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eMedicine Specialties > Dermatology > Parasitic Infections

Lice

Dirk M Elston, MD, Director, Department of Dermatology, Geisinger Medical Center

Updated: Sep 4, 2009

Introduction

Background

Louse infestation remains a major problem throughout the world.1 Head louse infestation among school children has reached epidemic proportions in many parts of the United States. Body lice are important vectors of disease. This article discusses pediculosis capitis (head lice), pediculosis corporis (body lice), and pediculosis pubis (pubic lice, crabs). All medical information must be interpreted in the context of the patient and the clinical situation. This article provides general medical information; it is not intended to be a guide for the treatment of any specific patient.

The head louse, <em>Pediculus humanus capitis,</e...

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




The pubic louse, <em>Pthirus pubis,</em> is ident...

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


Pathophysiology

Louse infestation is prevalent throughout the animal kingdom. Mallophaga, or chewing lice, are common pests of birds and domestic animals. Humans sometimes are affected as accidental hosts. All 3 types of human lice belong to the order Anoplura, the sucking lice. 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.2,3 All 3 types take periodic blood meals by piercing the skin of their host with narrow anterior mouthparts. The 3 types of human lice are Pediculus humanus capitis (head louse), Pediculus humanus corporis (body louse), and Pthirus pubis (crab louse).

Evidence exists to show that 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. This may explain, in part, the varying syndromes caused by closely related species of Bartonella organisms (eg, acute Oroya fever, Peruvian bacillary angiomatosis, bacillary angiomatosis of AIDS, bacillary peliosis hepatis, catscratch disease, infective endocarditis).4

Frequency

United States

Head louse infestation is a major problem, especially in urban areas. Major infestations are seen in all socioeconomic groups. The social stigma attached to louse infestation facilitates the spread of infestation. Families who are affected by such an infestation are reluctant to share information with their neighbors. Individual children are treated, but the community fails to address the infestation as a community-wide issue. School-wide and community-wide programs to eradicate lice are necessary to halt their continued spread.

In the United States, pubic lice generally are spread as a sexually transmitted disease (STD). Pubic louse infestation serves as a marker for other STDs, which may have been acquired simultaneously. Body louse infestation in the United States mainly affects the homeless. Body lice are vectors for Bartonella quintana, an agent of infective endocarditis among the homeless and the cause of many thousands of cases of trench fever and epidemic typhus during World War I.5 The organism that caused trench fever persists among the homeless in urban areas, spread from person to person by lice. Human reservoirs of typhus also exist in the population. 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.

International

Head lice are a major problem throughout the world. Black populations appear somewhat resistant to P humanus capitis infestation, although they may develop scalp infestation by P pubis. The patterns of pubic and body louse infestation throughout the world mimic those in US refugee populations, which commonly have a tremendously high rate of louse infestation. Louse-borne disease is a potential problem whenever body lice spread through a population.

Mortality/Morbidity

Morbidity results from the severe itching that is caused by lice infestation. Mortality may occur from infectious diseases transmitted by the body louse.

Race

Blacks have a lower incidence of infestation by the head louse but may experience scalp infestation by P pubis.

Sex

Males and females are equally at risk for lice infestation.

Age

Lice affect all age groups. Body lice are indiscriminate in regard to the age of their host. Head lice are common in young school children but much less common after puberty. Pubic lice infest body and pubic hair. Prepubescent scalp infestation by P pubis may occur in individuals with short, thick, curly scalp hair.6

Clinical

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, or a generic letter is sent home to parents indicating that they should inspect their children for lice.
  • Pruritus is the most common symptom of infestation. Children often have trouble sleeping because of intense pruritus at night when lice are feeding.
  • Children with infestation may be asymptomatic. Routine nit inspections by school nurses are standard in many parts of the United States.
  • Both combing and visual inspection have value in the diagnosis of lice infestation.7

Physical

Manifestations of head louse infestation include scalp pruritus, occipital lymphadenopathy, and impetigo. Examination of the scalp reveals excoriations, dark specks of louse dung, nits, and adult lice. The heaviest infestation typically is in the retroauricular scalp. Pruritus commonly leads to excoriation, secondary bacterial infection, and regional lymph node enlargement. A generalized exanthem rarely accompanies louse infestation (pityriasis rosealike pediculid).

