I] DEVELOPMENT OF PARASITE 9 



The possibility of other modes of infection, however, should 

 not be ignored, for they are known to occur in closely related 

 groups, and therefore it is considered advisable to include a 

 short account of some of the ways in which the infective agent 

 may be spread by its invertebrate host. 



The simplest case is that in which the parasite, without 

 undergoing any morphological or biological changes, merely 

 multiplies in the alimentary canal of its invertebrate host. 

 The parasite may then enter a vertebrate either by regurgi- 

 tation of the contents of the alimentary canal when the inverte- 

 brate host is feeding, or else by the faeces of the latter, containing 

 the parasites, entering the open wound caused by its bite. 

 The plague bacillus, which multiplies in the gut of the flea, is 

 probably transmitted in this way. 



Usually, however, the parasite undergoes a cyclical develop- 

 ment, passing through various stages that are generally incapa- 

 ble of living in the blood of vertebrates, and therefore during 

 this developmental period the host is non-infective. This 

 negative interval is known as the incubation period and may 

 vary from two or three days up to as long as 12 or 13 weeks, 

 its duration depending mainly on the temperature, as develop- 

 ment is accelerated by increased warmth and retarded, or even 

 completely arrested, by cold. 



The development of various species of trypanosomes in the 

 alimentary canal of the tsetse-fly includes some of the simplest 

 examples of cyclical evolution. In the case of Trypanosoma 

 cazalboui, when ingested by the tsetse some of the parasites 

 remain in the fly's proboscis. In this region they develop, 

 also undergoing certain biological changes, and for a period 

 of about seven days they are incapable of living in the blood 

 of vertebrates. At the expiration of this short incubation 

 period, however, the trypanosomes recover their infectivity for 

 vertebrates, and after this the tsetse is infective for a consider- 

 able period. In other trypanosomes, e.g. T. gambiense and 

 T. brucei, the evolution in the tsetse-fly is more complicated, 

 the parasites developing in the digestive tract and passing 

 through a crithidial stage before finally becoming infective, 

 but in every case during this development they are restricted 



