668 LEISHMANIOSIS 



be said that many others have been described in various species 

 of mammals, birds, and fishes, and that these are spread either 

 by flies or by leeches. The most interesting of those mentioned 

 is Dourine, a condition resembling in many ways nagana. It, 

 however, presents this peculiarity, that infection does not take 

 place by an intermediate host, but apparently directly through 

 coitus, as it occurs only in stallions and in mares covered by 

 these. 



In several of the trypanosomal infections of animals it appears 

 as if, as in the case of Tr. lewisi, the animal suffers little 

 inconvenience from the presence of the parasite in its blood, 

 and the view has even been put forward that in many pathogenic 

 trypanosomes there exists a host which acts as a " reservoir," and 

 carries the organism without being affected by its presence more 

 than, for example, is the rat by Tr. lewisi. Though no opinion 

 can be expressed on this point, it is necessary to bear the fact 

 in mind that either natural or acquired immunity can exist 

 against such protozoa. Not only is this important from the 

 point of view of the investigation of the conditions under which 

 such tolerance arises, but also from the bearing which the 

 existence of this tolerance may have on the spread in nature of 

 the parasiteS'to a susceptible species from immune animals which 

 still harbour trypanosomes in their blood. We are, however, 

 as yet quite ignorant of many of the processes at work in the 

 body during a trypanosomal infection, and of the causes of the 

 symptoms and other morbid effects. 



Leishmaniasis. 



Under this term there are grouped three human diseases, but 

 the exact zoological place of the parasites among the protozoa 

 cannot be said to be at present definitely settled. These 

 organisms are the Leishmania donovani, associated with the 

 human disease, kala-azar ; Leishmania infantum, derived from a 

 similar disease occurring in children ; and Leishmania tropica, 

 which has been found in a skin ulceration of widespread 

 geographical distribution. Microscopically the organisms are 

 practically identical, but at present it is convenient to look upon 

 the three species as being distinct. 



Leishmania Donovani. — Leishman noticed in several soldiers 

 invalided from India for remittent fever and cachexia that the 

 most careful examination of the blood failed to reveal the 

 presence of the malarial parasite. From the fact that such 

 patients had almost invariably been quartered during their 



