How old is a louse?

Lice are wingless insects that usually lodge in the coat of birds and mammals. Lice eggs are called nits and stick to the hair or feathers of the animal in which they are deposited. These insects have developed over time strong legs and claws to cling tightly to the host's skin, hair or feathers. At present we can distinguish between more than 3200 different species of lice. Some inhabit only certain animals and even certain parts of the body. Life expectancy of a lous.

The lice are insects very little lived and their life expectancy will vary greatly depending on the weather conditions and the animal in which they are housed. The average life expectancy of a louse is around 50 days, including its entire
. Life, that is, from the time the egg is laid until the adult louse dies. Eggs can live up to a maximum of 10 days without a host, while adult lice will only survive at most 2 or 3 days without food.The life cycle of a louse begins when the adult female lays the eggs near the fur or plumage of an animal. Females can lay a maximum of eight eggs per day. The egg will hatch in a period of time that oscillates between the 6 and 9 days, giving rise to a nymph that in about 7 days more, will happen to become an adult louse that will have a life expectancy of about 30 days.

Phthiraptera, commonly known as lice, are an order of apathetic insects (without wings in the adult, in this case secondarily lost) hemimetábolos (whose development consists of egg, several stages of nymph and adult), ectoparasites of birds and Mammals, including about 3250 species.1 Their eggs are called nits, which lice attach to the hair or feathers of their host.

They affect all birds and mammals except for monotremes (platypus and echidnas) and some orders of euphoria, such as bats (bats) and folidotos (pangolines). To this order belong insects that cause infestations of economic importance such as the human scalp lice (Pediculus humanus capitis, other lice that affect the human being the louse of the body Pediculus humanus corporis and the caterpillar Pthirus pubis), those affecting cattle (Damalinia (Bovicola) bovis, Haematopinus eurysternus, Linognathus vituli, Solenopotes capillatus2) and several species of "bird piojillos" affecting hens (such as Menacanthus stramineus, Menopon gallinae).

They are highly specific to your host or host and many species even prefer certain places in your body. Lice, unlike other ectoparasites like fleas, spend their entire life cycle on the host, from egg to adult stage and reproduction. They move from guest to guest walking, can spend a few hours or up to two days outside their primary host and waiting for the next. The adaptations to the parasitism are reflected in the adult in its size (from 0.5 to 8 mm), and the legs that in the more advanced suborders end in strong claws to firmly cling to the hair, skin and feathers; They have no wings or the ability to jump. Depending on the species they feed on skin remains, parts of feathers, sebaceous secretions or blood; May have a chewing or sucking mouth. Its color is variable. 



Lice are hemimetábolos insects. This means that its development consists of 3 phases: egg (in lice called liendre), nymph and adult. 

  • Liendre: Nits are the eggs of lice. The female deposits them in the hair near the root, where they remain firmly cemented. Nits need the warmth and ambience of the head at a set distance from the root to survive. They are visible to the naked eye during inspection of the head, and can be mistaken for dandruff or other compounds of which they differ because they do not easily detach from the hair. They have an oval shape and their color is yellowish to white. It takes between six and eight days to hatch, but the outer shell or chorion ( "the empty nits") remains attached to the hair until it is removed mechanically.

  • Nymph: the animal emerges from the lice in the form of a small louse called a nymph. It looks very much like an adult louse, it feeds, but it is smaller, and its reproductive organs are not yet mature. Nymphs take about 7 or 101 days to move to adulthood. The lice nymph feeds in the same way as the adult.

  • Adult: The adult louse measures 1 or 22 to 42 mm, and can already be reproduced. Like all insects it has 6 legs. The females are the oviposing and cementing the nits to the hair and, generally, they are bigger than the males. Lice feed on blood up to five times a day, pricking the host's skin with its small, piercing mouthparts. While they are being fed, they excrete an anticoagulant and vasodilator saliva that irritates the skin and causes itching. The adult can live for about 30 days, during that time each adult female can lay about 50 or 100 eggs at a rate of about 6 per day. Lice migrate with crowding, so a head may have about a dozen lice, but hundreds of nits in incubation1. If the louse falls outside the person or migrates and can not climb to another, it will live up to 21 days or maybe 42 without feeding, after which it will starve. Human scalp lice also do not survive in other animals such as dogs or cats. They move rapidly moving up to 23 cm per minute. In the abdomen they have six orifices called spiracles, which communicate the respiratory tract system with the environment Exterior. These openings can be closed voluntarily allowing them to float in the water and survive up to 36 hours, even in chlorinated water and also protect the parasite from the action of certain toxic substances.

Forms of contagion 


Lice do not fly and can not jump. The common form of contagion is direct contact, or sharing clothing or other contaminated objects. Lice live a few hours to two days or four days outside a host, depending on environmental conditions, after which they starve . Contact by infested objects may be: 

  • By wearing infested clothing (recently used by people with lice) such as hats, scarves, coats, sports uniforms, hair bands, etc.
  • When using combs, hairbrushes or infested towels.
  • When using a bed, mattress, clothing, pillow, rug or a stuffed toy that has recently been in contact with an infested person.
  • Sandboxes, swimming pools. 

The nits are not contagious, since if a nipper takes off from the hair, it has no way to climb to another head, and the nits have to be at a certain distance from the scalp to obtain the heat and humidity of it to mature, So that when removed they will no longer survive. Therefore, to be infected by the forms indicated above, it is necessary that these (caps, combs ...) have lice (nymphs or adults), because if they only have nits will not be possible the contagion.

Clinical picture and diagnosis


Characteristic symptoms:

  • Pruritus (itching) on the scalp. The bites are very small and generally can not be seen on a naked eye scalp inspection.
  • Eczema (scalp lesions) caused by scratching. These lesions can become infected by increasing irritation.

There is no proven transmission of diseases through human scalp lice at least in Europe. There may be complications such as bacterial or pediculide superinfections (a secondary papular reaction in the neck and back).

Lice and nits are visible on a naked eye inspection of the scalp. The lice are preferably lodged in the areas behind the ears and on the nape of the neck. Nits are more common in these areas, visible in an eye inspection of the hair near the roots. Head lice can cling to their hair with clawed paws, rarely found on other parts of the body or clothing. To make the diagnosis, live lice should be found in the nymph or adult phase, it is not enough to find nits, which may have left only the empty chorion or the embryo inside could be dead. The most effective way to check a Head is to pass a fine comb through the hair from birth to the tips, and fine comb teeth should be checked for live lice after each pass.Head lice infestation is usually benign and has no more manifestations than itching and scratching scalp injuries. However, in some rare cases it can lead to some general involvement with iron deficiency anemia and eosinophilia.


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