JULT 5, 1912] 



SCIENCE 



complete conquest of a hitherto uncon- 

 trolled scourge. 



Did time permit, some consideratioa 

 could be given to insect carriers, such as 

 the body louse, which has to do with the 

 transmission of relapsing fever and typhus 

 fever in man, and the trypanosome infec- 

 tion in the common rat ; the bed-bug, which 

 has often attracted attention in connection 

 with plague, relapsing fever and kala-azar ; 

 and to the sand-fly, which has recently been 

 shown to be responsible for the pappataci 

 fever, which is due, like dengue and yellow 

 fever, to a filterable, invisible virus. It 

 will be better to pass these by and consider 

 briefly a vastly more important carrier, the 

 tsetse fly. 



Curiously enough, the tsetse-fly disease 

 of South Africa was the first disease clearly 

 recognized as insect-borne. The natives, as 

 well as the early travelers, realized that the 

 bite of the tsetse fly caused sickness and 

 death of the animals thus bitten. The work 

 of Bruce in 1894 proved this relation and 

 at the same time demonstrated that the 

 disease was due to a blood parasite, since 

 named, Tr. Brucei. At that time Bruce 

 was led to believe that the fly was infective 

 for but a few hours, or at most a day or 

 two after an infective feed. In other 

 words, the fly was thought to be a passive 

 carrier, simply carrying the parasite from 

 one animal to another. This view has been 

 definitely set aside as a result of the studies 

 of the past two years. Mechanical trans- 

 mission, especially where interrupted feed- 

 ing occurs, is possible, but that this is the 

 natural way is no longer believed. That 

 the tsetse fly obtained its infection from the 

 wild animals was a reasonable supposition, 

 which was soon confirmed by special exam- 

 inations. The important fact was estab- 

 lished that the wild animal, recovering 

 from the infection, became a chronic car- 

 rier and as such served as a natural reser- 



voir of the virus. As long as such wild 

 animals existed, the fly could infect itself 

 and transmit the disease to the passing 

 domestic animal. The introduction of rin- 

 derpest into South Africa is said to have 

 brought about the destruction of wild ani- 

 mals to such an extent as to render this 

 disease a negligible quantity in that region. 



Especial interest centers about the tsetse 

 fly because of its relation to sleeping sick- 

 ness, which is caused by the Trypanosoma 

 gambiense. While the fly, Glossina pal- 

 palis, is chiefly responsible for the spread 

 of this disease, there is reason to believe 

 that other species of tsetse flies can likewise 

 serve as carriers. Attempts to eradicate 

 the disease by removing all of the natives 

 from certain regions have failed to accom- 

 plish the desired result. The tsetses un- 

 doubtedly obtain their infection from some 

 other source than diseased man. This nat- 

 ural reservoir of the virus has not as yet 

 been discovered, although from experi- 

 ments made on antelopes it is not unlikely 

 that these or related animals constitute the 

 chronic carrier from which the infection is 

 transmitted to man by the fly. 



The question as to whether the tsetse fly 

 could act as an active carrier was finally 

 settled by Kleine (1909) and confirmed by 

 Bruce and his co-workers. It is now 

 known that flies which feed upon infected 

 animals remain harmless for a period of 

 about three weeks. After that time, how- 

 ever, they become infective and apparently 

 remain so during the remainder of their 

 life. Only a small percentage of the flies 

 thus fed become infected. The changes 

 which the trypanosome undergoes in the fly 

 are but partly known. Whether this para- 

 site has an intracellular stage, such as has 

 been observed in fleas infected with Tr. 

 Lewisi, remains to be demonstrated. It is 

 certain, however, that Tr. gamhiense, when 

 it once adapts itself to the conditions in 



