THE MALARIAL PARASITE. 639 



natural food of this insect is the juices of plants, and, no doubt, a 

 vast majority of them never have a chance to fill themselves with 

 the rich red fluid from their human victims which they are so 

 eager to substitute for their normal diet when opportunity offers. 

 If, as we have suggested, the Plasmodium abounds upon the stems 

 and foliage of herbaceous plants in marshy localities, the mosquito 

 would be very likely to pick it up in following its everyday 

 method of gaining a livelihood. 



Parasitic protozoa, closely resembling the malarial parasite, 

 have been found in the blood of birds and of reptiles, and possibly 

 one or more species of lower animals may serve as an " interme- 

 diate host" for the hsematozoon under consideration. Laveran 

 has given a drawing, in his work on Paludism, of a parasite found 

 in the blood of the lark, which is evidently of the same family. 

 The fact that a parasite may develop in the blood, or elsewhere, 

 in one or more species of animals without giving rise to any evi- 

 dent symptoms of disease can not be taken as evidence that it is not 

 pathogenic for man, or for some other animal. On the contrary, 

 we have numerous instances which show that animals may have 

 a natural or acquired immunity to the pathogenic action of para- 

 sitic micro-organisms which are deadly for other animals of the 

 same or different species. 



Texas fever, an infectious disease of cattle which prevails as 

 an endemic disease in certain regions in the southern portion of 

 the United States, has been shown, by the researches of Theobald 

 Smith and other bacteriologists belonging to the Agricultural 

 Department, to be due to a blood parasite belonging to the pro- 

 tozoa {Pyrosoma higeminum of Smith). In this disease the tick 

 has been shown to be the intermediate host of the parasite. The 

 ticks which fall from infected animals give birth to a numerous 

 progeny in the pastures frequented by them, and these young 

 ticks attach themselves to other animals which subsequently feed 

 in the same pastures and transmit to them the fatal infection. 



The tsetse fly disease of Africa has recently been shown by the 

 researches of Bruce to be due to a flagellate infusorium which is 

 found in the blood of infected animals. This disease is fatal to 

 the ox, the horse, the dog, the sheep, and the ass, but not to the 

 indigenous wild animals in the region infested by the tsetse fly. 

 The researches of Bruce indicate that the fly acts as a carrier of 

 the parasite from diseased to healthy animals. He has shown by 

 experiment that after feeding on the blood of an infected animal 

 the tsetse fly can communicate the disease to a healthy animal by 

 its bite. After a short period of incubation the htematozoa appear 

 in the blood concurrently with the development of fever, and fol- 

 lowed by rapidly progressive anaemia, dropsy, and death. 



The so-called " surra disease,'^ which prevails in certain por- 



