MOSQUITOES AND FLIES
its being the carrier of the parasite of African sleeping sickness of man, and that of the related disease called nagana in horses and cattle.
African sleeping sickness is caused by a protozoan parasite of the genus Trypauosoma that lives in the blood and other body liquids. Trypanosomes are active, one-celled organisms having one end of the body prolonged into a tail, or flagellum. They are found as parasites in many vertebrate animals, but most of them do not produce disease conditions. There are at least three African species, however, whose presence in the blood of their hosts means almost certain death. Two cause the sleeping sickness in man, and the other produces nagana in horses, mules, and cattle. The two human species have different distributions and produce each a distinct variety of the disease.
Fig. 185. A tstse fly, Glossina palpalis, male (about five times natural size)
One is confined to the tropical parts of Africa, the other is more southern. The southern form of the disease is said to be much more severe than the tropical form, claiming its victims in a matter of months, while the other may drag along for years. The sleeping sickness and nagana trypanosomes are entirely dependent in nature on the tsetse files for their means of transport from one person or from one animal to another.
The tsetse fly (Fig. 185) is a larger relation of the horn fly and the stable fly, having the same type of beak and an insatiable appetite for blood. The tsetse fly genus is Glossina. There are two species particularly concerned with the transportation of sleeping sickness, corresponding with the two species of trypanosomes that cause the two
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