eMedicine Specialties > Infectious Diseases > Skin and Soft-Tissue Infections

Pediculosis

Author: Lyn Guenther, MD, FRCP(C), Professor, Department of Medicine, Division of Dermatology, University of Western Ontario; Chief of Dermatology, London Health Sciences Center; Medical Director, The Guenther Dermatology Research Centre, Canada
Coauthor(s): Sheilagh Maguiness, BSc, MD, Department of Dermatology, University of Alberta; Thomas W Austin, MD, Professor Emeritus, Department of Medicine, Division of Infectious and Sexually Transmitted Diseases, University of Western Ontario, Canada
Contributor Information and Disclosures

Updated: May 5, 2009

Introduction

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 are so ubiquitous that terms and phrases such as "lousy," "nit-picking," and "going over things with a fine-tooth comb" are part of everyday vocabulary.

Over the last 3 decades, the incidence of pediculosis has risen steadily, making the diagnosis and treatment of louse infestation one of the most common tasks in general medical practice. This article focuses on the pathophysiology and life cycle of 3 prevalent human ectoparasites: (1) Pediculus humanus capitis (ie, head louse), (2) Pediculus humanus corporis (ie, body louse), and (3) Phthirus pubis (ie, pubic louse). The clinical aspects of presentation, diagnosis, and treatment of these ancient and common human pests are also discussed.

Pathophysiology

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). Mammals are the hosts for all Anoplura, and, although lice prefer human hosts, P humanus is also known to live and reproduce on pigs.

The Anoplura are wingless and have 3 pairs of legs, each ending with a clawlike talus 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. 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 for approximately 35-45 minutes each time.

In each species, the female louse is slightly larger than her male counterpart. The life cycle of lice is 30-35 days from egg to adult. Early death is common, resulting from gut rupture during feeding or cementing of the female to the hair shaft during ovipositioning.

P humanus capitis

The average length of the head louse is 1-2 mm. 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. After incubation of the ova (8-10 d), the nymph molts 3 times before reaching its adult form (8-10 d later).

The female head louse lays as many as 10 eggs per 24 hours, usually at night. She positions her ova at the base of the hair shaft, within 1-2 mm of the scalp, with a predilection for the posterior hairline and postauricular areas. Egg and glue extrusion onto the hair shaft takes 16 seconds.

The female louse cannot survive for more than 3 days off the human head. Head lice can travel up to 23 cm/min. The head louse is unable to move on smooth surfaces (eg, glass, plastic). Lice can be dislodged by combs, towels, and air movement (including hair dryers in either low or high setting).² Hair combing and sweater removal may eject adult lice more than 1 meter from infested scalps. Lice lay eggs on most fabric, often within 5 minutes of contact, and more than 50% of the eggs typically hatch.

P humanus corporis (also known as Pediculus humanus humanus )

The body louse is larger than the head louse. Body lice range in size from 2-4 mm; the female lice are larger than the male lice. The body louse is also 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 lives in human clothing, crawling onto the body only to feed, predominantly at night. P humanus corporis prefers cooler temperatures, living and laying its 10-15 eggs per day some distance from the human body on the fibers of clothing, mainly close to the seams. The adult female body louse, unlike the head louse, can survive as long as 10 days away from the human body without a blood meal. The life cycle from nit to death is approximately 35 days, with 3 episodes of molting before maturity. On average, 20 adult female lice can be found on a person with an infestation.

P pubis

The pubic louse gets the nickname of "crab" from its shorter, broader body (0.8-1.2 mm) and large front claws, which give it a crablike appearance. The pubic louse is white to gray and oval and has a smaller abdomen than both P humanus capitis and P humanus corporis. The average life cycle of P pubis is also 35 days, although the period from ova to adult (15 d) is slightly longer than that of the other 2 forms. The average female pubic louse lays only 1-2 eggs per day. 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, and, occasionally, the periphery of the scalp. These insects are less mobile than P humanus and P corporis, mainly resting while attached to human hairs. 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 attaches to the base of the hair shaft or to fibers of clothing with a strong, highly insoluble cement. The nit is topped with a tough but porous cap known as the operculum. This porous sheath allows for gas exchange while the nymph develops in the casing. 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).

Frequency

United States

Pediculosis is extremely common; more than 12 million Americans are infested each year. Head lice infestation is more common in the warmer months, while pubic lice infestation is more common in the cooler months.³

International

Hundreds of millions of cases of louse infestation are reported annually worldwide, with an apparent increase over the past few decades. In a study of 6,169 Belgian school children aged 2.5-12 years, the prevalence of head lice was 8.9%.⁴ The prevalence in 1,569 school children in Izmir, Turkey, was 16.6%.⁵ In 2005, the incidence of pediculosis doubled in the Czech Republic.⁶ Live lice were detected in 14.1% and dead nits in another 9.8% of 531 children aged 6-15 years in 16 schools.⁶

Mortality/Morbidity

• Mortality with pediculosis occurs from the 3 infectious vector-borne diseases (ie, typhus, relapsing fever, trench fever) that are caused by P corporis. For more details, see Other Problems to be Considered.
• The morbidity associated with pediculosis no doubt relates to the social stigma attached to each of the 3 types of infestation. Pruritus, bite reactions, and secondary skin infections can also cause significant morbidity.

Race

• In North America, black persons are less commonly affected by head louse infestation than persons from any other racial group. This is probably due in part to 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.

Sex

• Girls are at higher risk of head louse infestation than boys because of social behavior (eg, social acceptance of close physical contact; sharing of hats, combs, hair ties).
• No sexual predilection exists in body or pubic louse infestation; males and females are equally likely to become infested.

