Lice: Infection, Transmission & Diseases

Lice are the name given to ectoparasites. Some species of them infest humans.

What are lice?

Lice, more specifically human lice (Pediculidae), are a family of insects descended from the animal lice (Phtiraptera). With their stinging proboscis, the parasites suck the blood of their victims and leave behind itchy wheals. Human lice can be distinguished from animal lice by their mouthparts. In addition, animal lice are very rarely found in humans because the lice specialize in their host. If an infestation of lice occurs in humans, doctors refer to it as pediculosis. It mainly affects hairy areas of the body such as the head, pubic hair and armpits. A distinction is made between three types of lice that can harm humans. These are the head louse (Pediculus humanis capitis), the felt louse (Phtirus pubis), and the clothes louse (Pediculus humanus humanus or Pediculus humanus corporis).

Occurrence, distribution, and characteristics

Since lice are parasitic insects, they require a host body to feed. In doing so, they live either inside or on the organism of their victim. Therefore, human lice belong to the group of ectoparasites, which also applies to leeches, fleas or ticks, for example. The lice occur permanently on their host, so they usually do not leave it unless they make a direct transition. The different types of lice specifically target a particular host. Thus, in addition to human lice, there are dog lice, pig lice, and seal lice. Among the most commonly encountered lice species is the head louse. In contrast, crabs and clothes lice are much less common. Whether the head louse and the clothes louse are a separate species or just a subspecies of the human louse has not yet been clarified. Lice have the property of feeding on the blood of their host. For this purpose, they have special mouthparts with which they penetrate the host skin and suck out the blood. They also secrete a little saliva into the sting canal, which prevents the blood from clotting. In addition, this process triggers the typical symptoms of a lice infestation. This principle is similar to that of mosquitoes. The louse’s blood meal takes place several times a day. Without a host, the parasite survives only a few days. Human lice settle in the hair or clothing of humans. There they hold on with their pairs of claws. The mating of the lice partly takes place intersexually. However, unisexual or virginal procreation (parthenogenesis) is also possible. The lice attach their eggs to the hair or the fibers of textiles. Because the eggs are insoluble in water, they cannot simply be washed out in the event of an infestation. Both eggs and the remaining chitinous shells that remain after the lice larvae hatch are called nits. In contrast, the larvae bear the designation nymphs. Externally, the larvae already bear a resemblance to the adult insects, but their size turns out to be so small that they are usually not visible to the naked eye. After passing through several stages of development, the lice larvae are sexually mature after ten days, which also allows them to lay eggs. Crabs and clothes lice are mainly found in poorer countries and crisis regions. In western climes, on the other hand, they occur only rarely. This does not apply to the head louse, however, as its spread can hardly be influenced by hygiene measures. While the louse nymphs have a size between 1 and 2 millimeters, adult head lice reach a length of about 3 millimeters. The shape of the male louse is somewhat smaller and more slender than that of female specimens. The louse carapace, made of chitin, is transparent, brownish or white-gray. As the digestive tract fills with blood after the sucking process, the chitin shell takes on a reddish coloration. The change in color of the chitinous envelope makes the lice more difficult to identify.

Diseases and ailments

If a lice infestation occurs in humans, it becomes noticeable through typical symptoms. These include skin reactions such as itching, which appears mainly at night. In most cases, red wheals and papules form on the affected areas of the body. Due to scratching, there is also a risk of eczema forming. The reason for the itching and reddening of the skin is the saliva of the louse, which is released during the biting process.In the case of clothes lice, the bright red spot at the puncture site later develops into an itchy bluish spot. The crab louse, on the other hand, causes small blue-grey spots to form in the puncture area. The part of the body where the symptoms appear depends on the type of louse and the region where it leaves its eggs. Crabs are mainly found on the hair of the anus and genitals. However, they sometimes also infest the beard, eyebrows, chest, armpits, abdomen or thighs. Infection with lice almost always occurs from person to person, since human lice almost never occur in animals. Since the parasites do not leave their host, direct physical contact is necessary for their transmission. However, transmission of the clothes louse can also occur through infested clothing. Clothes lice in particular are at risk of transmitting pathogens. These cause rickettsial diseases such as fifth-day fever or spotted fever.