In a study to determine the natural vectors of avian Plasmodium infection in Kern County, California 3,364 mosquitoes were examined for parasites during the summers of 1946, 1947, 1949 and 1950. The following average infection rates were demonstrated: Culex tarsalis, 199 of 2,074 (9.6 per cent); Culex stigmatosoma, 28 of 180 (15.6 per cent); and Culex quinquefasciatus, 14 of 746 (1.9 per cent). These differences are believed to reflect variation in vector efficiency. Of 364 Anopheles and Aedes examined, only 2 Anopheles franciscanus were infected with what appeared to be young oocysts of unidentified parasites.
The intensity of infection in the principal vector, Culex tarsalis, varied considerably in different years and in different areas. These variations are believed to be due to changeable factors such as climate, the effect of mosquito control on the vector population, and ecological differences between areas.
Plasmodium relictum was isolated from naturally infected Culex tarsalis and Culex stigmatosoma.
Present address: Langley Porter Clinic, University of California Medical Center, San Francisco, California.
Present address: Laboratory of Tropical Diseases, National Institutes of Health, Bethesda, Maryland.
Present address: Water Projects Section, Communicable Disease Center, Public Health Service, U. S. Department of Health, Education and Welfare, Salt Lake City, Utah.
Present address: Graduate School of Public Health, University of Pittsburgh, Pittsburgh, Pennsylvania.