560 



TRYPANOSOMES AND TRYPANOSOMIASES 



Method of Propagation. Dourine of the horse tribe probably never 

 spreads except through coitus, and in this respect it differs materially 

 from the other known trypanosomiases, which are propagated through 

 biting insects, particularly flies. Nagana is the trypanosomiasis in 

 which the mode of transmission by blood-sucking flies has been most 

 carefully studied by Bruce and others. It is the much discussed 

 tsetse fly (Glossina morsitans), which in biting and blood-sucking 

 spreads nagana from infected to non-infected animals. The tsetse 

 fly when at rest can easily be distinguished from other biting but 

 harmless flies. Its wings almost completely overlap like the blades 

 of a pair of shears. In other blood-sucking flies resembling the tsetse, 



FIG. 193 



Glossina palpalis, Rob. X 3%. 



the wings when the insects are at rest are always more or less separated. 

 In Africa the tsetse fly is usually found in low-lying, hot, humid 

 regions ; it is never seen far from water. Even in the so-called African 

 fly-belt it is not universally found, but is often strictly localized. It 

 bites most furiously during the day, less during the evening, rarely 

 during the night. Both sexes are blood-sucking. They follow the 

 big game in Central and South Africa. It has been found that many 

 wild animals in Africa harbor Trypanosoma Brucei in their blood 

 in small numbers. These animals are not sick, and are evidently in 

 a certain sense immune against the nagana trypanosomes, but the 

 latter when spread by the tsetse fly to domestic animals produce the 

 disease in its virulent form. It has already been said that cattle may 



