Diseases Acquired From Animals
( Originally Published 1936 )
How MALARIA IS CARRIED TO HUMANS BY MOSQUITOES
LONG chapters in the history of human life and misery could be written on the basis of the ability of mosquitoes to spread disease. Cities, and even countries, have undoubtedly been destroyed by malaria. and other diseases brought by mosquitoes. There are ruins of ancient towns in Italy, the inhabitants of which have long since either fled or been killed by this malignant scourge. The enormous ruins of a great city such as Angkor-Vat in Indo-China, and of great cities in Central America, upon which explorers have stumbled, were probably deserted by a population which could not stand the ravages of the disease.
Perhaps the greatest triumph of modern preventive medicine was the proved ability to render such territory as that of the Panama Canal safe for human habitation merely by destroying the disease carrying mosquito.
The story of how it was discovered that the mosquito transmitted malaria from one person to another is a triumph of scientific research and scientific imagination. In 1880 a French Colonial physician, Laveran, discovered the little animal parasite which causes malaria. He showed that the parasite lived inside the red cells of the blood, and went through a cycle of development there. The cause of the malarial chill was the destruction of the red blood cell as the parasites burst it open in their process of growth.
The question arose, how did the parasite get from one human being to another? It was easily proved that mere contact of individuals could not communicate it. An English surgeon in India, Sir Ronald Ross, working on the idea that it might be spread by mosquitoes, finally demonstrated a malarial parasite in one stage of its existence in the salivary glands of the mosquito.
The final proof was produced by a man in Rome and a man in London working together. Malaria is common in Rome, unknown in London. The Roman was Bastianelli, who procured some Anopheles mosquitoes in Rome and turned them loose on a man suffering from malaria. They, of course, bit the man several times, sucking his blood. These identical mosquitoes were corked up in a bottle, and sent to London, and there the English scientist, Manson, allowed them to bite him. In that malaria-free climate, Manson developed a Roman malarial fever, thus proving that its transmission depended entirely upon the presence of the mosquito as the intermediary host.
We know, therefore, that one part of the life cycle of the malarial organism is passed in the body of man. When an infected individual is bitten by a mosquito, the mosquito sucks some of these organisms into its digestive tract. They work into its body cavity and there undergo fertilization, a male and female element uniting. This initiates another stage in-the development, which takes place largely in the salivary glands of the mosquito. When a thus infected mosquito bites another human being, it injects some of its salivary secretion into the wound in order to dilute the blood to its satisfaction. But in doing so, it also injects a malarial parasite.
Destruction of the mosquito has, therefore, quite logically driven malaria out of the regions where such destruction is scientifically practiced.