits in another 9.8% of 531 children aged 6-15 years in 16 schools.^[18]

P capitis was found in 9.6% of adolescent schoolboys in Saudi Arabia.^[19] In Mali, the prevalence of head lice in children was 4.7%.^[20] Among attendees of an STD clinic in south Australia, pubic lice were found in 1.7% of men and 1.1% of women.^[21]

P corporis is now uncommon in developed countries except among homeless persons.^[22]

Racial differences in incidence

Louse infestation affects all races and ethnic groups. However, in North America, the reported incidence of head louse infestation is lower in African Americans than in any other racial group, probably in part because of the use of pomades and in part because the claw size of the head louse is more adapted to the round shape of the hair shaft found in white persons and Asian persons.^[23] However, blacks may experience P pubis scalp infestation.

Sex- and age-related differences in incidence

Girls are at higher risk of head louse infestation than boys because of social behavior (eg, social acceptance of close physical head-to-head contact and, less commonly, sharing of hats, scarves, combs, brushes, hair ties and lying on a sofa, carpet, or stuffed toy that has recently come in contact with an infested person); hair length is not a factor. No sexual predilection exists in body or pubic louse infestation; males and females are equally likely to become infested.

Children aged 3-11 years are most likely to become infested with head lice because of close contact in classrooms and day care facilities. Head lice are much less common after puberty. Body lice are more common in adults, but can affect all ages.^[24] Age is not a significant risk factor in body louse infestation; body lice are indiscriminate in regard to the age of their host. P pubis infestation is more common in people aged 14-40 years who are sexually active.

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Prognosis

Treatments are highly effective in killing nymphs and mature lice, but less effective in killing eggs.

Causes of therapeutic failure include the following:

• Misdiagnosis
• Inappropriate treatment
• Noncompliance
• Insufficient application of pediculicide (ie, amount, duration)
• Lack of ovicidal activity of pediculicide and failure to re-treat within 7-10 days
• Lack of removal of live nits
• Lack of environmental eradication
• Sharing clothing, bedding and towels used by a person infested with body or pubic lice
• Failure to treat close contacts
• Re-infestation
• Resistance to pediculicide

Frequent use of pediculicides may cause persistent itching. Body lice can be vectors for diseases such as epidemic (louse-borne) typhus, trench fever, and louse-borne relapsing/recurrent fever. Violation of the integrity of the skin from a bite can lead to bacterial infection with organisms such as methicillin-resistant Staphylococcus aureus (MRSA). More commonly, infestation with lice produces social embarrassment and isolation rather than medical disease.

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Patient Education

The social stigma associated with head lice infestation must be addressed. Poor hygiene is not a risk factor in acquiring pediculosis capitis, although it is for body lice.

Management of head lice must include examination of all individuals exposed (all household members and other close contacts) and treatment of all those who are infested. Individuals who have no evidence of infestation should not be treated; however, if they share a bed with an infested individual, it is reasonable to treat them prophylactically.^[4]

Education has been shown to reduce the number of lice infestations in schools. "No nit" policies exclude many children from the classroom, but they have not been shown to reduce the number of louse infestations.^[25] Schools with “no-nit” policies should be educated to abandon these policies. The Centers for Disease Control and Prevention (CDC), American Association of Pediatrics, and National Association of School Nurses recommend discontinuation of these policies.^[4]

Noncompliance is a common cause of treatment failure in all 3 types of lice infestations. Therefore, time is well-spent providing patients with detailed instructions regarding the application and timing of medications used in the treatment of lice. Fomites may harbor live lice and therefore should be treated to prevent re-infestation and infestation of other individuals.

To minimize acquiring head lice, during epidemics of head lice, children should be educated not to share combs, brushes, headbands, hats, and scarves.^[6] Hats and scarves should not be piled in a common area, but rather separated for each child.^[6] Shaving of hair is effective treatment of head lice, but not socially acceptable in most societies.^[26]

All sexual partners from within the previous month of a person infested with pubic lice should be treated.^[13] Sexual contact should be avoided until both parties have been successfully treated. Individuals infested with pubic lice are at risk for other sexually acquired diseases and should be screened for such.

In the case of body lice, infested clothing and towels need to be washed in hot water and with a hot dryer; pediculicides are usually not needed. The infested individual should be counseled on proper hygiene, changing clothing at least once a week, and proper laundering of clothing.^[10]

For patient education information, see the Parasites and Worms Center, as well as Lice and Crabs.

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History

Patients may come to the attention of a health care provider after discovering lice or nits. Parents and teachers typically make the initial diagnosis of head louse infestation. In the case of head lice, a school nurse usually discovers infestation (routine nit inspections by school nurses are standard in many parts of the United States), or a generic letter is sent home to parents indicating that they should inspect their children for lice, and concerned parents bring their children to their health care provider or the local emergency department.

Pruritus is the most common symptom of infestation. Children often have trouble sleeping because of intense pruritus at night. Areas affected in head louse infestation include the scalp, the back of the neck, and postauricular areas. Scratching can cause secondary infection with bacterial sores. However, lice infestation may be asymptomatic, particularly if it is the first infestation and if the infestation is light.^[4]

Patients infested with P corporis experience nocturnal pruritus, particularly in the axillary, truncal, and groin regions, when the lice move from the clothing to the body to feed. The investigating physician should inquire about the patient's socioeconomic status and living conditions, as body louse infestation generally affects people of low socioeconomic status.

