PROTOZOAL RESERVOIRS 731 



connected, sometimes very obscurely, with the parasitic habit. Repro- 

 duction, for instance, often takes place in a very aberrant manner, as 

 in the pupiparous flies and in the still more remarkable instance of 

 the bed bug. Generalizations from the study of allied but non-parasitic 

 forms have to be made with great caution. 



The relations between the protozoon and its two hosts are to a certain 

 extent specific, that is to say, a given species of parasite has for its 

 alternate hosts two particular species or their near 



allies. The parasite of malaria, for instance, occurs Relatlon8 between 



i u, ui j r -4.4. j i i Parasite and its two 



only in the blood of man and is transmitted only by host8 8pec jfj C 



certain species of mosquito of the genus Anopheles ; 

 the parasite of Yellow Fever is transmitted only by Stegomyia fasciata, 

 and there is no reason to believe that it occurs in any animal other than 

 man. There is seldom, however, a hard and fast line, either as regards 

 the vertebrate or the invertebrate host ; Filaria bancrofti, for instance, 

 can develop in several species of different though allied genera of 

 Culicidae ; according to Brumpt human trypanosomiasis (T. cruzi) in 

 Brazil can be conveyed by the bed bug as well as by Conorhinus 

 megistus, which was at first believed to be its only host ; several species 

 of Phlebotomus appear to be efficient in the transmission of Phlebotomus 

 fever ; Trypanosoma gambiense, usually transmitted by Glossina palpalis, 

 can also be transmitted by Glossina morsitans (Taute), while Trypano- 

 soma brucei, the usual host of which is morsitans, can develop in 

 palpalis. An equal latitude is to be observed in the relations between 

 the parasite and its vertebrate host ; Trypanosoma gambiense, for 

 example, is found in the blood of a considerable number of mammals in 

 the region in which the disease is endemic ; Piroplasma gibsoni, nor- 

 mally a parasite of the jackal, also occurs in the dog; most laboratory 

 animals can be infected with various species of trypanosomes and 

 spirochaetes. 



The occurrence of a given species of protozoal parasite in a number of 

 different vertebrates introduces a question of considerable importance, as 



each of these constitutes a source from which individuals 



Reservoirs of Infection 

 of another species may become infected. It will be 



observed that when a given species exists in several species of hosts, its 

 effect may vary with the kind of host in which it is living. In the instances 

 quoted above, the trypanosome which causes fatal sleeping sickness 

 in man is without effect on many species of antelope, while Piroplasma 

 gibsoni, which is fatal to most dogs, and particularly fatal to fox-hounds, 

 is harmless to the jackal, in which it normally occurs. From the point 



