Human Lice
There are three types of lice that feed only on humans: the head louse (Pediculus humanus capitis), the body louse (Pediculus humanus humanus or vestimenti), and the crab louse (Phthirius pubis). These lice are permanent parasites, meaning they must stay on a human host to survive. They die within a few hours without a host, even under ideal conditions. Lice are flat from top to bottom and have no wings. Their eggs, known as nits, are glued to hair or fabric. Lice develop through gradual changes, where the young, called nymphs, look like smaller versions of adults and go through several molts before reaching maturity.
1. Pediculus humanus capitis (Head Louse)
- Adults reach a maximum length of about 3 millimeters.
- They live and lay eggs on the hair of the human scalp.
- Female lice lay three to four eggs each day, with a total of fifty to one hundred fifty eggs in their lifetime.
- Each egg is oval and about 0.8 millimeters long, glued tightly to the hair shaft.
- The head louse does not carry or spread disease.
2. Pediculus humanus humanus (Body Louse)
- Adults measure between 2.3 and 3.6 millimeters in length.
- They live in the seams and folds of clothing, moving to the skin only to feed.
- Female body lice lay eggs on clothing, especially in areas that remain in contact with the body.
- These lice are known to transmit serious diseases:
- Epidemic typhus is caused by Rickettsia prowazekii, which lives in the gut of the louse. This disease can spread rapidly during times of war or disaster and is found in cooler mountainous regions of Africa, Asia, Central America, and South America.
- Louse-borne relapsing fever is caused by Borrelia recurrentis, which lives in the louse's body fluids.
- There is some uncertainty in the sources about whether fleas might also play a role in the spread of Rickettsia prowazekii, as infected flea waste may also be involved.
3. Phthirius pubis (Crab Louse or Pubic Louse)
- Adults are shorter and rounder than other lice, measuring up to 2 millimeters.
- They have large claws on their back legs, which they use to grip coarse body hair.
- Commonly found in pubic hair but may also infest eyelashes, armpits, or beard hair.
- They cannot jump or fly, relying on close physical contact for transmission.
- These lice feed on blood but do not spread any known diseases.