Adults infested with P pubis are usually sexually active and have groin and body hair involvement. Involvement with pruritus of the groin, axillae, eyelashes, or eyebrows can help differentiate P pubis infestation from head or body louse infestation. Parents of children infested with P pubis on the eyelids and/or eyebrows should be questioned about also being infested because the parents are usually the source of infestation.

Patients may describe associated features such as papules or wheals, indicating bite reactions.

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Physical Examination

A diagnosis of any type of pediculosis rests on the observation of eggs (nits), nymphs, or mature lice. Definitive diagnosis of active infestation requires detection of live lice. Nits are not diagnostic of an active infestation; however, if they are within 6 mm of the scalp, active infestation is probable. Viable nits may be found 8 or more inches from the scalp in warm climates.^[6] Nymphs and mature lice, although unable to hop or jump, can move rapidly through dry hair. Wetting the hair and using a fine-tooth "bug-busting" comb is useful to dislodge eggs and to remove live lice/nymphs. The use of a magnifying glass and knowing where to look for lice (based on the biology of each species) assists in the diagnosis.

Head lice can move rapidly, up to 23 cm/min.^[6] A helpful technique is to fasten a piece of transparent adhesive tape to the infested areas. Lice stick to the tape, which then becomes a convenient coverslip for a microscopic slide.

Mature lice are 3-4 mm long (approximately the size of a sesame seed), with an elongated body, 3 pairs of legs, and narrow anterior mouthparts. Wide crab-like bodies and claws distinguish pubic lice. Nits are approximately 1 mm in length, transparent, and flask-like in appearance. (See images of lice below.)

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The head louse, Pediculus humanus capitis, has an elongated body and narrow anterior mouthparts. Body lice look similar but lay their eggs (nits) on c....

View Image

The pubic louse, Pthirus pubis, is identified by its wide crablike body.

View Image

Nit on a hair. Note the thin, translucent cement surrounding the hair shaft. Photo courtesy of David Shum, MD Western University, London Ontario.

P humanus capitis

Manifestations of head louse infestation include scalp pruritus and, rarely, occipital lymphadenopathy and impetigo. Examination of the scalp reveals excoriations, dark specks of louse feces, nits, and adult lice. The heaviest infestation typically is in the retroauricular scalp.^{[4, 11]} For the diagnosis of P capitis infestation, the use of a louse comb is more efficient than direct visual examination of the scalp.^[27]

Pruritus commonly leads to excoriations, secondary bacterial infection, and enlargement of the posterior auricular and cervical nodes.^[11] A generalized exanthema (pediculid) similar to a viral exanthema or pityriasis rosea rarely accompanies louse infestation.^[6]

If excoriations are present, secondary infection (ie, impetigo) should be excluded and treated, if present. Bite reactions manifesting as pruritic red papules and/or wheals may be present, depending on the length of time since the blood meal.

Uncommonly, the hair of patients who are heavily infested and untreated is tangled with exudates, predisposing the area to fungal infection. This results in a malodorous mass known as a plica polonica. Numerous lice and nits are found under the matted hair mass.

Nits

Nits can be differentiated from dried hairspray and hair casts by attempting to separate the nit from the hair; hair casts and dried hairspray separate easily, while nits remain securely attached. If the physician remains unsure, a Wood lamp examination can be performed. Live nits are fluorescent white when illuminated with a Wood lamp; empty nits are fluorescent gray.

Eggs depend on body warmth to incubate, so nits are attached to the hair shafts just above the level of the scalp. Since human scalp hair grows at a rate of approximately 10 mm/month (0.37 mm/day), the distance of nits from the scalp can be used to estimate the duration of infestation. Nits found several millimeters from the scalp are nonviable empty egg cases. They indicate chronic infestation.

P humanus corporis

Physical examination findings in body louse infestation include multiple lesions from bites. Uninfected bites present as erythematous papules, 2-4 mm in diameter, with an erythematous base. Bites may be located anywhere on the body but tend to be concentrated in the axillae, groin, and trunk (ie, areas most often covered by clothing). Thus, the face, feet, and arms are not commonly affected. Body lice tend to avoid the scalp, except at the margins.

The finding of maculae cerulea is believed to be pathognomonic for infestation with lice. Maculae cerulea are blue-gray macules, which are actually a discoloration of the skin due to the insect's bite. Enzymes in the louse saliva are believed to induce the breakdown of human bilirubin to biliverdin, causing the change in skin color associated with maculae cerulea. An allergic reaction to louse bites may result in intense itching. Secondary infections due to excoriations may occur.

The diagnosis of body lice depends on the close examination of the patient's clothing for crawling lice and nits. The inner seams of clothing worn on the axillae and groin regions are common sites of residence. Occasionally, a body louse may be seen feeding or crawling on the skin. The number of body lice per host is usually approximately 10, although as many as 1000 lice can be present in clothing.^[11]

Body louse infestation is also known as vagabond disease, and individuals who have an infestation for many years can develop a condition termed vagabond skin. The skin becomes thickened and darkened after years of bites and subsequent rubbing and excoriations.^[10]

It is also important to examine for systemic illness that may be related to one of the vector-borne diseases associated with P corporis (see Complications).

P pubis

The primary symptom in patients with pubic lice is pruritus in the affected areas. Another clinical feature of pubic louse infestation is the presence of pathognomonic maculae cerulea (bluish-gray macules) secondary to bites.^[6] Crusts and pinpoint blood staining may also be noted on underwear.^[6]

Pubic hair is the most common site. Pubic lice and nits generally are plainly visible throughout the pubic hair. Because of the less-mobile nature of pubic lice, they are more likely to be found on affected areas clasping onto the hairs near the skin's surface.

