98 TRYPANOSOMES AND SLEEPING SICKNESS 



and three distinct types may be observed at once in the blood of 

 an infected animal, a long slender form with a long free flagellum, 

 a short stumpy form with a short flagellum and a form interme- 

 diate between these (Fig. 20). Some investigators regard the 

 trypanosome causing the mild sleeping sickness of Nigeria as a 

 distinct species, named T. nigeriense. 



Sleeping Sickness 



Transmission. — Sleeping sickness of either type, and also 

 many trypanosome diseases of lower animals, is transmitted 

 primarily by certain species of tsetse flies which act as inter- 

 mediate hosts for the trypanosomes. The Gambian parasite, as 

 first shown by Sir David Bruce, is normally transmitted by the 

 tsetse fly, Glossina palpalis, and its distribution is now almost 

 coincident with the range of this species. It occurs on the 

 west coast of Africa from the Senegal River to the State of 

 Mossamedes in Portuguese West Africa, including nearly all 

 the tributaries of the Niger and Congo Rivers. Eastward it 

 extends to the valley of the Upper Nile and Lake Victoria Nyanza 

 in Uganda and along the east shore of Lake Tanganyika. 



The Rhodesian trypanosome depends on the more widespread 

 and less easily controlled Glossina morsitans. This species of 

 tsetse fly occurs all the way from northeastern Transvaal to 

 northern Nigeria in West Africa and to southern Sudan in the 

 basin of the Nile in East Africa. So far, the disease caused by 

 the Rhodesian parasite is limited to a small portion of East 

 Central Africa but it is spreading both north and south. 



Experimentally other species of tsetses are able to transmit 

 the Gambian disease, but it is doubtful whether any except 

 G. palpalis are important transmitting insects in nature. As in- 

 timated above it is only the absence of tsetse flies in other parts 

 of the world that we have to thank for the fact that sleeping 

 sickness when introduced is not propagated. Experiments show 

 that it is also possible for the Gambian trypanosome to be trans- 

 mitted mechanically by the stable-flies, Stomoxys, though this 

 probably seldom happens in nature. Macfie has shown that in 

 Nigeria the human trypanosomes undergo developmental stages 

 in a stable-fly, .S. nigra, but circumstances did not allow him to 

 determine whether the salivary glands become infective as in 



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