146 IMMUNITY IN PROTOZOAL INFECTIONS 



the one from South America. There appear to be slight morphological 

 differences between the two, but whether these are sufficiently distinct 

 to justify the recognition of the South American form as a distinct species, 

 Babesia argentina, apart from the cross-immunity test, is open to question. 

 In connection with piroplasmosis of horses, Nuttall and Strickland (1910) 

 and du Toit (1919) showed that animals recovered from infections with 

 Babesia caballi were still liable to infection with Babesia equi. Here again 

 morphological characters enable the species to be distinguished. The 

 difficulty of accepting the test as a means of distinguishing species is 

 illustrated by the experiments of Laveran and Nattan-Larrier (1913) on 

 canine piroplasmosis. The disease occurs in dogs both in France and 

 North Africa, and on morphological grounds appears to be due to the same 

 parasite, Babesia canis, in both places. Yet dogs which have recovered 

 from infection with the French virus and are completely immune to 

 further inoculations are susceptible to the North African virus. It would 

 appear imj)ossible on these grounds alone to recognize two species of 

 parasite. 



As in the case of natural immunity, acquired immunity is dependent 

 on antibodies which appear in the blood, for the serum of the animals 

 which have recovered or have been infected for a length of time sufficient 

 to allow of the production of these substances can be employed as a 

 curative agent in the case of infected animals. Furthermore, the serum, 

 when injected into an animal before it is exposed to infection, may entirely 

 prevent an infection. In this case the immunity is known as passive 

 immunity, because the host itself has taken no part in the production 

 of the antibodies, which are merely introduced from another animal. 

 The extensive investigations of Rabinowitsch and Kempner (1899), and 

 of Laveran and Mesnil (1901a), on infections of rats due to TryjKinosoma 

 lewisi threw considerable light on this subject. Infected rats pass through 

 an acute phase followed by a chronic one, from which ultimate recovery 

 takes place. The animals are completely immune from reinfection. A small 

 quantity of the serum (0-5 c.c.) of a recovered animal, if inoculated into 

 the peritoneal cavity of a rat, will entirely prevent infection when trypano- 

 somes are inoculated twenty-four hours later. This property is possessed, 

 though to a less extent, by the serum of animals, such as goats and sheep, 

 which have recovered from infections with the pathogenic trypanosomes, 

 and animals, such as cattle, which are in a very chronic stage of infection. 

 Taliaferro has shown, in the case of T. leivisi, that this is due to the 

 appearance in the blood of the rat of a substance which inhibits the repro- 

 duction of the trypanosomes (see p. 467). 



Many attempts to produce an active immunity by other means than 

 actual infection and natural recovery have been made. So-called attenu- 