  • Pediculosis capitis: Although head lice may be found on any part of the scalp, they most commonly are found in the retroauricular scalp. Eggs depend on body warmth to incubate, so nits are attached to the hair shafts just above the level of the scalp. Human scalp hair grows at a rate of approximately 0.37 mm/d. Nits found several millimeters from the scalp are nonviable empty egg cases. They indicate chronic infestation.
  • Pediculosis corporis: Body lice infest the seams of clothing and take blood meals at night. Nits are found in the seams, not on human hairs. Maculae ceruleae, the hemosiderin-stained purpuric spots where lice have fed, suggest the diagnosis of body louse infestation.
  • Pediculosis pubis: Pubic lice and nits generally are plainly visible throughout the pubic hair, extending onto adjacent hairy areas of the body. Eyelash nits are a manifestation of pubic louse infestation, not head louse infestation.

Differential Diagnoses

Impetigo
Scabies

Other Problems to Be Considered

True nit infestation must be distinguished from hair casts (pseudonits). Hair casts are ringlike remnants of the inner root sheath of the hair follicle. They are amorphous and freely moveable along the hair fiber.

Many scalp conditions can cause pruritus. Seborrheic dermatitis presents with erythema and scale. It affects the scalp, eyebrows, nasolabial folds, and central chest. Acne necrotica presents with folliculitis with superficial pustules within scattered hair follicles. It is extremely pruritic, and patients pick at the lesions. Secondary follicular excoriations typically are noted on examination.

Free-living primitive psocid lice feed on decaying matter in leaves, old books, and animal habitats. They may cause human scalp infestation when children visit a library or doghouse that is infested. Psocids have large heads with massive jaws and are distinguished easily from Anoplura lice.

Workup

Laboratory Studies

  • The identification of adult lice or nits is diagnostic. The presence of bruiselike bites or dark specks of louse dung suggest the diagnosis of louse infestation.
  • Lice move rapidly. A helpful technique is to fasten a piece of transparent adhesive tape to the infested areas. Lice stick to the tape. The tape then becomes a convenient coverslip for a microscopic slide.
  • Mature lice are approximately the size of a sesame seed (3-4 mm) with an elongated body, 3 pairs of legs, and narrow anterior mouthparts. Wide crablike bodies and claws distinguish pubic lice. Nits are approximately 1 mm in length, transparent, and flasklike in appearance.
  • Nits and lice fluoresce with a Wood light.
  • 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 amount of blood present in a pubic louse. Lice recovered after an attack have the potential to provide evidence valuable in securing a conviction.

Other Tests

  • Evaluate individuals with pubic lice for other sexually transmitted diseases (STDs). Evaluations may include serologic testing for syphilis and AIDS, culture for gonorrhea, and antigen testing for chlamydia.

Histologic Findings

Louse bites demonstrate intradermal hemorrhage and a polymorphous wedge-shaped infiltrate rich in eosinophils.

Treatment

Medical Care

Reinfestation occurs unless louse infestation is addressed as a community-wide problem. Management must include examination of all individuals exposed and treatment of all those who are infested. 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 lice infestations.8

Fomite control is essential. Hats lined up on pegs or placed in adjacent cubbyholes provide an avenue for spread of the infestation. Cubbyholes can be sprayed with a permethrin spray or other insecticide, but the most effective method is for each child to "ground his or her clothing" (ie, hat, coat, scarves) under each individual chair or desk. Common cloakrooms may suggest an antiquated charm, but they should be viewed as merely antiquated and a site for spread of the infestation.

Combs, brushes, and headbands should not be shared. Shaving of hair is effective but not socially acceptable in most societies.9 Young nits do not have a nervous system and are immune to neurotoxic pediculicides.

Nit combs are provided with many products. Metal nit combs are more effective and can be purchased through the Internet. Chemical nit removers, such as distilled white vinegar and formic acid (GenDerm Step 2), can be helpful. An enzymatic nit remover (Clear) is also available, but the major action of each of these products may be to make combing easier. Little evidence indicates that they actually dissolve the nit sheath that attaches the nit to the hair shaft. Advances in topical therapy will include lotions that specifically dissolve the attachment of the nit to the hair.10

For body lice, when feasible, removal of infested clothing is all that is required. Laundering in hot water, ironing with a hot iron, or drying in hot dryer also is effective. In mass epidemics, other treatments may be more practical. Body lice may respond to oral or topically applied pediculicides. None of these agents currently is labeled or marketed for treatment of body lice in the United States. Topical agents should be applied to clothing, especially the seams. Published data suggest that permethrin spray can help prevent body lice reinfestation.
In some cultures, monkeys are used as patient nit pickers to groom the hair and to remove adult lice and nits.