The infestation may spread to hair around the anus, abdomen, axillae, chest, upper arms, eyebrows, and eyelashes.^[23] Rarely, facial hair (ie, beard, moustache, eyebrows, eyelashes) is a site of infestation.^[23] Scalp involvement is rare and is usually confined to the marginal areas. In adults, eyelash involvement in the absence of genital involvement is rare. In prepubertal children, the eyebrows and eyelashes are the typical sites of infestation. Eyelash nits are a manifestation of pubic louse infestation, not head louse infestation.

In children, P pubis infestation may be acquired from an infested parent or during sexual exposure or abuse; the child should be examined for signs of abuse.^[13]

Excoriations are common. Inguinal lymphadenopathy and axillary lymphadenopathy have also been reported with pubic louse infestation.

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Complications

There is no evidence indicating that any species of louse has the ability to transmit HIV. However, lice may carry S aureus and group A Streptococcus pyogenes on their surface and transmit these coagulase-positive pathogens to humans.

Pubic louse infestation is usually spread as a sexually transmitted disease (STD). Thirty percent of infested individuals may have other concurrent STDs (eg, HIV infection, syphilis, gonorrhea, chlamydia, herpes, genital warts).^[6]

Louse-borne disease is a potential problem whenever body lice spread through a population. The body louse, P humanus corporis, is a known vector of 3 major bacterial diseases, all of which have caused epidemics: louse-borne typhus, trench fever, and louse-borne relapsing fever. Evidence shows that some infectious organisms are altered by their arthropod vector and that disease manifestations may be vector-specific. For example, bartonellosis spread by a louse has different manifestations from bartonellosis spread by a flea or biting fly.^[28]

In a 2018 study of pathogens infecting 524 body lice collected from homeless persons in Algeria, several emerging pathogenic bacteria were found. B quintana, which causes trench fever, was found in 13.35% of lice specimens. In addition, Coxiella burnetii was found in 10.52% of specimens, Anaplasma phagocytophilum in 0.76%, and Acinetobacter species (Acinetobacter baumannii, Acinetobacter johnsonii, Acinetobacter bereziniae, Acinetobacter nosocomialis, Acinetobacter variabilis) in 46.94%. Neither Rickettsia prowazekii nor B recurrentis was detected.^[29]

Louse-borne typhus

The intracellular pathogen R prowazekii causes typhus. Typhus fever epidemics have consistently been related to times when overcrowded conditions and body louse infestations were prevalent. For example, mass migration, refugee camps, and times of war have been linked to body louse infestations and secondary epidemics of typhus. Human reservoirs of typhus also exist. Following natural disasters, body lice have the potential to spread rapidly throughout the population, causing great epidemics similar to those seen during World War I.

The illness begins with a high fever and progresses over hours to days to malaise, backache, headache, and myalgia. A petechial rash appears approximately on day 4, beginning in the flank and axillary regions and quickly spreading to the trunk and extremities. By the second week, the fever begins to wane, profuse sweating occurs, and convalescence ensues. CNS involvement during this period places the patient at high risk of mortality.

Trench fever

The extracellular pathogen B quintana causes trench fever and infective endocarditis. Although rarely fatal, this disease has been the cause of many epidemics, including trench fever during World War I.^[30] Infection in humans results from autoinoculation of louse feces into abraded or scratched skin. The infection has a 10- to 30-day latency period and results in a fever similar to that of typhus, with headache, myalgia, and pain in the back and the legs.

Louse-borne relapsing fever

The spirochete B recurrentis causes relapsing fever. This disease is highly fatal in malnourished persons. Although not common in North America, epidemics have been described during the last few decades in Asia, South America, Africa, and Europe.

Human infection with this spirochete occurs only when a crushed louse comes into contact with an abrasion or if it is ingested. The bacteria replicate in the louse hemolymph, not in the gut; therefore, no transmission occurs through the salivary glands or via the feces. The bacterial infection causes a high fever, headache, dizziness, and myalgia. Rash and sweating also appear and wane approximately on day 5. As the name indicates, this fever often returns several times.

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Approach Considerations

Because the diagnosis of infestation requires identification of a live louse and/or a viable nit, examining suggestive particles under the microscope confirms the diagnosis. Cellulose tape can be applied over an infested area to pick up lice and place them on a microscopic slide to be examined. A Wood lamp examination of the area considered to be infested shows yellow-green fluorescence of lice and nits. Dermoscopy can be used to reliably differentiate nymph-containing eggs from empty cases or pseudonits.^[31]

Infestation with P pubis is an STD, and 30% of these patients have an additional STD. Thus, it is appropriate to screen these patients for other STDs, including human immunodeficiency virus (HIV), syphilis, gonorrhea, chlamydia, genital herpes, and trichomoniasis.

Scrapings for a fungal culture can be collected if dermatophyte infection is in the differential diagnoses. This is useful when the diagnosis is unclear (ie, no nits or lice have been identified).

Properly evaluating persons who have been raped is essential. Evidence should be collected in such a way as to avoid contamination and to ensure a legal chain of custody. Human DNA can now be identified in the blood present in a pubic louse. Lice recovered after an attack have the potential to provide evidence valuable in securing a conviction.

Histology is rarely required for diagnosis. Examination of a bite shows intradermal hemorrhage and a deep, wedge-shaped infiltrate with many eosinophils and lymphocytes.

