TRANSMISSION OF PATHOGENIC TRYPANOSOMES 513 



Hornby (1921) found that mechanical transmission of trypanosomes 

 amongst domestic stock in Rhodesia is by no means uncommon. A few 

 animals which have acquired infection in tsetse-fly areas, if brought into 

 close contact with animals in a tsetse-free district, may lead to the infection 

 spreading through the stock. In such cases infection is spread by flies 

 other than tsetse flies, and presumably in a mechanical manner. All the 

 pathogenic trypanosomes which are transmitted by tsetse flies have been 

 shown by various observers to be capable of mechanical transmission by 

 mosquitoes or species of Stomoxys and Tabanus. In the case of T. evansi 

 and the forms allied to it both in the Old World and America, this is the 

 only method of transmission which has been demonstrated, unless the 

 claim made by Cross and Patel (1921) regarding the transmission of 

 T. evansi by ticks in India indicates a cycle of development com- 

 parable with that in tsetse flies in Africa. Mechanical transmission 

 of T. evansi {T. hippicum) by the house fly was proved to be possible 

 by Darling (1912). 



Direct inoculation of blood from an infected to a healthy animal will 

 bring about infection, and it is by this means that the various laboratory 

 strains of trypanosomes have been maintained for experimental work. 

 Many strains have been kept in rats or guinea-pigs for numbers of years, 

 but it must always be remembered that such artificially maintained strains 

 may acquire peculiarities which they did not originally possess in the 

 normal host. There is a variation in the animals inoculable with any one 

 trypanosome, and, furthermore, after successive passages the virulence 

 may become much increased. Intraperitoneal and intravenous inocula- 

 tions lead to infections more readily than subcutaneous ones. It is 

 highly probable that after long maintenance in animals like rats in the 

 unnatural conditions of direct passage, without any fly intervention as 

 occurs in nature, trypanosomes become profoundly altered, not only 

 morphologically, but also physiologically, so that care has to be exercised 

 in comparing such forms with those recently isolated from their natural 

 hosts. Bruce et al. (19136) expressed the opinion that " it is absurd to 

 expect to arrive at any classification at all approaching a true one by the 

 study of strains of trypanosomes kept for many years and undergoing 

 many vicissitudes in our European laboratories." 



Bruce (1897) noted that a dog which had eaten a piece of the congealed 

 heart blood of a heifer which had died of nagana contracted the disease, 

 while many instances are on record of animals becoming infected after 

 eating the organs of infected animals. Experimental work has demon- 

 strated the infective power of blood introduced into the mouth, stomach, 

 conjunctival sac, and vagina. Under natural conditions it is known that 

 T. equiperdum is transmitted through mucous membranes, while rats 



I. 33 



