An evolving pattern of human hydatid disease transmission in the United States.

Echinococcus granulosus infection was being acquired in the contiguous United States by Virginia sharecroppers and small-holders by the turn of the century. The last recorded human infection from that general area was diagnosed in 1947. By 1920 human infections were also being acquired in the lower Mississippi valley. Apparently, infection in both of these areas was maintained chiefly in swine. By 1940 a third transmission area definitely existed in the Central Valley of California, with its reservoir intermediate host sheep. Seemingly, infection has been disseminated from these California foci into Utah and more recently into northern New Mexico and Arizona. Human populations now at unusual risk in the western United States are transhumant sheep ranchers, including Basque-Americans in California, Mormons in central Utah, and Navajo and Zuni Indians in New Mexico and Arizona. Conditions highly favorable to E. granulosus transmission, intensification and spread now exist throughout relatively large areas of the American West.