140 IMMUNITY IN PROTOZOAL INFECTIONS 



under ordinary conditions by a definite cyclical development in the tsetse 

 fly, is greatly increased. 



It is noteworthy that Blanchard and Blatin (1907) have shown that 

 the marmot during hibernation at a temperature of 6° C. becomes resistant 

 to trypanosomes, with which it can readily be inoculated when it is in 

 an active condition. Brumpt (1908a) found that the dormouse showed 

 a similar immunity during hibernation, though it was observed that the 

 trypanosome (T. blanchardi) with which it may be naturally infected per- 

 sists in its blood during the hibernation period. The natural susceptibility 

 or the resistance of animals to infection with parasites has been advocated 

 as a means of differentiating species. The method has been mostly 

 used in the case of trypanosomes, but it has been also applied to other 

 parasitic Protozoa. As an example may be quoted the effect of inoculating 

 into rats the two trypanosomes T. congolense and T. nanum, which in 

 their natural hosts are morphologically indistinguishable from one another. 

 When inoculated into rats T. congolense gives rise to an infection, while 

 T. nanum does not, and it is claimed by the advocates of the specific 

 value of this test that the dift'erence justifies the separation of the two 

 species. That the test is not as straightforward as at first it might appear 

 is illustrated by the fact that if T. congolense is inoculated into a goat, 

 it will be found to have lost its power of infecting rats. It follows, there- 

 fore, that distinction of species based solely on the ground of resistance 

 of certain animals is zoologically unsound. Another application of the 

 same test was made by Adler (1924), who discovered a coccidium in the 

 intestine of the civet cat in West Africa. Morphologically it resembled 

 Isospora rivolta, a parasite of dogs and cats. Attempts to infect dogs 

 and cats with the parasite of the civet cat having failed, it was thought 

 justifiable to establish a new species. Looking at the question from the 

 reverse point of view, the susceptibility of a number of different hosts 

 to a parasite derived from one host is strongly suggestive of the identity 

 of the parasites which may occur naturally in a variety of hosts. Thus, 

 birds are very liable to natural infection with a malarial parasite, Plas- 

 7nodium prcecox. The demonstration that the parasites from one bird 

 can be inoculated into birds belonging to other species is a valuable 

 indication that the one parasite may, under natural conditions, occur in 

 a variety of hosts. The converse is not necessarily true, for development 

 in one host may bring about such a change in the parasite that it is no 

 longer able to infect a host which was originally susceptible to it. The 

 example of passage of Trypanosofna congolense through the goat, referred 

 to above, is a case in point. 



In connection with natural immunity it has to be remembered that 

 much depends upon the number of parasites — the dose of virus — intro- 



