that the blood of persons suffering from malaria contains the parasitic organisms, and that these, on being taken into the stomach of the proper kind of mosquito, undergo certain changes and later penetrate the wall of the stomach to form vesicular swellings. Within these they multiply, and finally on the bursting of the nodule are set free in the body cavity and find their way to the salivary glands. After becoming infected, a period of twelve to twenty days are required for these changes in the mosquito. Then for a period of several weeks the virulent organisms remain in the salivary glands and if the mosquito bites a second person the parasites are introduced with the salivary secretion, through the puncture into the circulation. Here they multiply and produce another case of human malaria, which develops from ten days to three weeks after inoculation.
As previously stated, only certain mosquitoes can transmit malaria, for when the parasites are ingested by other species of mosquitoes they do not continue their development, but die without passing through the stomach into the salivary glands. In the United States only one form, Anopheles maculipennis, is capable of harboring malaria, but in other parts of the world, especially in the tropics, other species of Anopheles and related genera act as hosts for the several kinds of Plasmodium.
Although in temperate regions the number of deaths from malaria is rather small, in spite of the wide-spread occurrence of the disease, the economic loss is very great, due to the debilitated condition which invariably occurs in the population of malarial districts. In the tropics, however, malaria in its various forms causes an enormous number of deaths and predisposes its victims to so many other dread diseases that it ranks as perhaps the most important human disease.
Fortunately prophylactic measures against malaria are not difficult, although they have been shamefully neglected in our own country. They consist in the elimination of anopheline mosquitoes, which is best accomplished by the destruction of mosquitoes in general. The larval or preparatory stages of anopheline, and of practically all other mosquitoes, are passed in the water of small quiet ponds, puddles, exposed vessels containing water, rain-barrels, etc., and it is during this period that they are most easily controlled. This is accomplished by oiling the water with either crude or refined petroleum or with some miscible oil. The petroleum forms a film over the surface of the water through which the larvæ can not extrude their breathing tubes and they are thus suffocated. The application of miscible oils is efficacious, but attended with some danger, since it destroys fish and predatory insects which are themselves some of the most important natural enemies of mosquitoes. Very frequently even oiling is not necessary, as much swamp land may be permanently freed from mosquitoes by very simple systems of drainage ditches which prevent the accumulation of the stagnant water in which the larvæ occur.