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The notorious prevalence of malaria on the frontier is due to the introduction of the plasmodium into a region swarming with mosquitoes, where there are few window-screens or two-story houses. No known race has any real immunity against malaria.
This discovery, due to the Englishman, Ross, in connection with birds, and to the Italian, Grassi, in connection with man, consists in having found out that the plasmodium of malaria, which produces the malady, is inoculated in man and in the various animals subject to it, by a special kind of mosquito. Let us inquire what was the state of science prior to this discovery.
But here will challenge some twentieth-century Gradgrind: "This is all very pretty from the point of view of abstract science, but what is the practical value of it? The discovery of the plasmodium and its peculiarities has merely shown us the how and the why of a fact that we had known well and utilized for centuries, namely, that quinine will cure malaria." Just listen to what follows.
It was not until 1880, that Laveran, a French army surgeon stationed in Algeria, announced the discovery in the blood of malarial patients of an organism which at first bore his name, the Hematozoon-Laveran, now known as the Plasmodium malariæ. This organism, of all curious places, burrowed into and found a home in the little red corpuscles of the blood.
It wasn't long before he was, in the immortal language of Mr. Devery, "caught with the goods on"; and in 1895 Dr. Ronald Ross, of the Indian Medical Service, discovered and positively identified the plasmodium undergoing a cycle of its development in the body of the mosquito.
The very sickle-shape of the plasmodium turned itself into an interrogation mark. The first clew that was given was the new and interesting one that this organism was a new departure in the germ line in that it was an animal, instead of a plant, like all the other hitherto known bacilli, bacteria, and other disease-germs.
Being an animal, the plasmodium naturally would not grow upon culture-media like the vegetable bacilli and bacteria, and this very fact had delayed its recognition, but raised at once the probability that it must be conveyed into the human body by some other animal.
The parasites that cause the malarial fevers are Sporozoans and belong to the genus Plasmodium. Other names such as Hæmamoeba and Laverania have been used for them, but the term Plasmodium is the one now most commonly employed. The three most common species are vivax, malariæ and falciparum, causing respectively the tertian, quartan and remittent fevers.
The negro and other colored races, it is true, are far less susceptible; but this we now know applies only to adults, as the studies of Koch in Africa showed that a large percentage of negro children had the plasmodium in their blood. No small percentage of them die of malaria, but those who recover acquire a certain degree of immunity.
In the meantime studies of other parasites more or less closely related to Plasmodium showed that the sexual stage occurred outside the vertebrate host. The remarkable work of Dr. Smith on the life-history of the germ that causes the Texas fever of cattle had a strong influence in directing the search for this other stage of the malarial parasite.
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