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Approach Considerations

Treatment of pediculosis has 2 aspects: medication and environmental control measures. Increasing emphasis is being placed on understanding the life cycle of lice in order to provide effective treatment.

Not all treatment preparations are ovicidal. For weakly ovicidal or non-ovicidal pediculicides, routine retreatment is recommended typically 7-9 days after the first treatment. For strongly ovicidal pediculicides, retreatment is recommended only if live (ie, crawling) lice are still present after treatment.^[4] Retreatment should ideally occur after all eggs have hatched but before new eggs are produced.^[4] It is extremely important to use medications as directed to ensure total eradication of the lice through their life cycle. Different head lice medications should not be used at the same time. In addition, all infested persons in a household and their infested close contacts and bedmates should be treated at the same time. If an approved treatment has been properly applied and live lice are still present, a full course of treatment of another class of topical medication should be used.^[32] The Canadian Pediatric Society recommends pyrethrins and permethrin as first-line treatments and isopropyl myristate and dimethicone as second-line therapies in cases of treatment failure.^[32]

Head lice have been found on hats, scarves, brushes, combs, hair accessories, linens, towels, and stuffed animals. Since exposure to these fomites could result in infestation, it is recommended that such items used by the infested person within 2 days prior to pediculicide treatment be machine washed with hot water and dried with hot air since the lice and eggs are killed after 5 minutes of exposure to temperatures greater than 53.5°C (128.3°F).^[4] Items that cannot be laundered can be dry-cleaned or sealed in a plastic bag for 2 weeks.^[4] The floors and furniture should be vacuumed in order to remove hairs from an infested individual, which might have been shed with viable nits attached.^[4] Children should also be educated not to share combs, brushes, hair accessories, and towels.^[4]

In the treatment of body lice, medications are less essential than environmental measures. Patients with body lice should have infested clothing, bedding, and towels laundered with hot water (at least 130°F) and then dried in a dryer using a hot setting.^[10] For items that cannot be washed in a washing machine, the CDC recommends dry-cleaning or sealing and storing for 2 weeks in a plastic bag.^[13] If the patient maintains hygiene with regular appropriate laundering of clothing, changes into clean clothing at least weekly, and avoids the sharing of clothing, beds, bedding, and towels used by other infested individuals, pediculicides are generally not required. If hygiene cannot be maintained, treatment with a pediculicide used to treat head lice may be necessary. Fumigation or dusting with chemical insecticides is occasionally needed to control and prevent spread of louse-bourne infections.^[10]

Related clinical guideline summaries include the following:

• British Association for Sexual Health and HIV -United Kingdom National Guideline on the Management of Phthirus pubis infestation^[33]
• CDC: Parasites - Lice^{[4, 10, 13]}

• References

Pesticides

Various topical pediculicidal agents are available for the treatment of head and pubic lice. Pyrethrins with piperonyl butoxide (A-200, Pronto, R&C, Rid, Triple X), permethrin 1% lotion (NIX), dimethicone (LiceMD Pesticide Free) and mineral oil–based products (Nix Ultra shampoo) are available over the counter; malathion 0.5% (Ovide), lindane 1% shampoo and lotion, ivermectin 0.5% lotion (Sklice), spinosad 0.9% (Natroba), and benzyl alcohol 5% lotion (Ulesfia) are prescription agents.

Pubic lice can be treated with over-the-counter permethrin 1% lotion and a mousse containing pyrethrins and piperonyl butoxide (RID foam).^[13] Lindane shampoo (1%) is FDA approved as second-line treatment of pubic lice (crabs). Dimethicone, malathion, ivermectin, spinosad, and benzyl alcohol are currently not US Food and Drug Administration (FDA) approved to treat pubic lice.

Pyrethrin and permethrin kill live lice, but not unhatched eggs. A second treatment 9-10 days after the first treatment is recommended in order to kill any newly hatched lice before they can produce new eggs. Pyrethrin is derived from chrysanthemums and is approved for use in children aged 2 years or older. It should not be used by individuals who are allergic to chrysanthemums or ragweed. Piperonyl butoxide has been added to pyrethrin products to enhance efficacy and to minimize the potential for resistance.^[6]

Examples of pyrethrin/piperonyl butoxide combination formulations include A200 Maximum Strength, A200 Lice Control, A-200 Lice Treatment, A200 Time-Tested Formula, A200 Lice Killing Shampoo, Good Sense Lice Killing Shampoo, Step 1, Lice Treatment gel, Lice-X, Licide shampoo, Leader Lice Solution, Medi-Lice maximum strength, Pronto Plus Lice Killing shampoo, Pronto Maximum strength, Pronto Lice Kill System, Pyrinex, Pyrinyl II, Pyrinyl Liquid Shampoo, Pyrinyl Liquid, R&C Lice Treatment Kit, RID Pediculicide, RID shampoo/spray kit, RID foam, RID gel, and Tisit shampoo.

Permethrin is a synthetic pyrethroid similar to the naturally occurring pyrethrins from the chrysanthemum flower. Permethrin 1% lotion is approved for use in children aged 2 months or older.

Malathion is an organophosphate that has proven to be more ovicidal than permethrin and has a higher lethal effect and decreased frequency of re-infestation, if used properly. It is approved for use in individuals aged 6 years or older. Malathion 0.5% lotion (Ovide) is flammable and should not be used in the presence of hot hair care products or near individuals who are smoking. If lice are still present at 7-9 days, a second treatment should be given.

