Page:Popular Science Monthly Volume 81.djvu/549

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543
INSECTS AS AGENTS IN SPREAD OF DISEASE

Anti-malarial work of this sort has been undertaken and successfully prosecuted in many parts of the world; in the Federated Malay States, in the Suez Canal region, at Havana, Cuba, and in the Panama Canal Zone. Many other regions might be mentioned, but we must look in vain for such concerted and sustained work in the United States. Attempts have been made by many entomologists and public-spirited citizens to inaugurate measures against mosquitoes on account of their relation to malaria, but with the excepion of the most successful work accomplished by Smith in New Jersey and by others on Long Island little has been done to aid the efforts of the energetic few. We have, however, reason to believe that such apathetic contemplation on the part of the American public will some day develop into an active interest, and that the population of our extensive malaria-ridden areas will gradually see the possibilities of improvement in public health through the destruction of the malarial mosquito.

Another mosquito-borne disease which has aroused more interest in America on account of its spectacular appearance and higher mortality is yellow fever. This is due to a filterable virus, concerning the nature of which we can only speculate at the present time, although enough has been ascertained through experimental work to demonstrate that the virus is a living organism which undergoes a development of definite periodicity in mosquitoes of a single species known as Stegomyia calopus. This mosquito enjoys a very wide distribution in many parts of the world, mainly in the tropics, but also extends into the warmer temperate regions. Yellow fever is not so extensively distributed, being absent in many places where Stegomyia occurs, but it is nevertheless present in many parts of the tropics in both hemispheres and all that is necessary for the development of a possible epidemic in a region where Stegomyia occurs is the introduction of a human case in the early stages of the fever.

The larval habits of Stegomyia are in quite marked contrast to those of Anopheles. The adults are strictly domestic mosquitoes and occur almost entirely in the neighborhood of human habitations. Their larvae occur in the same places, breeding preferably in vessels containing small amounts of water, rain barrels, cisterns, stray tin cans filled with rain water, etc. On this account, extermination work against the yellow-fever mosquito resolves itself mainly into the examination and treatment of cities, towns and the immediate environment of smaller settlements.

A Stegomyia feeding upon the blood of a person suffering from yellow fever becomes infected only during the first three or four days after the onset of the fever; later than this mosquitoes do not obtain the virus. An incubation period of at least twelve or fourteen days in the mosquito is now necessary before the mosquito can infect a second