Page:Popular Science Monthly Volume 81.djvu/555

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549
INSECTS AS AGENTS IN SPREAD OF DISEASE

can not be said, and the same is true to a greater or less degree of cholera, pink-eye, yaws, syphilis, and many other diseases which can not be considered as typically insect-borne.

One other disease which has been increasing at an alarming rate in our own country during the past several decades is infantile paralysis. This malady occurs in certain parts of Europe, whence it is probable that it was brought to America. As a rule it affects children during the first few years of life and, although the mortality is not so very great, a majority of the children affected are left permanently lame after recovery. The virus of this disease is an ultramicroscopic organism which causes lesions of the spinal cord that sometimes lead to paralysis. At the present time it appears probable that infantile paralysis is insect-borne, and it has been suggested by Brues and Sheppard that the stable fly, Stomoxys calcitrans, acts as a carrier of the virus,[1] although it is quite possible that some other insect also may be concerned.

No account of insect-borne diseases, however brief, could be complete without some reference to animal diseases. A few of these have already been referred to incidentally as affecting both man and animals, and it is quite likely that other human diseases whose etiology is at present obscure, will in the future be shown to bear some relation to those of animals. Apart from this, the economic loss occasioned by such affections of domestic animals is enormous, although it is in great part preventable.

A wide-spread disease of cattle in the southern part of the United States, known as splenetic fever, or "Texas fever," is the most important insect-borne animal disease that occurs in this country, and is particularly interesting since it was the first disease of any kind shown to be carried exclusively by insects or ticks. It occurs very generally throughout the gulf states as far north as the thirty-sixth parallel of latitude and is the cause of immense pecuniary loss to this region, not only on account of the cattle lost, but as a result of the greatly weakened condition of the animals in general. Southern cattle are usually immunized by an attack at an early age, but northern animals die in large numbers when exposed to the disease.

Smith and Kilborne showed, in 1893, that the protozoan blood-parasite, Piroplasma bigeminum, which Smith had discovered several years earlier to be the cause of the disease, is carried by ticks. The common cattle-tick of the southern United States, Margaropus annulatus, acts as the exclusive vector, becoming infected during its period of engorgement when feeding on the blood of a diseased animal and then trans-

  1. Since the above was written, it has been shown by experiments with monkeys by Rosenau and Brues, that Stomoxys can actually transmit this disease, and their results have been confirmed by Anderson and Frost.