ACQUIRED IMMUNITY 145 



employed as a means of avoiding the risk of the disfiguring natural 

 infection on an exposed part such as the face. Another illustration of 

 absolute immunity conferred by a single infection occurs in the case of 

 East Coast fever of cattle due to infection with Theileria parva. Animals 

 which have recovered from one attack are immune for the rest of life. 

 The same remark applies to rats which have recovered from an infection 

 with Trypanosoma lewisi. Again, in the case of many of the disease- 

 producing trypanosomes it has been found that certain animals, such as 

 the goat and sheep, though acquiring an infection, eventually recover to 

 such an extent that trypanosomes can no longer be detected. In this 

 condition they are immune to further inoculations with the same trypano- 

 some. As in the case of naturally immune animals, these actively im- 

 munized hosts have been employed as a means of differentiating species. 

 If it is desired to distinguish two trypanosomes which resemble one 

 another morphologically, one of them is inoculated into a goat. When 

 the animal has recovered and is no longer susceptible to inoculation with 

 this trypanosome, it is inoculated with the other. If infection occurs, it 

 is assumed that the trypanosomes are different. Though the experiment 

 undoubtedly indicates a physiological difference between the trypano- 

 somes, it is far from clear that they belong to distinct species. The test 

 has been applied by Laveran and Mesnil and others to a group of trypano- 

 somes which resemble Trypanosoyna evansi, with the result that a number 

 of species of very doubtful value has been created. Similarly, in the 

 case of piroplasmosis the test has again been applied. Animals which 

 recover from an acute attack pass into a chronic phase, during which the 

 parasites show a gradual diminution in their numbers, till finally they can 

 no longer be detected except by the inoculation of comparatively large 

 quantities of blood into a susceptible animal. It has been shown by 

 Ed. Sergent and his co-workers (1924) that in the case of Babesia bigemina 

 it is possible to produce a superimposed infection in wdiich parasites 

 appear in the blood, but this is unaccompanied by symptoms. The infec- 

 tion, moreover, is less intense than the original one, the parasites quickly 

 disappearing again. Stockmann and Wragg (1914) showed that cattle 

 which had recovered from an infection with B. bigetnina, and were 

 immune to further inoculations with this parasite, were nevertheless 

 susceptible to Babesia bovis, and behaved, as regards symptoms and 

 intensity of infection, as animals at their first infection. In this instance 

 there were morphological differences which justified the separation of 

 the two parasites as distinct species. On the other hand, a form of piro- 

 plasmosis in cattle in South America is due to a parasite resembling 

 B. bovis. Brumpt (1920) showed that cattle which had recovered from 

 the infection with this parasite were still susceptible to inoculation with 

 I. 10 



