Insects As Carriers Of Disease

Within fairly recent years careful study has been given to insects as carriers of human disease, with the result that astounding facts have been discovered. We know today that several of the serious and fatal diseases that afflict man, and several others to which domestic animals are subject, are carried or transmitted by insects; and in some cases the disease is carried in no other way. The study of these facts and possibilities constitutes the new Medical Entomology.

House Flies

Beyond doubt the commonest and the worst offender is the house fly. Both observed facts and careful experiments have proved that this insect is instrumental in the spread of typhoid fiver, tuberculosis, and certain intestinal diseases, and there is every probability that further study will reveal others.

The habits of the fly in its choice of breeding places, its irrepressible tendency to enter our houses and walk over our food, and the structure of its body, especially its feet and its tongue, form the chain of circumstances by which the transfer of disease germs is brought about. The same fly that spent its larval life as a maggot in filth or infected excrement later comes through our opened doors or unscreened windows; its hairy feet loaded with dangerous germs, and alight on the food set on our dinner table. Or, coming from the street, where it has been feeding on the sputum of some unfortunate victim of tuberculosis, it brings in the deadly bacteria in the ridge and hollows of its tongue.

Mosquitoes

In a wholly different manner the mosquitoes of certain species have been proved to transmit malarial fever. Indeed it is known that this disease never is transmitted in any other way. Here, in contrast to the fly which simply carries germs mechanically on some part of its body, we have an insect that serves as an intermediary host to the organism, the latter going through a definite part of its life round within the body of the insect, the remainder within the body of man. The mosquito itself is infected by sucking blood of a human being suffering from malaria. The organism that causes the disease, being thus transferred to the stomach of the insect, goes through certain changes, and eventually collects in large numbers in the salivary gland of the mosquito. If, now, this insect bites another person, the organisms are transferred, and shortly develop in the blood, giving rise to the characteristic chills and fever, recurring at regular intervals, according to the particular type of organism with which the mosquito has been infected.

As a direct result of this knowledge it has been possible to bring about phenomenal results in fever ridden districts, by careful screening, and by isolating fever patients so that mosquitoes could not get at them while they were suffering from the disease. In places where this work has been carefully done the death rate from malaria has been reduced to a small fraction compared to the amount that was seen before.

Other Diseases Transmitted

Yellow fever is transmitted solely by certain species of mosquitoes. In Montana and Idaho a disease known as spotted fever is carried by a tick. The terribly fatal bubonic plague is transmitted mainly by fleas. A species of fly has been found to be the means of spread of the sleeping sickness that has been ravaging some sections of Africa. Yet this is only a part of the known list.

Among domestic animals, a striking example is found in the disease known as Texas fever, which has caused tremendous losses among owners of cattle. The organism causing this disease is carried by a species of tick, and infection invariably takes place only through the bite of this tick. In Asia and Africa other serious or fatal diseases of domestic animals have been traced to insect carriers, and it is probable that similar discoveries will be made here.