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Louse (ukun) common name applied to small wingless, ecotoparasistic insects of two distinct orders Anoplura (sucking lice) and Mallophaga (chewing lice). The sucking lice are exclusively parasitic on mammals, while the chewing lice are mostly parasitic on birds and only a few species on mammals.

Both the sucking lice and chewing lice are dorsoventrally flattened, majority being about 2-3 mm in length, with short legs adapted for swift running within the hairs or feathers of the host. The immatures or nymphs resemble adults and have similar habits. When the lice are numerous they cause great irritation to the host, some species transmit diseases.

Louse in human body P. humanus corporis
Crab louse Phthirus pubis

The sucking lice or the anoplurans have mouthparts fitted for piercing and sucking. The thoracic segments are fused, and the head is narrower than the thorax. The Anoplura comprise nearly 250 species belonging to six families, three of which contain species of medical and veterinary importance. The parasitic lice of man belong to the family Pediculidae and the genera Pediculus and Phthirus. Both the genera are cosmopolitan in distribution. The species P. humanus has two subspecies, P.h. humanus, the head louse and p.h. corporis, the body louse. Phthirus pubis is commonly known as pubic louse or crab louse. The head louse is very common in Bangladesh. Almost 100% children and woman folks of rural Bangladesh, and people living in slum areas of metropolis harbour this louse.

The body louse is not common, and the pubic louse is virtually non-existent in this country. The body louse may infest the clothing where it comes in close contact with the body. Its preferred sites of infestation however, are the armpits, the waistline, neck and shoulders. When heavily infested the head louse may cause serious irritation, and the skin may also become deeply pigmented. The common sucking lice infesting the livestock in Bangladesh are Haematopinus eurysternus, the cattle louse H. asini, the horse louse and H suis, the louse of pig. Different species of Linognathus also infest cattle, sheep, goat, and dog. In addition to man and domestic animals, a wide variety of wild animals are also infested by the sucking lice.

The Mallophaga or the chewing lice constitute a separate order of the class Insecta, although they were previously classified as a suborder of Anoplura. They are mostly ectoparasitic on birds and hence known as bird lice. Nearly 3,000 species are known worldwide. Their mouthparts are chewing type and never suck blood directly, but feed on cuticular materials, bits of hairs, and feathers. None of them is parasitic on man or vector of human disease. The dog louse, Trichodectes canis however is an intermediate host of the cestode, Dipylidium caninum, and transmits this parasite from dog to dog.

The common poultry lice of Bangladesh are Menopon gallinae, Menacanthus stramineus, Goniocotes gigas, Lipeurus caponis, and Chelopistes meleagridis. The cattle biting louse, Bovicola bovis is also fairly common in this country. All birds irrespective of habit and habitats, including the pigeon and cage birds, are infested by a wide variety of chewing lice, and these are mostly transmitted from one bird to other through physical contacts. Because of wide distribution of birds, with them the mallophagans have similar distribution pattern. [SM Humayun Kabir]