Page:Popular Science Monthly Volume 81.djvu/547

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541
INSECTS AS AGENTS IN SPREAD OF DISEASE

through eating moldy corn or corn products, and investigators went so far as to describe a certain fungus as the specific cause of the disease. This hypothesis was never satisfactorily in accord with the facts, and has been abandoned very generally in favor of a belief that pellagra is insect-borne. This has not yet been sustained by actual proof, and is far from being generally accepted, but Sambon and others have adduced much evidence to show that the "black-fly" (Simulium) may be the carrier for the virus of pellagra. These flies are widely distributed throughout the world, always occurring in proximity to rapidly flowing streams of water in which the larva? live. The adults, though small, are vicious biters. They appear mainly in the spring, more rarely in the fall, and agree in seasonal distribution with the incidence of pellagra. The causal organism has never been found and is evidently an ultramicroscopical or filterable virus.

One of the best known insect-borne diseases, and one which is of great importance in many parts of our own country is malarial fever, variously termed ague, chills and fever, etc. This was the first human disease traced directly to insect carriers and gave the impetus which has led to the unraveling of the facts connected with other insect-borne diseases. There are many types of malarial fevers, due to a number of similar but different blood parasites and the disease is most common in tropical regions, although in our own country it extends well into the northern states, even quite commonly into Massachusetts. The protozoan blood parasites that cause malaria were first demonstrated many years ago, in 1880, by a French surgeon, Laveran, who discovered them in the blood of persons suffering from malaria. Five years later an Italian, Golgi, distinguished three kinds, each associated with one of the more familiar types of malaria. They were found to go through a regular life cycle in the red blood corpuscles and, from analogy with other known Protozoa, it was suspected that in addition to their non-sexual generations in the human blood there must be a sexual development in some cold-blooded animal. Manson was led to suspect that some insect might be the secondary host and, working on this hypothesis, Eoss in India first found the malarial parasites in a certain kind of mosquito in 1898. He had worked for nearly three years on a common mosquito belonging to the genus Culex without result, but finally in a mosquito of the genus Anopheles was able to trace the development of the parasite. His epoch-making discovery has been since amply confirmed and extended by experimental proof till we now know that the various types of malarial blood parasites complete their life-cycles in anopheline mosquitoes, the latter acting as the sole carriers of the disease.

The details of growth and development of these parasites, which belong to the Protozoan genus Plasmodium, are extremely interesting, but far too complicated to discuss briefly. In general it may be said