Lindane is an organochloride that should be reserved for treatment of individuals in whom alternative treatment has failed or is intolerable.^{[4, 13]} Seizures may result from abnormal absorption or gross overuse of the product. It should not be used in patients with a defective cutaneous barrier, premature infants, people with seizure disorders, or pregnant or breastfeeding women.^[13] Many authors recommend that it not be used in elderly persons or people who weigh less than 110 pounds.^[13] Because of safety concerns, retreatment should also be avoided.

In February 2012, topical 0.5% ivermectin (Sklice) was approved in the United States as a single-dose, 10-minute application without the need for nit combing in individuals aged 6 months or older. Although it is not ovicidal, it appears to prevent nymphs from surviving.^[4]

Spinosad 0.9% suspension (Natroba) was approved by the FDA in 2011 for the treatment of head louse infestation in patients aged 6 months and older. The product is applied to dry hair as a cream rinse, left in for 10 minutes, and then shampooed out. Spinosad is derived from the soil bacterium Saccharopolyspora spinosa. It has ovicidal activity. Retreatment is necessary only if live (ie, crawling) lice are seen one week after the first treatment. A phase 3 study showed that spinosad was more efficacious in clearing head lice than permethrin (P< 0.001).^[34]

• References

Occlusive and Nonpesticide Therapy

Agents that work by clogging the respiratory spiracles of lice offer an alternative to neurotoxic pediculicides.^{[35, 36]} This is the mechanism of action of benzyl alcohol lotion 5% (Ulesfia), which is approved by the FDA for treatment of head lice in patients aged 6 months or older; the benzyl alcohol inhibits lice from closing their respiratory spiracles, allowing the lotion to obstruct the spiracles. The lotion is given in 2 applications 1 week apart for 10 minutes; it needs to be applied twice because it kills only lice, not nits. Benzyl alcohol lotion may be an easier and safer alternative to lindane and malathion. Because its mechanism of action is physical rather than chemical, development of resistance should not be a concern. In clinical studies, more than 75% of those treated with benzyl alcohol lotion became lice-free.^[37]

Isopropyl myristate (Resultz) is available in Canada and Europe and received FDA approval for treatment of head lice in 2017, although it is not yet marketed in the United States. It is a non-insecticide–based drug that contains isopropyl myristate, an ingredient commonly used in cosmetics. Its mode of action is a mechanical process that weakens the waxy shell of lice, resulting in internal fluid loss and dehydration.^[38] It requires only a 5-minute application time.

Dimethicone is available in the United States as an over-the-counter liquid gel formulation containing 100% dimethicone (LiceMD Pesticide Free) and in Canada and Europe as NYDA dimeticone 100 cSt Solution, 50% w/w. Dimethicone replaces the air in the breathing system of lice, nymphs, and nits (eggs); thickens quickly; and then causes suffocation and death.^[39] Dimethicone can be used in individuals aged 2 years or older. LiceMD should be applied for 10 minutes; the hair is then combed with the included lice comb while LiceMD is still in the hair. Afterward, the hair is shampooed with regular shampoo and warm water. In contrast, NYDA is left on for 8 hours. After it has been on for 30 minutes, the dead lice and eggs should be combed out using the included NYDA lice comb. The hair should not be washed for 8 hours. Although NYDA should eliminate head lice after one application, the product monograph recommends a second application after 8-10 days to ensure complete removal of head lice. A small open-label US study showed that, after 14 days following 1-3 treatments with the 100% dimethicone gel regimen, 96.5% were free of live lice and 80.7% of viable eggs.^[40] In another study, NYDA was found to be superior to 1% permethrin lotion.^[41]

The mineral oil–containing shampoo Nix Ultra shampoo suffocates lice. It also contains fragrance, laureth-4, MIPA laureth sulfate, and propylene glycol. It should be applied to dry hair for 10 minutes. Afterward, water is added to the hair, which is lathered and rinsed off. It is used in infested individuals aged 1 year or older and is not intended for pubic lice.^[42]

Other occlusive therapy techniques, such as vinegar, mayonnaise, petroleum jelly, olive oil, butter, isopropyl alcohol, and water submersion as long as 6 hours, have been advocated, but most have not been scientifically evaluated. However, a dry-on, suffocation-based pediculicide (DSP) lotion was found to be effective in open trials.^[35]

Various botanical agents have been used. Essential oils demonstrate variable efficacy and may be contact allergens.^[43] In general, the evidence supporting their efficacy is of poor quality.^{[44, 45]} Monoterpenoids are promising agents.^{[44, 46, 39]}

• References

Oral Agents

Oral anthelmintics, including ivermectin, levamisole, and albendazole,^{[47, 48]} have been found to be effective against head louse infestation, but are not approved by the FDA to treat lice. Oral ivermectin has also been successfully used to treat pubic and head lice, but is not FDA approved for these conditions.^[13] Administration should be repeated in 7-10 days to kill lice emerging from nits that may have survived the first treatment. Trimethoprim-sulfamethoxazole was initially reported to be effective; however, controlled studies have shown only minimal efficacy.

Resistance

Resistance of lice to the most commonly used medications for treatment of infestation (permethrin and pyrethrin) is increasing.^{[49, 50, 51, 52]} Resistance has been reported in the United States as well as among countries in South America and Europe.

A possible mechanism of drug resistance development includes mutations of target enzymes (eg, acetylcholinesterase) so that the enzymes no longer bind the organophosphate permethrin with the same affinity. Another possible mechanism is increasing the metabolism of the insecticides through an increase in monooxygenase enzyme activity, turning them into harmless compounds before they can cause damage.^{[53, 54, 55, 56, 57, 58]}

Resistance to malathion has been reported in the United Kingdom; however, the UK formulation contains only malathion, whereas, in the US formulation, malathion is coupled with isopropyl alcohol and terpineol.

