Penn State Extension


Human Lice

Pediculus humanus capitus (Human Lice Head louse), Pediculus humanus humanus (Body louse), Pthirus pubis (Crab or pubic louse)


Head Louse Adult  Head Louse Nit   Crab Louse Adult

      Head louse adult                         Head louse nit                   Crab louse adult


Since no verified fossils of lice have ever been found we can only speculate when they originated. We do know the ancient Egyptians and Greeks wrote of them and they were found on prehistoric American Indian mummies. There are three kinds of lice which feed on man.

  1. The head louse ( Pediculus humanus capitus )
  2. The body louse ( Pediculus humanus humanus )
  3. The crab or pubic louse ( Pthirus pubis )

Head and body lice are very much alike, however, body lice are slightly larger. These lice have abdomens longer than they are broad and their six legs are equal in size. In contrast, the abdomen of the crab louse is about as wide or even slightly wider than its length, and the second and third pairs of legs are thicker than the first pair. Crab lice are much smaller than head and body lice.

Depending on temperature, lice eggs (nits) usually hatch into nymphs within ten days. There are three nymphal stages, the third followed by the adult stage. All stages must have blood meals if they are to survive and continue their development. Adult lice live about 30 days.

Public Health Implications

Louse-transmitted diseases are presently not a serious threat in the United States. In other parts of the world, however, lice do transmit typhus fever (a disease which has killed many millions of people), epidemic relapsing fever, and trench fever.

Lice cause much discomfort to humans due to their bites, which are irritating, causing sleeplessness. Scratching often leads to secondary bacterial infections.

Head Lice The female head louse will deposit from 50 to l50 eggs or nits in her lifetime. Head lice glue their eggs to the base of hairs. Only one nit is glued to any one hair. Eggs hatch in five to ten days when the temperature is between 95× and 100×F. Below 74×F, most eggs will not hatch.

People finding nits on hairs wonder if they are old nits which have already hatched or new nits which may still hatch. Since human hair grows about 1/2 inch per month, any nits found on a hair 1/4 of an inch from the scalp would be approximately 16-days old, and would have hatched already, or will not hatch. These nits are glued tightly to the hair shaft and can only be removed by combing. A special fine-toothed comb must be used which has teeth 0.l mm apart. A vinegar rinse is not effective.

The life cycle for the head louse (from egg to adult) takes from 16 to 21 days.

Body Lice Body lice spend most of their time on the underclothing, next to the body, particularly along the seams. From here they periodically visit the hosts’ body for blood. They usually glue their eggs (up to 300 in a lifetime) to the fibers of clothing, but may occasionally glue them to body hairs as does the head louse.

Crab Lice These lice are found mostly in the hair of the pubic area. They may also be found under the armpits, in the beard or mustache and on the eyebrows and eyelashes. Their development from egg to adult normally requires from 30 to 41 days. Adult crab lice live about 30 days. Females deposit 30-50 eggs (in her lifetime).

How Do People Get Lice?

Anyone can get lice no matter how clean they are about their personal hygiene and their homes. Lice do not feed on dirt; they feed on blood! People get lice from people. They don’t come from pets. (Dog and cat lice do not infest man.)

Head lice are transferred between persons who share items such as hats, hairbrushes, combs, or wigs.

Body lice spread from clothing, particularly underclothing. If infested clothing is left lying about, body lice can migrate and thus, spread to other clothes and people.

Crab lice usually are transmitted from person to person by sexual contact, however, they can be found on toilet seats and in beds, and from there, spread to people.


Shampoo products containing either prescription or over-the-counter preparations are the standard treatment for head lice. Body lice and crab lice are controlled using lotions which contain an insecticide. Contact your physician or pharmacist to determine which of these products will be most effective.

Washing clothes in hot water, 123 degrees F. or higher, will kill any body lice and their nits. Clothes which can’t be laundered should be dry cleaned. Body lice can often be controlled by frequent changes and washings of clothes. Head lice, on the other hand, are not killed by frequent showers. Cutting the hair will not eliminate head lice (unless the head is actually shaved).


Pesticides are poisonous. Read and follow directions and safety precautions on labels. Handle carefully and store in original labeled containers out of the reach of children, pets, and livestock. Dispose of empty containers right away, in a safe manner and place. Do not contaminate forage, streams, or ponds.

Authored by: Steve Jacobs, Sr. Extension Associate

October 1998 Reviewed February 2015

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