292 FLIES AND DISEASE 



by mosquitoes, but the next important disease to 

 be considered, Sleeping Sickness, is transmitted by 

 a large blood-sucking fly, known as the Tse-tse fly. 

 This disease, which is the cause of a very great 

 annual mortality in parts of Africa, is due to a 

 large protozoon parasite, known as a Trypanosome, 

 which swims freely in the blood and multiplies by 

 division. There is a periodic increase and decrease 

 in the number of trypanosomes in the blood, and 

 accordingly the infectivity of the patient for the 

 fly varies. In the fly the parasite passes through 

 certain developmental stages and eventually the 

 insect is capable of infecting a man, when it feeds 

 on him. One of the most important facts recently 

 discovered in regard to Sleeping Sickness is that 

 wild game, antelopes and reed-bucks, serve as a 

 reservoir for the parasite, remaining infective for 

 long periods without showing any signs of the disease. 



The problem of preventing this disease is a 

 peculiarly difficult one, and much has yet to be 

 learnt in regard to the habits of the fly and of its 

 natural enemies before the disease can be success- 

 fully dealt with in all the regions it affects. In 

 certain restricted areas, for example the Island of 

 Principe in the Gulf of Guinea, the measures for 

 destroying the fly have been completely successful 

 and the spread of the disease has been checked. 



Blood-sucking flies of various kinds transmit 

 several other species of disease-producing organisms 

 to men and animals, and other blood-sucking insects 