• References

Emerging Agents

Abametapir 0.74% lotion is a metalloproteinase inhibitor that is not yet commercially available. It is used as a single 10-minute treatment for head lice in individuals aged 6 months and older. Two studies (n=704) showed that 81.5% of treated individuals were free of lice 2 weeks after a single application versus 49.1% treated with vehicle (P< 0.001). The efficacy of vehicle is higher than what is typically seen in studies and deserves further study. Nonetheless, inhibition of metalloproteinases is a novel treatment approach that might be available in the future.^[59]

• References

Treatment of Head Louse Infestation

Medicated lotions or shampoos may be used to eliminate head lice. Empiric treatment of all contacts is not needed; only contacts with confirmed infestation should also be treated; however, prophylactic treatment of bedmates of an infested individual is prudent.^[4] Re-treatment after 7-10 days is recommended with many agents to eradicate any lice that hatched from nits after the initial treatment. It is important to read the instructions before applying the pediculicide to ensure that the medication has been left on for the appropriate amount of time and washed off properly.

Hair conditioner and combination shampoo/conditioner products should not be used prior to pediculicide application, and the hair should not be re-washed for 1-2 days after the pediculicide is removed.^[4] Hair conditioners may prevent binding of the pediculicides to the hair shafts. After treatment, the infested individual should put on clean clothing.

Mechanical removal and shaving

The CDC, American Association of Pediatrics, and National Association of School Nurses recommend that school “no-nit” policies be discontinued.^[4] Nits are cemented to hair shaft and unlikely to be successfully transferred to others.^[4] Nonetheless, some schools still have a no-nit policy. In such cases, nits can be mechanically removed with fine-tooth combs. Soaking the hair in a solution of equal parts water and white vinegar and then wrapping the wet scalp in a towel for at least 15 minutes may facilitate removal. Commercial products include an 8% formic acid preparation (GenDerm Step 2) and an enzymatic nit remover (Clear).

Most studies have shown that mechanical removal alone (ie, wet-combing every 2-3 days for a minimum of 2 weeks) is not as effective as mechanical removal combined with a pediculicide.^[57] Proper treatment with medication is advised.

Shaving is effective but is usually not necessary or socially acceptable. However, in resistant disease, it may be a consideration.

Environmental eradication

Since head lice can sometimes be spread by sharing hats, hair bands and accessories, and towels that have been in contact with an infested person’s hair, any object that the infested child or parent has come into contact with should be considered a potential fomite. It may be beneficial to launder potential fomites (eg, towels, pillowcases, sheets, hats, toys) in hot water, followed by machine drying using the hottest cycle. Temperatures exceeding 130°F (55°C) for more than 5 minutes kill eggs, nymphs, and mature lice. Items that are not machine washable may be placed in a dryer at high heat for 30 minutes. Dry cleaning may be an effective alternative.

Because adult lice cannot survive for long if separated from a host and because eggs hatch in 6-10 days and nymphs die without a blood meal, carefully sealing potential fomites in plastic bags for 2 weeks can be effective. This technique works well for objects such as stuffed animals that do not tolerate laundering or dry cleaning. Vacuuming selected areas of the home, such as couches used by infested patients, is recommended by some as an adjunctive control measure.

Combs and hair brushes can be discarded or soaked for at least 5 minutes in very hot water (>130°F [>55°C]).

Chemical insecticide sprays used in the home environment have not been shown to be effective in the control of head lice.

Providing education to children about the sharing of hats, combs, and hair-ties is also prudent. Giving children separate areas to store their belongings in the classroom may help prevent the spread of lice.^[4]

• References

Treatment of Pubic Louse Infestation

Many of the pediculicides used for head louse infestation are also used for pubic louse infestation. In addition, P pubis infestations of the eyelashes are treated with occlusive therapies.

Pubic lice can be treated with over-the-counter permethrin 1% lotion and a mousse containing pyrethrins and piperonyl butoxide (RID foam).^[13] Although malathion lotion 0.5% has been used to treat pubic lice, it is currently not FDA approved to treat pubic lice. Lindane shampoo causes neurotoxicity and should not be used as a first-line agent; it should be reserved for cases of resistance or when other medications are not tolerated.^[13]

Sexual partners of a person infested with pubic lice should be informed that they are at risk for infestation and should be treated if the sexual contact occurred within a month prior to diagnosis.^[13] Infested persons should avoid sexual contact until they have both been successfully treated.^[13] If live lice are still found, treatment should be repeated in 9-10 days.^[13] Individuals with pubic lice should also be checked for other STDs.^[13]

The infested areas should be washed and towel dried. The instructions on the pediculicide should be read and carefully followed. The pubic hair and other infested areas (excluding eyebrows and eyelashes) should be thoroughly saturated with the pediculicide. After leaving on the pediculicide for the recommended time, it should be removed according to the package instructions. Clean clothing and underwear should be put on after treatment. Towels, clothing, and bedding used within the 2-3 days before treatment should be machine-washed in water of at least 130°F and dried in a hot dryer.^[13] Items that cannot be laundered should be dry-cleaned or stored in a sealed plastic bag for 2 weeks.^[13]

Ophthalmic-grade petrolatum (2-4 times a day for 10 days) is often used, with good results, as an asphyxiant for eyelash infestation.^[13] The petrolatum covers the lice and their nits, preventing respiration. The dead lice are removed mechanically, usually with tweezers. Mercuric oxide ointment is also useful in the treatment of eyelash infestation with P pubis. If only a few live lice and nits are on eyelashes and/or eyebrows, it may be possible to remove them with a nit comb or fingernails.^[13]

Fluorescein dye strips, which are used in the diagnosis of corneal abrasions, may be used in combination with white petrolatum. The strips are applied to the eyelashes for 3 nights.

