INSECTS AS AGENTS IN SPREAD OF DISEASE 545 



states in a much milder form with only 5 per cent, mortality. A com- 

 mon 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 con- 

 taining 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. 



