Insects and disease

Relatively few insect species cause disease directly in humans. Some parasitize humans, living under the skin or on the body surface (see lice; chigoe; myiasis). The most troublesome insects are flies and biting insects. Flies can carry disease organisms from human or animal excrement via their feet or legs and contaminate food or wounds. A number of serious diseases are spread by biting insects. These include malaria and filariasis (transmitted by mosquitoes), sleeping sickness (tsetse flies), leishmaniasis (sandflies), epidemic typhus (lice), and plague (rat fleas). Mosquitoes, sandflies, and ticks can also spread illnesses such as yellow fever, dengue, Lyme disease, and some types of viral encephalitis. Organisms picked up when an insect ingests blood from an infected animal or person are able to survive or multiply in the insect. Later, the organisms are either injected into a new human host via the insect’s saliva or deposited in the faeces at or near the site of the bite. Most insect-borne diseases are confined to the tropics and subtropics, although tick-borne Lyme disease occurs in some parts of the UK. The avoidance of insect-borne disease is largely a matter of keeping flies off food, discouraging insect bites by the use of suitable clothing and insect repellents, and, in parts of the world where malaria is present, the use of mosquito nets and screens, pesticides, and antimalarial tablets.