^00 SLEEPING SICKNESS, FLIES, AND DISEASE 



spread of tropical diseases, we are not surprised to 

 learn that this disease is spread by the agency of an 

 insect. Tliis time a fly — the well-known tsetse fly. 

 Bruce had previously proved that the fly disease of 

 horses and cattle known as Nagana in Africa -was 

 communicated from animal to animal by tlie bite of 

 a species of fly {Glomiui mors'itans) ; lie concluded, 

 moreover, that the transference was mechanical — that 

 is to say that, unlike the case of the malarial parasite 

 where the anophelines play the part of intermediary 

 hosts, the fly simply became mechanically infected by 

 the parasite adhering to the mouth parts, so that when 

 it bit a healthy animal it transferred to the wound the 

 adherent parasites, in a manner somewhat analogous 

 to the way in which the domestic house-fly carries in- 

 fection on its body. The mode of infection having been 

 shown in Nagana, it was not long before it was deter- 

 mined that an allied species of fly {Glossiiui pa/palis) was 

 probably the agent which transmitted sleeping sickness. 



There was here, as in the case of yellow fever 

 and malaria, the most significant fact that sleeping 

 sickness was found only in districts where the fly was 

 found. No tsetse, no sleeping sickness : just as no 

 anophelines, no malaria ; no stegomyia, no yellow fever. 



In the AVest Indies, where although there are 

 many species of biting flies there is no tsetse, sleeping 

 sickness has not occurred, although sleeping sickness 

 was introduced time and time again with the slaves 

 during the slave trade period. 



The next great question then arose, AVas the 

 tTansfer.ei^ce of infection i^echg-nical ? Or, as m 



