Lice are small, wingless insects that feed on human blood.
There are three species that infest humans:
- Pediculus humanus capitis (the head louse),
- Pediculus humanus corporis (the body louse), and
- Phthrirus pubis (the crab, or pubic, louse).
All have flattened bodies and may measure up to 3 mm across.
Head lice live on the scalp and are spread by direct contact. (The lice prefer clean hair, and their spread is not due to poor hygiene.) Their tiny eggs (nits) are attached to hairs close to the scalp. The bites from head lice cause intense itching.
Body lice live and lay eggs on clothing next to the skin. They can transmit epidemic typhus and relapsing fever.
Crab lice live in pubic hair or, more rarely, in armpits, beards, or eyelashes; they are usually transmitted during sexual contact (see pubic lice).
Various preparations can be applied to kill lice and their eggs. In addition, lice on clothes, bedding, and items such as hairbrushes can be killed by washing the items in very hot water.
Lice or Louse infestation in more detail
Lice are obligate parasites of animals. They bite using piercing mouthparts to feed on blood or tissue fluids. Three species, of cosmopolitan distribution, are associated with humans: the pubic louse Pthirus pubis, the body louse (or clothing louse) Pediculus humanus, and the head louse P. capitis. Body and head lice are morphologically similar and are treated by some authors as subspecies or forms of P. humanus. Lice complete their life cycle on their host. Adult females deposit eggs (nits) on hair shafts (pubic and head lice) or on clothing (body louse). Larvae hatch after about 1 week, begin to feed and over the course of about 2 weeks, undergo several moults before reaching adulthood. Adult females live for about 1 month and may lay about 100 eggs. Egg cases remain where attached and may persist after successful treatment of infestation. Most infestations are probably acquired through close contact with an infested case, but some cases may result from contact with clothing, bedclothes, or hairbrushes containing living lice or their eggs, which may be attached to shed hairs. In addition to the aesthetic and social drawbacks of louse infestation, medical problems common to all three taxa relate to sensitization of the host to louse antigens from bites and the resulting pruritus which may lead to excoriation and secondary infection. Louse bites have a central punctum and surrounding small red macule. Body lice may transmit a number of agents, including those of endemic typhus (Rickettsia prowazekii), trench fever (Bartonella quintana), and relapsing fever (Borrelia recurrentis).
Pubic lice (crab lice)
The lice (Pthirus pubis) attach themselves to pubic hairs. Rarely, lice may be found on eyebrows, eyelashes (phthirosis palpebrarum), axillary, head, or chest hair. Eggs are deposited on hair shafts. Most infestations are probably acquired through sexual contact with an infested case. Children may acquire phthirosis at atypical sites through close contact with adults. Lice seldom stray from the body. Transmission is possible but unlikely without close contact with an infested case. The main symptom is pruritus, sometimes with excoriation and secondary infection. Grey patches (maculae caeruleae) may occur on the skin. Diagnosis is by observation of the lice, which may be difficult to find, or of eggs or egg cases attached to hair shafts. Adults are 1 to 2 mm long. The anterior legs are smaller than the other two pairs. The body is squat and crablike (body length, excluding head, c.1.2 times body width). The original description contained a printing error (pthirus) for phthirus (Greek: louse).
Aqueous carbaryl, permethrin, phenothrin, or malathion is applied to the whole body and left on for 1–2 days. This is repeated a week later to kill newly hatched larvae. Sexual contacts must be treated.
Head lice infest the scalp and rarely other body sites. They lay their eggs at the base of hair shafts. Infestation is more common in children than in adults and more common in females than in males. Prevalence rates vary but may be very high in certain communities or institutions, such as schools. Prevalence rates may be high despite good standards of hygiene. Most cases probably occur as a result of close contact. The main symptom is pruritus which may be associated with excoriation, secondary infection and lymphadenopathy. Diagnosis is by observation of lice, which generally remain close to the scalp, or of eggs or egg cases, attached to hairs. A fine comb (nit comb) may be used to collect material to make the diagnosis. Adults are 3 to 4 mm long.
Insecticide lotion (malathion, permethrin, phenothrin, dimeticone, or carbaryl) is applied to the scalp overnight. This is repeated a week later to destroy newly hatched larvae. Permethrin failure has been reported from many parts of the world. Compared with laboratory reference strains, lice collected from infestations failing to respond to permethrin have shown relative resistance to the agent. In Israel, there is evidence that permethrin resistance may be due to monooxygenase plus nerve insensitivity resistance mechanisms. Malathion resistance has been reported and may be due to a malathion-specific esterase. Pediculocides should be used with caution in children and asthmatics. Regular and fastidious use of a nit comb may be used (on its own or in combination with a pediculocide) to treat infestation. There is much anecdotal evidence, that combing can be effective, and it avoids concerns of pediculocide toxicity and resistance, but a study in Wales showed combing to be less effective that chemical treatment. In institutions, coordinated treatment campaigns may be required to prevent reinfestation.
Body lice infest clothing and body hair. They lay their eggs on clothing, often along seams. Body lice are morphologically like head lice but slightly larger. Body louse infestation is associated with poor hygiene and social deprivation, as may occur in wartime. Transmission occurs as a result of close contact or through contact with infested clothing. Bites occur on the body, resulting in pruritus which may be associated with excoriation, eczematization, and secondary infection. Diagnosis is confirmed by finding lice, usually on clothing.
Infestation may be treated by topical application of carbaryl or malathion to the whole body, repeated a week later to kill newly hatched larvae. Hot washing of clothing will destroy adults and early stages.