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314 capable of transmitting the infection. The proportion of infected ticks in natural conditions is small; only 1 in 296 in Ricketts' experience.

In conformity with the seasonal prevalence of the tick, the disease in man is commonest in April, May, and June.*

Treatment and prophylaxis.— In the absence of a specific remedy, treatment must be conducted on general principles, having regard to the natural history and nature of the disease. Attempts at prophylaxis are now being carried out, based on the above-described hypothetical method of transmission of the disease. War is being waged on the ground squirrel and the woodchuck; domestic stock and goats are being systematically dipped to prevent the spread of and, if possible, to exterminate the tick. Fricks, finding that if D. venustus is placed on the fleece of an unshorn sheep it either dies or remains unimpregnated, and is manifestly on an uncongenial host, has suggested, in addition to the measures just mentioned, turning the badly infected districts into sheep-runs.