• References

Treatment of Body Louse Infestation

Use of a pediculicide is usually unnecessary with P humanus corporis infestation because the lice live on the clothing. Treatment of clothing and bed linens includes laundering in hot water, ironing with a hot iron, or drying in a hot dryer. Dry cleaning is also effective for killing lice and their nits on clothing.

Education about hygiene and accessibility to laundering facilities are important in preventing the spread of body lice and re-infestation.^[58]

Topical agents can be applied to clothing, especially the seams. Published data suggest that permethrin spray can help prevent body louse re-infestation.

In cases of heavy pediculosis, treatment of the body with a pediculicide shampoo or lotion may be beneficial, especially if the patient also has confirmed or suspected concomitant head or pubic louse infestation. Oral ivermectin 12 mg given as 3 doses 7 days apart has also been shown to be effective in a cohort of homeless men.^[21] None of these agents is currently labeled or marketed for treatment of body lice in the United States. In some cultures, monkeys are used as patient nit pickers to groom the hair and to remove adult lice and nits.

• References

Treatment of Contacts

The treatment of infested family members, friends, and/or other close contacts at the same time as the infested individual is important in helping to prevent further spread of head lice and in preventing re-infestation. In the case of pubic lice, all sexual partners from within the previous month should be treated.^[13]

Patient education regarding treatment of contacts is essential. Parents with children who are infested should be advised to treat all infested family members with a pediculicide and to provide environmental fomite control.

Education about hygiene and accessibility to laundering facilities are important in preventing the spread of body lice and re-infestation.^[58]

• References

Deterrence and Prevention

To prevent head lice re-infestation, all household members and contacts of a patient should be examined and treated at the same time if infested. Bedmates should also ideally be treated even if they do not show evidence of infestation. Washing combs, brushes, and other fomites may reduce re-infestation.

Children should be educated not to exchange or use another child's hat, comb, or brush. Some parents choose to extend this prohibition to use of "common" headwear such as is available in the dress-up area of various play spaces or public libraries.

Once an infestation has been identified and treated, ongoing vigilance with close, direct visualization of hair and scalp at periodic intervals is recommended.

Sexual partners within the previous month of individuals with pubic lice should be treated at the same time as the infested individual. Sexual contact should be avoided between sexual partner(s) until they have both been successfully treated.^[13] Bed linens, towels, and underwear and clothing should be washed at the same time as treatment with medication.

Prevention of re-infestation with body lice can be accomplished by ensuring that infested clothing, bedding, and towels were appropriately laundered with hot water (at least 130°F) and then dried in a dryer using a hot setting to destroy the lice, improving the individual’s hygiene, regularly laundering clothing, and changing to clean clothing at least weekly.^[10]

• References

Medication Summary

The goal of therapy is to eliminate lice and eggs. Chemical pediculicides are the mainstay of pharmacotherapy. With most medications, treatment should be repeated in 7-10 days (the time needed for the eggs to hatch) because nits are less effectively killed than adults. Eyelash infestation can be treated effectively with petrolatum ointment (eg, Vaseline).

• References

Pyrethrins/piperonyl butoxide (A-200 Maximum Strength, LiceMD Complete, Pronto Plus Complete Lice Removal System, RID Lice Killing)

• Dosing, Interactions, etc.

Clinical Context:  Pyrethrins are first-line treatment in head, pubic, and severe body louse infestation. They are available as a mousse or shampoo. These agents stimulate the parasitic nervous system, causing seizures and death of parasites. This is an older OTC agent that still appears to be effective. It lacks the residual action of permethrin and is more likely to require repeated applications. Pyrethrin products are contraindicated for patients with contact allergy to ragweed, turpentine, or chrysanthemums. Piperonyl butoxide has been added to pyrethrin products to enhance efficacy and to minimize the potential for resistance.

Pyrethrin/piperonyl butoxide products should be applied to dry hair and left on for 10 minutes, then rinsed off. If live lice are noted after 9 days, a second application is recommended.

• References

Permethrin (Nix, Elimite)

• Dosing, Interactions, etc.

Clinical Context:  Permethrin is the drug of choice recommended by most authorities as the first line of treatment in head, pubic, and severe body louse infestation, especially for infants older than 2 months and small children. This agent is a neurotoxin that causes paralysis and death in ectoparasites. It is more effective than crotamiton in treating symptoms and reducing the chances of secondary bacterial infection. One benefit of permethrin is a residual effect in the hair for several hair wash cycles. The product should be applied to recently shampooed hair that is damp and left on for 10 minutes then rinsed off. A second application in 7 days is recommended.

Resistance probably has developed in many areas, however. Physicians in some countries select different pediculicides on a rotating basis to discourage development of resistance.

Permethrin is very effective in killing adult lice and nymphs but is not as effective in killing nits (eggs). Patients should wash hair with a nonmedicated shampoo.

An over-the-counter (OTC) 1% concentration (permethrin lotion) may be insufficient for treatment of pubic lice and for some cases of head lice. The 5% prescription preparation marketed for scabies (Elimite) may be more effective in some cases. Strict adherence to the treatment regimen is essential.