Age

• Children aged 3-11 years are most likely to become infested with head lice because of close contact in classrooms and daycare facilities.
• Age is not a significant risk factor in body louse infestation.
• P pubis infestation is more common in people aged 14-40 years who are sexually active.

Clinical

History

Infestations are underreported because of the social stigma attached, namely the preconceived notion that lice of any kind are related to dirt and poor personal hygiene.

• P humanus capitis
  • Parents of school-aged children often seek an assessment after an outbreak of head louse infestations at school is reported.
  • Pruritus is the major symptom, and parents may note the lice and nits in the hair of the child.
  • The duration of the problem is often valuable information because most children are infested with head lice for as long as 2 months before their discovery.
  • Areas affected in head louse infestation include the scalp, the back of the neck, and postauricular areas.
• P humanus corporis
  • Patients infested with P corporis (generally people of low socioeconomic status) experience nocturnal pruritus, particularly in the axillary, truncal, and groin regions. The lice move from the clothing to the body at nighttime to feed, causing intense pruritus.
  • The investigating physician should inquire about the patient's socioeconomic status and living conditions.
• P pubis
  • Involvement with pruritus of the groin, axillae, and eyelashes or eyebrows can help to differentiate P pubis infestation from head or body louse infestation.
  • Adults infested with P pubis are usually sexually active and have groin and body hair involvement.
  • Children have eyelash and eyebrow involvement. Parents of children infested with P pubis should be questioned about being infested because the parents are usually the source of infestation.

Patients may describe associated features such as papules or wheals, indicating bite reactions. Patients may have a history of secondary infection after excoriation, which may help to confirm the presence of an infestation.

A diagnosis of any type of pediculosis requires the finding of live specimens of lice and/or a viable nit (ie, one located at the base of the hair shaft <2 mm from the scalp). The practitioner should assess the patient's risk factors for infestation (eg, age, sex, race, social and/or economic status, living environment).

Physical

• P humanus capitis
  • Patients infested with head lice generally present with an itchy scalp. The back of the neck and postauricular areas are commonly involved.
  • If excoriations are present, secondary infection, (ie, impetigo) should be excluded and treated, if present.
  • In patients infested with head lice, lymphadenopathy in the posterior auricular and cervical nodes is not uncommon.
  • Bite reactions manifested as pruritic papules and/or wheals may be present, depending on the length of time since the blood meal. Healed bites may reappear when new bites occur in other areas.
  • Close inspection of the scalp in affected patients may reveal the nits, live lice, and small dark specks of insect feces. True nits can be differentiated from dried hairspray and hair casts by attempting to separate the nit from the hair; the hair casts and dried hairspray separate easily, while nits remain securely attached. If the physician remains unsure, a Wood lamp examination reveals yellow and/or green fluorescence of the lice and their nits.
  • 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.
• P humanus corporis
  • Physical examination findings in body louse infestation include multiple erythematous papules (bites) located anywhere on the body but concentrated in the axillae, groin, and trunk (ie, areas most often covered by clothing). Thus, the face, feet, and arms are not commonly affected.
  • Maculae cerulea may be present as 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 cause the breakdown of human bilirubin to biliverdin, causing the change in skin color associated with maculae cerulea. The finding of maculae cerulea is believed to be pathognomonic for infestation with lice.
  • The development of secondary infections due to excoriations is also possible. The diagnosis of body lice depends on the close examination of the patient's clothing for lice, nits, and insect feces. The seams of clothing worn on the axillae and groin regions are common sites of residence. The number of body lice per host is usually approximately 10, although as many as 1000 lice can be present.
  • Body louse infestation is also known as vagabond disease, and patients 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.
  • Individuals with P corporis infestation should also be examined for the presence of pubic and head lice. Examining the individual for systemic illness that may be related to one of the vector-borne diseases associated with P corporis is also important (see Other Problems to be Considered).
• 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.
  • The groin, axillae, eyebrows, eyelashes and, rarely, facial hair may be sites of infestation. Scalp involvement is rare and is usually confined to the marginal areas. In adults, eyelash involvement in the absence of genital involvement is rare.
  • Excoriations are common.
  • 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.
  • Inguinal lymphadenopathy and axillary lymphadenopathy have also been reported with pubic louse infestation.

Causes

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

• Risk factors for infestation with P humanus capitis
  • Factors that predispose to head louse infestation include young age (see Age), close crowded living conditions, sex (see Sex), race (see Race), and warm weather. The risk of nosocomial transmission is low, unless close patient-to-patient contact (eg, playrooms, institutions) is present.
  • Based on an 11-year study of the Israel Defense Force, the head lice infestation rate is highest during the warmer summer months.
• Risk factors for infestation with P humanus corporis
  • The risk factors for body louse infestation include the presence of close, crowded living situations (eg, crowded buses and trains).
  • Social circumstances in which the washing and/or changing of clothing is not possible are also significant risk factors for body lice.
  • P corporis can also be acquired via bedding 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.
• Risk factors for infestation with P pubis
  • Risk factors for infestation of the pubic louse also include crowded living conditions.
  • Intimate or sexual contact with an individual who is infested is another common risk factor.
  • Because these organisms are most often spread through close or intimate contact, P pubis infestation is classified as a sexually transmitted disease (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 is usually contracted from a parent who is infested (sexual transmission to children is rare). In most cases of infestations in children, transmission is due to shared bed linens and close nonsexual contact.

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