84 INFECTION AND IMMUNITY. 



It is possible that this is purely a mechanical trans- 

 mission. In many of the experiments rather large 

 numbers of flies have been used on a single animal, 

 and in this case an infective quantity of the para- 

 sites might well be introduced mechanically, where- 

 as a single fly might be able to produce infection 

 only after the parasite had multiplied in its tis- 

 sues, and, perhaps, had reached the salivary glands. 

 It is well known that an intermediate "insect 

 stage" of the trypanosome is not required for infec- 

 tivity, as appears to be the case with malaria. 

 Trypanosomatic diseases may be carried directly 

 from one animal to another by the injection of 

 blood for an indefinite period. A sexual stage of 

 the trypanosome has not yet been shown to occur 

 in the tsetse fly. 

 Type A unique type of insect transmission was de- 

 scribed recently by Miller 13 in the case of a dis- 

 ease of rats which is caused by a protozoan organ- 

 ism, Hepatozoon perniciosum, a new genus as well 

 as a new species. The transmitter is a mite, 

 Lelaps echidninus. The latter derives its infection 

 by sucking the blood of diseased rats, in which the 

 micro-organisms, at a particular stage of their 

 development, occur within large mononuclear leu- 

 cocytes. A sexual phase of the parasite takes place 

 in the stomach of the mite, forming an ookinet. 

 The latter penetrates the stomach wall, reaches the 

 body cavity of the mite, and becomes encysted 

 (oocyst stage). Within the cyst the parasite sub- 

 divides to form a number of sporoblasts, each of 

 which eventually contains from fourteen to twenty 

 sporozoites. Miller readily produced the disease 



13. Bull. No. 46, Hygienic Laboratory, U. S. Public Health 

 and Marine-Hospital Service, Washington, D. C. 



