Page:Popular Science Monthly Volume 81.djvu/129

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that the malaria of birds was transmitted by a species of mosquito (Culex) and when Grassi, Bignami and Bastianelli (1898-99) likewise demonstrated that malaria of man is transmitted by another species of mosquito (Anopheles) . But before this, Bruce's study (1894-97) of the South African disease of cattle, which you may remember Living- ston refers to as the "tse-tse fly disease," resulted in the discovery of the protozoan origin of the disease and the importance of the tse-tse fly (Glossina morsitans) in its transmission.

Soon followed (1900-01) the discovery by the United States Army Yellow Fever Commission — Keed, Carroll, Agramonte and Lazear — of the transmission of yellow fever by a third species of mosquito, the Stegomyia, and in 1903 Bruce announced that the sleeping sickness of Africa, due to a trypanosome, is transmitted by the tse-tse fly (Glossina palpalis). So also certain closely allied diseases of the far east, known as dum-dum fever, kala-azar, oriental sore, etc., were shown to be due to protozoa and to be probably transmitted by an insect.

The importance of these discoveries for prophylaxis was far reach- ing. It had long been known that malaria could be cured by quinine, but physicians in face of constant infections and reinfections were helpless. Now, the knowledge that the disease is transmitted by a mos- quito, and by but one genus of mosquito, the Anopheles, allows the health officer to step in and by draining the breeding places of the mos- quito to destroy the agent of transmission or, if this is impossible, to prevent contact with the mosquito by screens and other mechanical means. As far as we know, the parasite of malaria exists only in infected man and in infected mosquito. Perpetuation of the disease is due to the perpetuation of the cycle, man to mosquito, mosquito to man. If the parasite is destroyed in man or the Anopheles is not allowed to breed, the disease disappears. Not only has this been demonstrated experimentally, but it is in many communities a commonplace of sanitation.

Yellow fever is a disease, the causal agent of which is unknown, but so carefully has its prophylaxis been worked out on the basis of its transmission by the mosquito, as a result of the work of Read, Carroll, Lazear and Agramonte, that an epidemic of yellow fever would now be considered as due to ignorance or criminal carelessness on the part of those responsible for the public health. It is unnecessary for me to remind this audience of the heroism of Lazear and his associates and of the non-immune enlisted American soldiers, who offered themselves for experimental inoculation through the bite of mosquitoes infected with yellow fever. To their labors we, as a people, owe the present magnificent progress in the Canal Zone, the absence of yellow fever in the Gulf ports, an increase in human comfort and happiness and an increase in national prosperity and national progress ; but still more, to