Three types mainly affect humans. Adult lice are tiny, about 1/16 to 1/8 inch long, Dorso-ventrally flattened, and whitish gray in color. They have no wings, small blunt antennae, small heads, chewing mouthparts, and a single hook-shaped tarsi on each leg. Young lice look similar to adults but are smaller. Head lice appear exclusively on the hair and eggs (nits) are laid attached to head hair. Body lice look like head lice, but they attach their nits to human clothing. Pubic lice are about 1/16 inch long and oval in shape. They are adapted to human hair that is coarse and spaced far apart, including that in the pubic region, armpits, eyebrows, and beards. They attach their nits to this type of body hair.

Signs of infestation:

• Presence of nits or lice in the hair or on clothing

• Itching and lesions at the back of the head, nape of the neck, or around the ears (head lice)

• Itching and lesions on other parts of the body (body and pubic lice)

• Nits in clothing (usually present in specific conditions of poverty, homelessness, or times of disaster and when people are forced to live in areas without heat and with poor bodily hygiene)

Head Louse: All stages live on the human host and do not survive for long off the host. Females lay 50 to 100 eggs that are glued to hair shafts close to the scalp. Eggs are ⅓ inch and visible. They require the warmth and humidity of the human scalp for survival and hatch within 5 to 10 days. First-instar lice will feed immediately. There are 3 juvenile stages, and lice mature with. in 3 weeks. Adults live for another 3 weeks.

Body Louse: This species is thought to have evolved from head lice and adapted to living on human clothing. They tend to occupy seams of clothing closest to the body. Females lay up to 200 eggs at a rate of 5 to 8 eggs a day during times when the host is resting. Eggs are 1/32 inch long and white. Body lice will come and go with clothing. If the hosts sleep in their clothes, the life cycle of a body louse is faster due to warmth and access to feeding. Hygiene is therefore the most important factor in body lice infestations. A change into clean clothes and washing or discarding infested clothing breaks the cycle. For this and other reasons, body lice are rare in the United States and limited to indigent, homeless individuals and those in need of care.

Crab Louse: This species will occupy the pubic region primarily, although they have been found on the eyelashes of children. They are transmitted through intimate person-to-person contact. Crab lice will die within 24 hours off the host. Females layup to 3 eggs every day. Eggs are cemented to the hairs near the skin and hatch within a week. Nymph crab lice have a longer development time, resulting in a 5- to 6-week life cycle.

Lice have piercing-sucking mouthparts, and their feeding causes itchy lesions on the skin. Left unattended lice infestations will only worsen and spread to others. Body lice are vectors of typhus, relapsing fever, and trench fever.