The Blood Flagellates 589 



tection. Certain other species, such as G. morsitans and G. swynnertoni , 

 have a wide range over bush land and must be controlled by other 

 methods. Destroying or driving out game may be effective, with reduction 

 in numbers or even practical elimination of tsetse flies in the area. This 

 method has been practiced successfully in Southern Rhodesia for the 

 control of G. morsitans. Aside from the decrease in sleeping sickness, the 

 reduction in incidence of nagana has favored the maintenance of cattle. 

 The local use of DDT sprays also seems to have some value. The spraying 

 of domestic animals not only protects them from flies, but may help also 

 in reducing the numbers of tsetse flies. In addition, the spraying of DDT 

 from airplanes has been effective in practical tests over bushy areas of a 

 few square miles. 



A rather interesting delayed effect on the tsetse fly population may 

 be produced by hybridization. It seems that mating will occur readily 

 between subspecies or species in certain combinations, leading to hybrid 

 offspring which are often sterile or of low fertility. Biological warfare of 

 this type has been suggested as a possibility in controlling tsetse flies. 



Chagas' disease 



Distribution. This disease was first described by Chagas (12) in 

 Minas Geraes, Brazil, and for many years this appeared to be the only area 

 in which cases were at all common. Prior to 1937, only 113 cases had 

 been reported outside Brazil, although these were distributed through 

 Argentina, Guatemala, Panama, Peru, El Salvador, and Venezuela (77). 

 Increasing interest, coupled with the extensive use of xenodiagnosis and 

 complement-fixation tests, has revealed that the disease is far from rare in 

 South America. In Chile (50), xenodiagnosis has shown an incidence of 

 12 per cent in 12,581 individuals; complement-fixation tests, 17 per cent in 

 8,142. Surveys of smaller groups have indicated a comparable or higher 

 incidence in other areas: Argentina, 23-42 per cent; Bolivia, 6-31 per cent; 

 Brazil, 15-51 per cent; Uruguay, 6 per cent; Venezuela, 27-46 per cent. 

 Cases also have been reported in Colombia, Ecuador, Mexico, and 

 Paraguay. Such data accentuate the need for intensive surveys throughout 

 the known range of Trypanosoma cruzi. Perhaps Chaga's disease, once 

 considered geographically as well as historically Brazilian, will prove to 

 be more nearly an ail-American disease. 



The causative organism. Trypanosoma cruzi is unusual in that, in the 

 vertebrate, the trypanosomal form invades various tissue cells in such 

 organs as the heart, striated muscles, central nervous system, thyroid 

 and lymph glands, bone marrow, ovaries, and testes. Invasion is fol- 

 lowed by metamorphosis into the leishmanial stage. Repeated fission then 

 occurs, so that the host cell is distended into a relatively thin membrane 

 ("cyst") enclosing leishmanial stages (Fig. 12.3), the number of which 

 varies with size of the host cell (41). Metamorphosis into trypanosomal 