• References

Malathion (Ovide)

• Dosing, Interactions, etc.

Clinical Context:  Malathion is approved by the FDA for the treatment of head lice. It is an irreversible cholinesterase inhibitor that is hydrolyzed and therefore detoxified rapidly by mammals but not by insects; it is both ovicidal and pediculicidal. It binds to hair and provides some residual protection after therapy. Malathion is available as a 0.5% lotion for use in individuals aged 6 years or older. Malathion 0.5% lotion (Ovide) is flammable and should not be used in the presence of hot hair care products or near individuals who are smoking.

• References

Ivermectin topical (Sklice, Soolantra)

• Dosing, Interactions, etc.

Clinical Context:  Topical ivermectin causes parasite death by selective, high-affinity binding to glutamate-gated chloride channels located in invertebrate nerve and muscle cells. It is a topical pediculicide that treats head lice with a single 10-minute application without nit combing in adults and children aged 6 months or older.

• References

Ivermectin (Stromectol)

• Dosing, Interactions, etc.

Clinical Context:  Ivermectin binds selectively with glutamate-gated chloride ion channels in invertebrate nerve and muscle cells, causing cell death. Its half-life is 16 hours; it is metabolized in the liver. Ivermectin is the drug of choice for onchocerciasis and strongyloidiasis; it has been shown to be effective against pediculosis and has been used in mass epidemics. It is not effective against nits.

Few serious adverse effects have been reported when ivermectin is used to treat lice or scabies. This agent is not associated with evidence of selective fetotoxicity in pregnant women inadvertently exposed (based on limited data). Limited animal data also fail to show evidence of selective fetotoxicity.

Ivermectin is available in the United States as an oral 6-mg pill marketed for treatment of Strongyloides. Physicians have used this drug for lice and scabies (off-label use) in cases where such therapy was in the best interest of patients and conventional therapy failed. Health care providers in the United States are encouraged to read the FDA statement concerning off-label use of approved drugs, which appears in the Physicians' Drug Reference. 

• References

Lindane

• Dosing, Interactions, etc.

Clinical Context:  Lindane stimulates the nervous system of parasites, causing seizures and death. It is a chlorinated insecticide available as 1% lotion, cream, and shampoo. It is a second-line treatment if other agents fail or are not tolerated.

In March 2003, the FDA issued a public health advisory warning of increased risk of adverse effects of lindane treatment in persons who are young, small, or elderly. Heightened caution should be exercised if lindane is used in such patients.

• References

Benzyl alcohol (Ulesfia)

• Dosing, Interactions, etc.

Clinical Context:  Benzyl alcohol inhibits lice from closing their respiratory spiracles, allowing the lotion to obstruct the spiracles, which ultimately results in asphyxiation. It does not elicit ovicidal activity. This product contains 5% benzyl alcohol and is approved by the FDA for treatment of head lice in patients aged 6 months or older. The lotion is applied twice (initially and 1 week later) for 10 minutes.

• References

Spinosad (Natroba)

• Dosing, Interactions, etc.

Clinical Context:  Spinosad causes neuronal excitation in insects, followed by hyperexcitation, paralysis, and death. It is a pediculicide indicated for topical treatment of head lice infestations in individuals aged 4 years or older. This agent is available as a 0.9% suspension. The product is applied to dry hair as a cream rinse, left in for 10 minutes, and then shampooed out.

• References

Isopropyl myristate (Resultz)

• Dosing, Interactions, etc.

Clinical Context:  Dissolves the wax covering on the exoskeleton of head lice. This in turn causes dehydration and subsequent death. Owing to the physical mode of action, lice are unlikely to develop resistance to this product.

• References

Mineral oil topical (Nix Ultra Shampoo)

• Dosing, Interactions, etc.

Clinical Context:  Mineral oil can be used as a safe and effective alternative to insecticide-based pediculicides. There is less risk for development of resistance because of the physical mode of action of mineral oil.

• References

Dimethicone topical

• Dosing, Interactions, etc.

Clinical Context:  A highly effective head lice treatment, with a low risk of adverse effects. May serve as a potentially less toxic and an alternative to resistance-prone pesticide-containing products.

• References

Class Summary

Treatment options include pyrethrins with piperonyl butoxide (A-200, Pronto, R&C, Rid, Triple X), permethrin 1% lotion (NIX), permethrin 5% (approved to treat scabies but occasionally used to treat lice; Actin, Elimite), malathion 0.5% (Ovide), lindane 1% shampoo and lotion, ivermectin 0.5% lotion (Sklice), spinosad 0.9% (Natroba), benzyl alcohol 5% lotion (Ulesfia), isopropyl myristate (Resultz), dimethicone (LiceMD Pesticide Free, NYDA) and a mineral oil–based shampoo (Nix Ultra shampoo).

Parasite biochemical pathways are different from the human host; thus, toxicity is directed to the parasite, egg, or larva. Mechanism of action varies within the drug class. Antiparasitic actions may include the following:

- Inhibition of microtubules causing irreversible block of glucose uptake

- Tubulin polymerization inhibition

- Depolarizing neuromuscular blockade

- Cholinesterase inhibition

- Increased cell membrane permeability, resulting in intracellular calcium loss

- Vacuolization of the schistosome tegument

- Increased cell membrane permeability to chloride ions via chloride channels alteration

- Inhibition of metalloproteinases

Resistance to pediculicides has increased over recent years. Therapeutic agents can be rotated to slow the emergence of resistance.

• References