Page:Popular Science Monthly Volume 81.djvu/551

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545
INSECTS AS AGENTS IN SPREAD OF DISEASE

states in a much milder form with only 5 per cent, mortality. A common wood-tick of that region, Dermacentor venustus, seems to be the only carrier under natural conditions, but recently Maver has shown experimentally that other ticks can transfer the' virus. One of these is Amblyomma americanum, a common form in the eastern states, and two others are members of the genus Dermacentor which occur also in the east. Whether spotted fever will eventually become established beyond its present range must remain a matter of conjecture, although there appears to be nothing that precludes such a grave possibility.

One of the most important insect-borne human diseases which does not exist in the new world is African sleeping sickness. In recent years this malady has decimated the native population in certain parts of eastern equatorial Africa and any extension of its range would be most serious. It seems very unlikely that America will ever have to face an epidemic, for the introduction of sleeping sickness together with its carrier is not at all probable, and the possibility of its becoming established, even after introduction, is still more remote. As is well known, sleeping sickness depends for its spread entirely upon certain biting flies known as tsetse-flies belonging to the same family as our common house fly and stable fly. The genus Glossina in which these flies are included is restricted to the African continent, but is there represented by a number of species, several of which have been shown to act as carriers for trypanosome diseases in animals. One only, Glossina palpalis, is known to carry the trypanosome of human sleeping sickness, Trypanosoma gambiense. The disease appears to have been originally endemic only in West Africa, but was found in eastern equatorial Africa something over ten years ago, and it is in this latter region that its ravages have been so pronounced. Owing to certain peculiarities in the habits of the tsetse-flies, the distribution of sleeping sickness is limited to very definite areas in the region where it occurs. The fly, which has a sharp needle-like beak for sucking blood, resembles our own, stable-fly (Stomoxys calcitrans) in general appearance but is considerably larger, measuring about half an inch in length. It is found only in the dense brush which grows along the edges of streams, ponds and lakes. In such places persons and animals may be bitten by the flies and it is exclusively through such bites that these insects may obtain virus of sleeping sickness from the blood of a person or animal suffering with the disease. Should the fly obtain a meal of blood containing trypanosomes, these may multiply in the body of the fly, although not always, for only about one in twenty of such flies becomes infectious. A considerable period must now elapse before the infected fly is in condition to inoculate a new patient, usually thirty or forty days, but after this for at least seventy-five days it remains infectious, and may introduce the trypanosomes into the blood of any animal upon which it feeds during this period.