The human louse, Pediculus humanus. P.19. The louse sucks blood through contact. There are two species of human lice: the head louse, Pediculus humanus capitis, which lives in the scalp hair and the body louse, Phcorporis, which lives on the body and clothes.

The crab louse, Phthirus pubis. P.3. The crab louse sucks blood and lives in the body hair of humans, especially on the abdomen. The crab louse is transmitted by intimate contact.

The bed bug, Cimex lectularius. P.41. The bed bug feed on human blood, which they suck from us at night. They are found in heated homes and are transmitted when infested house hold effect are moved.

The human flea, Pulex irritans. P.92. It is found in moist, dirty houses. It sucks blood from humans. It is rare and is transmitted by transfer of adult fleas and when moving infested household effects.

There are two species of follicle mites, Demodex folliculorum and Demodex brevis. P.123. They live in hair follicles, especially on the nose, feed on sebum and rarely causes symptoms. They are transmitted by body contact.

The itch mite, Sarcoptes scabiei. P.115. The itch mites feed on the top layer of the skin. They are especially found between fingers and on wrists. The itch mites are transmitted by intimate contact.


The treatments used against crab lice are the same as the once you use to eradicate head lice and body lice. Shampoos are most suitable because they do not sting the thin-skinned places. Apply on the body and on the scalp hair and eyebrows if these are also infected, however, do not apply on the eyelashes. For eradication of lice on the eyelashes, apply the eye ointment Ophtha twice a day for 8 days. The ointment as well as the other treatments for crab lice is available over the counter at pharmacies. Treatment for pubic lice should be applied on days 1, 5 and 10 (for the exact same reasons mentioned in the chapter about head lice).

Treat the whole family simultaneously. You can still infect others until after the third treatment. The surroundings do not need treatment, however, it makes sense to change clothes and linens every time you have treated the lice, i.e. on day l, 5 and 10 immediately after the treatment.


Many people only experience a little itching discover the crab lice by accident. For other, itching, especially in the crotch, is an outstanding symptom of crab lice. Itching in the groin area can have many other causes (among the animals, the flagellate Trichomonas vaginalis or pin worm Oxyuris vermicularis, may cause itching. These two kinds of animals are not affected by crab louse treatment).

The easiest way of detecting lice in beards, armpit hairs, chest hairs and pubic hair is to comb the hair with a fine-tooth comb. The comb will pull out the hairs which put up resistance and it is usually hairs with nits on them.

Spreading of crab lice

Crab lice are spread by intimate contact – either through intercourse or simply by sharing a bed with an infected person. However, there are cases of crab lice which cannot be explained by intercourse or sharing a bed. The lice rarely leave their host but they cause itching which can result in them being scratched off along with the hair they cling to. During the first few hours, those homeless crab lice might climb on to the other people who come into contact with the bedding, the clothing or the toilet seat on which the lice landed. Crab lice are no longer as common as they have been. This is attributed to the shaving of the pubic hair, which has become increasingly common and reduces the amount of hair for lice to live on.

Crab louse bites

The crab louse does not move much. When it has chosen its sucking place, it keeps the mouth inside the skin – also in the periods where it does not suck blood. Crab lice suck blood almost all the time. That is probably why they do not tolerate being away from their host for long periods of time. A single day without access to blood will cause them to die of hunger.

Crab louse bites and their symptoms are similar to those from the head louse. However, crab louse bites are characterized by blue-colored spots, which may appear where the lice bite. The spots, which are deep in the skin, appear a few hours after the bites. They may be 0.2 to 3 mm in diameter and will stay on the skin for a long time.


Crab lice in natural size

Fig. 9. Twelve medium-sized and large crabs as silhouettes in natural size.

The crab louse has adapted to sparse hair. The scalp hairs are too close (about 220 hairs / cm 2). The pubic hair (about 34 hairs / cm 2) is better suited for crab louse. The crab louse primarily live in pubic hair but is also seen in the hair on the chest and in armpits. On the head, crab louse live in the hair lines (neck and temples), in the scalp hair (thin-haired men), in beards and (especially with children) in eyebrows and eyelashes. There can be many crab lice in the eyelashes – up to 100. When there are many there may be inflammation of the eyelid edge.


An adult female lives about a week. It lays three nits a day and only mate once in her short lifetime. The nits hatch after 7-8 days. The three nymph stages lasts a total of 13-17 days. The total development time from nit to nit is therefore 20-25 days.

The crab louse

Crab louse

Crab louse

Crab louse

Crab louse

The crab louse is called Phthirus pubis or pubic lice. It’s easy to tell the difference between human lice and crab lice. The human louse has a large abdomen. The abdomen of the crab louse is wider than it is long and has small protuberances on each side. Crab louse nits are smaller than those of the head louse and the body louse, and the crab louse uses more glue to stick the nit to the hair.

Crab louse

Crab louse

Crab louse

(Latin: Phthirius pubis)

A crab louse is almost as broad as it is long. Its ‘claws’ are extraordinarily well developed and together with the shape of the body give it a crab-like appearance. Its preferred habitat is among the body hairs and particularly among the pubic hairs.

The large claws are well adapted for gripping these very strong hairs. Crab lice can also occur in the armpits, in beards and sometimes on eyelids and eyebrows, and indeed they have even been found among the very fine hair on the heads of infants.

A female crab louse lays about 25 eggs, each firmly fixed to its own hair. The development from egg to adult takes about three weeks.

Crab lice are sedentary. Having found a suitable place a louse will seize the host’s hair, bore into the skin with its mouth parts and suck blood several times in succession, with only short intervals. It will die within about a day if removed from its host. There is no doubt that crab lice are mostly transmitted from one human to another during sexual intercourse. There are, however, records of small children carrying crab lice, so they can be transmitted by other means.