Excerpt from Pediculosis

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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.

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