The Path Between the Seas

by David McCullough

Not until the following year, however, was he able at last to prove the mosquito theory by locating the mosquito’s salivary gland and determining that the expanding parasite within the mosquito’s stomach eventually penetrates all parts of the mosquito’s body, including that gland. “The door is unlocked,” he wrote in an exultant letter to England, “and I am walking in and collecting the treasures.” The solution was this: Anopheles, comparatively large, brown mosquitoes with little black dots on their wings, transmitted malaria only after having bitten someone already infected with the disease. The mosquito drew the blood containing the parasite, the parasite multiplied in the stomach of the mosquito, then moved to the salivary gland whence the parasite was delivered to the bloodstream of whomever the mosquito bit next. The insect was not the source, in other words, only the agent of conveyance. But this particular insect was the only means of conveyance. Furthermore, it could cause damage only when there was infected blood to feed on, when there were people about who were sick with malaria. So the way to stamp out malaria was not simply to get rid of the Anopheles mosquito, but to make it as difficult as possible for the Anopheles mosquito to get at anyone who had the disease. The idea of preventing malarial epidemics by exterminating the Anopheles was first put forth in a letter addressed by Ross to the government of India on February 18, 1901.