Resistance to pediculicides is emerging.11,12,13,14 In the United States, malathion retains the best efficacy among chemical pediculicides at present. Permethrin appears to have a wide margin of safety, although some data suggest a possible connection between insecticides and leukemia.15,16 Better agents that work via clogging of respiratory spiracles rather than via neurotoxicity would be valuable additions to the armamentarium.17,18 Valuable adjunctive treatments include wet combing and forced air. One such agent is benzyl alcohol lotion, which was recently approved by the US Food and Drug Administration.

The safety and effectiveness of benzyl alcohol lotion 5% was demonstrated in 2 multicenter, randomized, double-blind studies of 628 people, aged 6 months and older, with active head lice infestation. Individuals received two, 10-minute treatments of either benzyl alcohol lotion or topical placebo, 1 week apart. When observed 2 weeks following the final treatment, more than 75% of those treated with benzyl alcohol lotion were lice free compared with the placebo vehicle (4.8-26.2%).19

Desiccation of lice is possible with forced air, but this process takes roughly 30 minutes. Combing regimens must be repeated frequently over a period of days.20 Various botanical agents have been used. Essential oils demonstrate variable efficacy and may be contact allergens.21 In general, the evidence supporting their efficacy is of poor quality.22,23 Dimethicone and monoterpenoids are promising agents.22,24,25

Related clinical guideline summaries include the following:

  • British Association for Sexual Health and HIV - United Kingdom national guideline on the management of phthirus pubis infestation26
  • Centers for Disease Control and Prevention - Ectoparasitic infections. Sexually transmitted diseases treatment guidelines 200627
  • University of Texas, School of Nursing, Family Nurse Practitioner Program - Guidelines for the diagnosis and treatment of pediculosis capitis (head lice) in children and adults 200828

Medication

Pediculicides

Treatment options include Malathion, permethrin cream, and pyrethrins. Less toxic agents (eg, benzyl alcohol lotion) are being developed or are now FDA-approved that aim to occlude the respiratory spiracles of the louse and kill via asphyxiation.29 Reports suggest the possibility of neurotoxicity from ivermectin in a population of nursing home patients treated for scabies.30,31,32,33,34


Permethrin (Nix, Elimite)

DOC recommended by most authorities. Resistance probably has developed in many areas. Physicians in some countries select different pediculicides on a rotating basis to discourage development of resistance. Very effective in killing adult lice and nymphs but not as effective in killing nits (eggs). OTC 1% concentration 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. One benefit of permethrin is a residual effect in the hair for several hair wash cycles.

Dosing

Adult

Wash hair with nonmedicated shampoo; apply as cream; leave in place for 10 min, then rinse off; unless every nit is removed, may apply second application 7-10 d after initial therapy

Pediatric

Administer as in adults

Interactions

Enzymatic nit removal systems may inactivate permethrin (use before, not after, permethrin)

Contraindications

Documented hypersensitivity

Precautions

Pregnancy

B - Fetal risk not confirmed in studies in humans but has been shown in some studies in animals

Precautions

May exacerbate redness, swelling, and itching, at least temporarily


Pyrethrins (RID Mousse, RID Shampoo, A200, Barc)

Treatment of P humanus infestations. Stimulates nervous system, causing seizures and death of parasite. Older OTC agent that still appears effective. Lacks residual action of permethrin.

Dosing

Adult

Wash hair with nonmedicated shampoo; apply as cream; leave in place for 10 min (or apply overnight), then rinse off; unless every nit is removed, may apply second application 7-10 d after initial therapy

Pediatric

Administer as in adults

Interactions

None reported

Contraindications

Documented hypersensitivity to ragweed or turpentine; actual incidence of cross-reaction is uncertain, but many other agents are available

Precautions

Pregnancy

C - Fetal risk revealed in studies in animals but not established or not studied in humans; may use if benefits outweigh risk to fetus

Precautions

Do not apply to eyes, face, or mucous membranes; retreatment in 7-10 d necessary to kill newly hatched nymphs


Fluorescein dye strips (SoftGlo Strips)

Used with white petrolatum. Helpful in the management of eyelash nits. Dye strips are used as if looking for a corneal abrasion (off-label use).

