The Human Head Louse
Pediculus humanus capitis

The human head louse, Pediculus humanus capitis, is distinct from, but closely related to, the body louse, Pediculus humanus humanus and crab or pubic louse, Pthirus pubis. In past centuries, head and body lice were simply a fact of life. It is believed that body lice evolved from head lice after humans began wearing clothes. Head lice and body lice are nearly indistinguishable, although the head louse is slightly smaller (up to ~3 mm long) and darker in colour than the body louse (up to ~3.5 mm long):

During the twentieth century, human louse infection in industrialized countries plummeted, at first due to the introduction of insecticides (starting with DDT after World War II) and later with improved hygiene standards. However, towards the end of the century, infestation with head lice increased rapidly in incidence, and millions of school children in developed countries are infested with head lice, a condition which affects people of all social and economic levels.

Lice or their eggs are transmitted from person to person on shared hats, coats, scarves, combs, brushes, towels, bedding, upholstered seats in public places, but most importantly, by close personal contact - common among school-age children. Head louse infestation (pediculosis) can spread rapidly and may reach epidemic proportions if untreated. Length of the hair does not appear to be a significant factor, but may make detection and treatment more difficult. Head lice prefer clean hair and are not a sign of poor personal hygiene! Head lice are highly dependent upon human body warmth and will die if separated from their host for more than 24 hours. (Body lice are more hardy since they live on clothing and can survive if separated from human contact for up to a week without feeding.)

Head louse infestation is usually signalled by intense itching and scratching of the scalp and the back of the neck or when there is a known infestation in a community. When infestation is discovered, it is advisable to examine all members of the family, especially other children, who have been in contact with the infested person to check whether they have become infested. Both the immature forms and adult lice feed on human blood. To feed, the louse bites through the skin and injects saliva which prevents blood from clotting; it then sucks blood into its digestive tract. Although the body louse is the vector of three human diseases (epidemic or louse-borne typhus, caused by Rickettsia prowazeki, trench fever, caused by Rochalimaea quintana and louse-borne relapsing fever, caused by Borrellia recurrentis) head lice are not associated with any specific disease. However, persistent scratching resulting from infestations may result in bacterial and fungal skin infections.

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