INSECTS AND HUMAN DISEASE 133 



infective. Should the infected fly bite a healthy person 

 within twenty-four hours, mechanical transmission takes 

 place ; the trypanosomes adhering to its mouth parts are 

 simply transferred to the blood of the new host and sleep- 

 ing sickness ensues. At the end of twenty-four hours 

 there is no fear of mechanical transmission ; from this time 

 till some days after the infective meal, further bites are 

 harmless, even to a healthy person. During this time the 

 trypanosomes taken into the system of the Glossina are 

 undergoing development in its gut an incubation period, 

 in fact. Eleven to fifteen days after the fly has had its 

 infective meal, it is again able to infect those whom it 

 bites, this time with the organisms it has harboured in its 

 body, provided that the atmospheric temperature has been 

 fairly high ; for it is a curious fact that only under such 

 conditions do the parasites develop within the tsetse flies, 

 but, once infective, they may remain so for sixty days. 

 So much for the transmission of sleeping sickness from 

 man to man. But man is not the only host ; the trypano- 

 somes are to be found in the blood of domestic animals 

 and a number of wild animals. Trypanosoma gambiense 

 is said to exist in the blood of cattle without detriment to 

 health, but Trypanosoma rhodesiense, on the other hand, is 

 the cause of enormous mortality in horses, donkeys, goats, 

 and dogs. Six hundred and ninety-eight animals were 

 examined at the Nawalia laboratory, including elephant, 

 rhinoceros, hippopotamus, lion, buffalo, caracal, galago, 

 squirrel, hunting dog, giant rat, wild rabbit, fourteen 

 different kinds of antelope, twenty-five monkeys, thirty- 

 five head of domestic stock, one hundred and forty-two 

 wild rats, and fifteen mice. Approximately, the percentage 

 of infected big game was fifty, being very high among 

 the antelopes, of which waterbuck, haartebeeste, reedbuck, 

 and duiker have been proved dangerous neighbours to man. 

 Big game are the reservoirs of the trypanosomes of man 

 and domestic animals. Frequenting the fly belts, and 



