Page:Popular Science Monthly Volume 50.djvu/659

Rh 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 "intermediate host" for the hæmatozoon 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 evident 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 parasitic 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 protozoa (Pyrosoma bigeminum 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 hæmatozoa appear in the blood concurrently with the development of fever, and followed by rapidly progressive anæmia, dropsy, and death.

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