Dosing

Adult

Apply to eyelashes 3 successive nights, and wash 24 h after each application

Pediatric

Administer as in adults

Interactions

None reported

Contraindications

Documented hypersensitivity

Precautions

Pregnancy

C - Fetal risk revealed in studies in animals but not established or not studied in humans; may use if benefits outweigh risk to fetus

Precautions

Petrolatum obscures vision but is safe and useful in young infants


Ivermectin (Stromectol)

Used in mass epidemics of lice and scabies. Few serious adverse effects have been reported when drug is used to treat lice or scabies. 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. Available in United States as oral 6-mg pill marketed for treatment of Strongyloides. Physicians have used 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 FDA statement concerning off-label use of approved drugs, which appears in the PDR.
One report suggests the possibility of neurotoxicity from the drug in population of nursing home patients treated with ivermectin for scabies. Patients also had been treated with other neurotoxic agents (eg, lindane). An unexplained decrease occurred in death rate on other wards that coincided with unexplained increase in death rate on the ward where ivermectin was used. Several authors have questioned whether deaths in this report had any real relationship to ivermectin. Health care providers are referred to articles and letters cited in the list of references.

Dosing

Adult

150-200 mcg/kg/d PO as single dose; 12 mg PO repeated in 7-10 d

Pediatric

<5 years: Not established
>5 years: Administer as in adults

Interactions

May interact with other ligand-gated chloride channel (eg, those gated by GABA)

Contraindications

Documented hypersensitivity

Precautions

Pregnancy

C - Fetal risk revealed in studies in animals but not established or not studied in humans; may use if benefits outweigh risk to fetus

Precautions

Treat those who intend to breastfeed only when risk of delayed treatment outweighs possible risks to the newborn caused by ivermectin excretion in milk; repeat courses of therapy may be required in immunocompromised patients; may cause nausea, vomiting, mild CNS depression, and drowsiness


Malathion (Ovide)

Approved by US FDA in 1999 to treat head lice. Irreversible cholinesterase inhibitor that is hydrolyzed (and therefore detoxified) rapidly by mammals but not by insects; ovicidal and pediculicidal. Binds to hair, and provides some residual protection after therapy. Used as a 0.5% lotion and 1% shampoo for pediculosis and scabies.

Dosing

Adult

Apply copious amounts of lotion to dry hair and massage; leave on 8-12 h, rinse, and remove nits with fine-tooth comb (repeat in 7-10 d if lice present)

Pediatric

<2 years: Not recommended
>2 years: Administer as in adults

Interactions

None reported; potential for interaction with aminoglycosides and antimyasthenics

Contraindications

Documented hypersensitivity

Precautions

Pregnancy

B - Fetal risk not confirmed in studies in humans but has been shown in some studies in animals

Precautions

Contains flammable alcohol, so do not expose lotion or wet hair to open flame or electric heat, such as hair dryer (allow hair to dry naturally and uncovered following application); avoid contact with eyes (flush eyes immediately with water if contact occurs)


Benzyl alcohol lotion

Benzyl alcohol inhibits lice from closing their respiratory spiracles, allowing the lotion to obstruct the spiracles, which ultimately results in asphyxiation. Does not elicit ovicidal activity. Contains 5% benzyl alcohol.

Dosing

Adult

Apply lotion to dry hair, using enough to completely saturate scalp and hair; rinse off with water after 10 min; repeat treatment in 1 wk

Pediatric

<6 months: Do not use
>6 months: Apply as in adults

Interactions

None reported

Contraindications

Documented hypersensitivity

Precautions

Pregnancy

B - Fetal risk not confirmed in studies in humans but has been shown in some studies in animals

Precautions

May cause irritation to skin, scalp, and eyes (avoid eye exposure and flush immediately with water if contact occurs); application site anesthesia and hypoesthesia may occur; serious adverse events (eg, respiratory distress, seizure, coma) and death with benzyl alcohol have been well documented in premature infants; IV administration of products containing benzyl alcohol has been associated with neonatal gasping syndrome consisting of severe metabolic acidosis, gasping respirations, progressive hypotension, seizures, CNS depression, intraventricular hemorrhage, and death in preterm, low birth weight infants; neonates (ie, <1 mo or preterm infants with a corrected age <44 wk) could be at risk for gasping syndrome if treated

Follow-up

Further Outpatient Care

  • Treatment of the patient environment (control measures) is important; reinfestation occurs if the problem is not addressed on a school-wide and community-wide basis.
    • Potential fomites (eg, towels, pillowcases, sheets, hats, children's stuffed animals) may benefit from laundering in hot water and machine drying (using the hottest cycle). Temperature exceeding 131°F (55°C) for more than 5 minutes kills eggs, nymphs, and mature lice.
    • Combs and brushes can be treated by soaking for longer than 5 minutes in very hot water (>131°F or 55°C).
    • Because adults cannot survive for long if separated from a host and eggs hatch in 6-10 days, carefully sealing potential fomites in plastic bags for 12-14 days can be effective. This technique works well for objects, such as stuffed animals, that do not tolerate laundering or dry cleaning.

