Body louse

The body louse, sometimes called the clothing louse or informally a cootie, is a blood-feeding external parasite that lives on humans. It is one of three kinds of lice that infest people, alongside the head louse and the crab (pubic) louse. These insects can spread disease, carrying pathogens responsible for epidemic typhus, trench fever, and relapsing fever.

Body lice cannot jump or fly, so they move from person to person through direct contact, or less often via infested clothing or bedding. Most of their eggs are attached to clothing rather than to the host’s hairs, though eggs may also be laid on hairs. In wealthier countries, infestations mainly occur in conditions of poverty, poor hygiene, crowded living, and limited access to clean clothes. Outbreaks also happen when large groups are forced into unsanitary situations—such as in prisons, among homeless populations, during refugee crises, or after natural disasters when proper sanitation is unavailable.

The body louse and the head louse look identical and can interbreed in a lab, but in the wild they live in different habitats and rarely meet. An adult louse can feed up to five times daily and live about thirty days, but it dies within two days if separated from its host. Under good conditions, reproduction is rapid: after the final molt, males and females mate immediately, and a single female can lay 200 to 300 eggs in her lifetime.

The life cycle has three stages: egg, nymph, and adult. Eggs, also called nits, are oval and yellow to white. The female glues them to clothing or hairs with a secretion, and at the right temperature and humidity, they hatch in six to nine days. A nymph is the immature louse that emerges from the egg. It starts feeding on blood right away, then returns to clothing until its next meal. The nymph molts three times and becomes an adult in about nine to twelve days. Adult body lice are 2.5 to 3.5 millimeters long, wingless, tan to grayish-white, with six legs. Their heads are short, with two five-segmented antennae, a compact thorax, and a seven-segmented abdomen with side plates.

It was once thought that the body louse split from the head louse about 107,000 years ago, which would mark the latest possible date for humans to have started wearing clothing. But more recent transcriptome studies cast doubt on this, finding that body and head lice are almost genetically identical. The differences that allow body lice to live on clothing may come from regulatory or epigenetic changes triggered by the environment. Carl Linnaeus first described the body louse in the 10th edition of *Systema Naturae*. Its genome was sequenced in 2010, and at that time it was the smallest known insect genome. The body louse belongs to the phylum Arthropoda, class Insecta, order Psocodea, and family Pediculidae. Of the roughly 5,000 described louse species, about 4,000 parasitize birds and 800 parasitize mammals. Lice on mammals likely originated from a common ancestor that lived on Afrotheria, which acquired them from an ancient bird host.

An infestation can involve thousands of lice, each biting up to five times a day. This causes intense itching, especially early on, which can lead to skin damage and secondary infections. Long-term infestations may also cause apathy, lethargy, and fatigue.

The simplest treatment is to change clothes and bedding regularly. Infested items should be washed in water at least 50°C (122°F) and dried on a hot cycle. Itching can be relieved with topical or systemic corticosteroids and antihistamines. If a secondary bacterial infection occurs, antibiotics may be needed. When regular washing isn’t possible, insecticides can be used on infested items.

Unlike other human lice, body lice can transmit diseases. The key pathogens they carry are *Rickettsia prowazekii* (epidemic typhus), *Borrelia recurrentis* (relapsing fever), and *Bartonella quintana* (trench fever). Epidemic typhus can be treated with a single dose of doxycycline; without treatment, the fatality rate is 30%. Relapsing fever is treated with tetracycline, and if untreated, its fatality rate ranges from 10 to 40%. Trench fever is treated with doxycycline or gentamicin; without treatment, the fatality rate is less than 1%.