An outbreak of introduced malaria in California possibly involving secondary transmission.

During 1974, 12 cases of Plasmodium vivax malaria were reported from an agricultural area in California's Sacramento Valley. At least three of these cases resulted from local mosquito transmission. The imported cases were in Punjabi immigrants except for one in an American-born visitor to the Punjab. This is the 11th reported outbreak of introduced malaria in the United States since 1952, and the first in California since 1957. A unique aspect of this outbreak is the likelihood that secondary transmission occurred. Extensive surveillance activities, including a house-to-house case-finding survey in a 15-square-kilometer area of suspected transmission, yielded no new malaria cases, but the activities may have increased awareness of malaria among both the medical community and the public, and thus facilitated the detection of later cases. The occurrence of local malaria transmission coincided with unusually high numbers of Anopheles freeborni. The increase in imported malaria cases probably reflects a resurgence of malaria in Punjab State, India.