4 INDIRECT TRANSMISSION [CH. 



It is, of course, quite possible for a biting-fly to transmit 

 any particular infection both directly and indirectly, as in the 

 case of the transmission of Nagana {Trypanosoma brucei) by 

 Glossina pallidipes. Bruce shewed that if a tsetse fed on 

 blood containing these trypanosomes, the fly remained infec- 

 tive for about forty-eight hours, during which period, if it 

 bit another animal, the latter became infected. It has since 

 been proved, however, that in addition this trypanosome 

 develops in the alimentary canal of the tsetse-fly and after a 

 certain incubation period, during which the fly is non-infective, 

 it again becomes infective. This infection, therefore, is trans- 

 mitted both directly and indirectly, but the epidemiology of 

 the disease is strongly against the view that transmission is 

 usually effected by the direct method. 



(6) Indirect transmission. 



When the pathogenic agent causing the disease undergoes 

 some developmental cycle in the biting-fly, resulting in the 

 latter becoming more or less permanently infective after this 

 development has taken place, the transmission is said to be 

 indirect or cyclical. In these cases there is always a definite 

 biological relationship between the biting-fly and the parasite 

 which it conveys, and the latter is only capable of development 

 within the members of certain species or families of insects. 

 The best known example of this indirect method of transmission 

 is that of the malarial parasite by the mosquito. When the 

 parasite, at a suitable stage of its life-history, is taken into 

 the stomach of a susceptible species of mosquito, it undergoes 

 a complicated cycle of development in its new host, finally 

 resulting in the salivary glands of the latter becoming invaded 

 by a stage of the parasite adapted for entry into the blood of 

 the next person that the mosquito bites. 



It will be noticed that in this, and all other cases of indirect 

 transmission by biting-flies, the parasite develops in two hosts, 

 vertebrate and invertebrate, respectively. The host in which 

 the parasite undergoes its sexual life-cycle is called the 

 definitive host. In the case of the majority, if not all, 