Patient Education

  • In cases of school-wide infestations, the social stigma associated with infestation must be addressed. The community must address the problem honestly and openly, or the infestation will continue. Louse infestation is a community-wide problem.
  • Patients must realize that pediculicides are ineffective against young nits that lack a nervous system.
  • For excellent patient education resources, visit eMedicine's Parasites and Worms Center. Also, see eMedicine's patient education articles Lice and Crabs.

Miscellaneous

Medicolegal Pitfalls

  • Follow manufacturer's instructions for lindane treatment.35,36
    • It may not be suitable for use in patients with a defective cutaneous barrier.
    • Seizure may result from abnormal absorption or gross overuse of the product.
    • Many authors recommend that it not be used as a first-line therapy.
  • Pyrethrin products are contraindicated for patients with contact allergy to ragweed or turpentine.
  • Failure to recognize that pubic lice in children may be an indication of sexual abuse is a potential medical/legal pitfall.
    • Children with eyelash nits have been exposed to an adult with pubic lice. This is not proof of abuse, merely an indication that the child has had contact with an adult with pubic lice.
    • Pubic lice commonly infest the adult's body hair, as well as pubic hair.
    • Although lice can be transferred easily in the absence of abuse, examine patient for other signs of abuse.

Multimedia

The head louse, <em>Pediculus humanus capitis,</e...

Media file 1: The head louse, Pediculus humanus capitis, has an elongated body and narrow anterior mouthparts. Body lice look similar but lay their eggs (nits) on clothing fibers instead of hair fibers.

The pubic louse, <em>Pthirus pubis,</em> is ident...

Media file 2: The pubic louse, Pthirus pubis, is identified by its wide crablike body.

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Keywords

lice, lice infestation, pediculosis, louse infestation, nits

Contributor Information and Disclosures

Author

Dirk M Elston, MD, Director, Department of Dermatology, Geisinger Medical Center
Dirk M Elston, MD is a member of the following medical societies: American Academy of Dermatology
Disclosure: Nothing to disclose.

Medical Editor

Daniel J Hogan, MD, Clinical Professor of Internal Medicine (Dermatology), NOVA Southeastern University; Investigator, Hill Top Research, Florida Research Center
Daniel J Hogan, MD is a member of the following medical societies: Alpha Omega Alpha, American Academy of Dermatology, American Contact Dermatitis Society, and Canadian Dermatology Association
Disclosure: Nothing to disclose.

Pharmacy Editor

David F Butler, MD, Professor of Dermatology, Texas A&M University College of Medicine; Chair, Department of Dermatology, Director, Dermatology Residency Training Program, Scott and White Clinic, Northside Clinic
David F Butler, MD is a member of the following medical societies: Alpha Omega Alpha, American Academy of Dermatology, American Medical Association, American Society for Dermatologic Surgery, American Society for MOHS Surgery, Association of Military Dermatologists, and Phi Beta Kappa
Disclosure: Nothing to disclose.

Managing Editor

Edward F Chan, MD, Clinical Assistant Professor, Department of Dermatology, University of Pennsylvania School of Medicine
Edward F Chan, MD is a member of the following medical societies: American Academy of Dermatology, American Society of Dermatopathology, and Society for Investigative Dermatology
Disclosure: Nothing to disclose.

CME Editor

Catherine Quirk, MD, Clinical Assistant Professor, Department of Dermatology, Brown University
Catherine Quirk, MD is a member of the following medical societies: Alpha Omega Alpha and American Academy of Dermatology
Disclosure: Nothing to disclose.

Chief Editor

Dirk M Elston, MD, Director, Department of Dermatology, Geisinger Medical Center
Dirk M Elston, MD is a member of the following medical societies: American Academy of Dermatology
Disclosure: Nothing to disclose.

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