Insects and disease ? Medical Webbook

Insects are six-legged animals with a pair of antennae, a firm exterior skeleton, and, in many cases, wings. They include such animals as ants, bees, cockroaches, fleas, flies, lice, and mosquitoes, but not mites, ticks, or spiders, which belong to another animal group, the arachnids. The insects and arachnids belong to a larger animal group, the arthropods.

Disease caused by insects

There are about 1 million known species of insects; most are either harmless or positively beneficial to humans. The majority of the harmful species cause sickness by attacking crops or stored food, thus contributing to malnutrition and famine. Other insects are a more direct cause of illness or disease. Certain insects directly parasitize humans, living underneath the skin or on the body surface (see Lice; Chigoe; Myiasis). Still others will sting if provoked, causing moderate discomfort (in most cases) to a severe life-threatening reaction (see Insect stings).

The most troublesome insects are flies and various biting insects. Many types of flies settle first on human or animal excrement and then on food to lay eggs or to feed. They can transmit disease organisms from excrement to food via their feet and legs. This is probably important in the spread of intestinal infections such as typhoid fever and shigellosis. Insect bites are irritating in themselves, but the much more serious risk is of an insect spreading infectious organisms as a result of its bite.

Serious diseases spread by biting insects include malaria and filariasis (transmitted by mosquitoes), sleepin:5 sickness (tsetse flies), leishmaniasis (sand flies), epidemic typhus (lice), and plague (rat fleas). Also, various mosquitoes, sand flies, and ticks spread a group of viral illnesses calfed the arthropod-borne or arbo viruses. They include yellow fever, dengue, and some types of viral encephalitis. In each case, 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 feces at or near the site of the bite and later rubbed in by the victim. Most of these diseases (of which?malaria is by far the most important) are confined to the tropics and subtropics. However, some cases of plague and arthropod-borne encephalitis occur each year in theUS. Leishmaniasis can be contracted by sand-fly bites in theMediterranean.

Avoidance

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 areas of the world where malaria is present, the use of mosquito nets and screens, pesticides, and anti malarial tablets.

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