8 CONDITIONS AFFECTING TRANSMISSION [CH. 



vertebrates, but are usually insect parasites (e.g. Trypanosoma 

 boylei Lafont), and such forms as Trypanosoma gambiense, which 

 have become well-adapted to a parasitic mode of life in many 

 of the larger vertebrates. In the latter case the tsetse-fly, 

 the insect host of T. gambiense, has apparently developed an 

 immunity against infection with this flagellate, for only a com- 

 paratively small proportion of the flies becomes infected after 

 ingesting blood containing the parasites. 



When once a biting-fly becomes infected with the par- 

 ticular species of blood parasite of which it is the definitive 

 host, it generally remains infective for a very considerable 

 period, often for the remainder of its life. It is obvious, 

 therefore, that when a parasite is transmitted by a true defini- 

 tive host, it is much more difficult to eradicate than when it is 

 spread mechanically. In the latter case it is only necessary 

 to protect all infected vertebrates from the bites of flies for a 

 period of not more than three days and the insects cease to 

 be infective. In the former case, however, the eradication of 

 the parasite can only be accomplished effectively by the destruc- 

 tion of the insect host. In fact the prevention of protozoal 

 infections, which include many of the more important tropical 

 diseases, is mainly an entomological problem, as it depends on 

 the destruction of insects and to do this a knowledge of their 

 habits and life-cycles is essential. 



In the case of yellow fever the destruction of the invertebrate 

 host, Stegomyia fasciata, is comparatively simple, and as a 

 result this disease is rapidly disappearing from the more 

 civihzed parts of the world. On the other hand, no effective 

 means have yet been discovered of destroying Glossina mor- 

 sitans, one of the most important disease carriers in Africa. 

 Yet there is little doubt that when we have acquired a complete 

 knowledge of the bionomics of this redoubtable insect, it will 

 be possible to devise some means of considerably reducing its 

 numbers. 



An infected invertebrate host may spread infection from 

 one vertebrate to another in a variety of ways, but in the case 

 of biting-flies, the infected insect whilst feeding generally 

 inoculates the parasite directly into the body of the host. 



