Body lice ( Pediculus humanus humans ) are larger than head lice, with longer bodies and antennae. They are mostly found in areas of clothing that continuously come in contact with the body, such as underwear, fork of trousers, armpits, waistline, and near neck and shoulders. If the temperature becomes high near the skin, layering in the clothing allows lice to move to the upper layers of clothing. As a result, body lice can survive starvation longer (3.5 days) at room temperature (23 to 24°C) than head lice. Dirty clothes and unclean bodies provide a stable environment for body lice. Body lice cannot withstand high humidity. In hot weather, they move to the upper layers of clothing, resulting in increased transmission.
Body lice feed on human blood. Most feeding takes place in areas where the clothing comes in contact with the body.
Two days after blood feeding, body lice deposit eggs in the seams of clothing that are in contact with the host's body. Female body lice lay 50-150 eggs in their lifetimes, but some females lay 275-300 eggs. The time required for incubation of eggs and emergence of nymphs is directly dependent on incubation temperature, which is dependent on the proximity of the eggs to the body. Eggs take 5-7 days to hatch. Eggs only hatch at temperature 23°C-38°C. In the body lice's life cycle, eggs are the most resistant stage to changing environmental temperatures.
Nymphs are miniature replicas of their parents. They start feeding frequently (day or night) just after hatching. The nymph stage lasts 16-18 days, including three molts. Females start laying eggs 1-2 days after maturity. Body lice live from 30-40 days and survive best at the body temperature of humans. A four to five degree rise in temperature is fatal for lice. They prefer cold environments where clothing layers provide a humid to dry gradient. They also flourish best in hot dry climates where a temperature gradient exists in clothing.
Body lice are transferred between people by close interpersonal contact. For example, in close, crowded conditions, such as in subways and buses, high temperature and humidity close to the body forces body lice to crawl out of the inner layers of clothing to outer layers and may facilitate transfer from one person to the next. An increase in body temperature from a high fever or the death of a host also results in emigration of the body lice from sick people to healthy people.
Body lice initially cause little irritation. Continuous infestation with body lice results in hardened and deeply pigmented skin (Vagabond's disease or Morbus errorum). Body lice serve as hosts of Rickettsia and Spirochetes and transmit diseases that are caused by these pathogens (organisms that cause diseases). The three diseases transmitted by body lice are epidemic typhus, trench fever, and epidemic relapsing fever. If you would like further information about these diseases please visit the web sites listed at the end of this document.