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Summary St. Louis encephalitis virus was isolated on 23 occasions from mosquitoes collected between September 7 and October 25 at widely separated localities of the four-county Tampa Bay area during the St. Louis encephalitis epidemic of 1962. Twenty-two of these isolations were obtained from 485 pools of Culex nigripalpus; the 23rd was from 91 pools of Melanoconion, a subgenus of Culex. The mosquitoes were collected alive in battery-powered light traps and traps baited with ducklings or chicks. These isolations indicate that infected vectors were prevalent over a vast area exposing the human population to infection throughout the region.

Tensaw virus, a new member of the Bunyamwera arbovirus group (named for the river bordering the area in south Alabama where the prototype strain was found), is sensitive to sodium deoxycholate, relatively heat-stable at 37°C, and filterable through membranes with an APD of 220 millimicrons, but not through 100 millimicrons. The virus produced clinical disease and death in suckling and adult mice but did not produce disease in rabbits, guinea pigs, or hamsters. The virus grows well in hamster-kidney, HeLa, duck-embryo fibroblast, and monkey-kidney tissue-culture systems, producing cytopathic effects and plaques. Detailed antigenic comparison of eight strains of Tensaw virus with five known Bunyamwera-group viruses was made and the relation of the Tensaw strains to other Bunyamwera viruses was discussed.

Summary A total of 25,240 mosquitoes was captured in the Tampa Bay area during September and October 1962, using miniature, battery-operated light traps. These were tested in suckling mice in 1,114 pools. A total of 19 isolations of St. Louis encephalitis virus was made, 18 from Culex nigripalpus and a single one from Anopheles crucians. Twenty-nine other virus isolations were made. Twenty-eight were from An. crucians and have been identified as a new virus related to Cache Valley virus of the Bunyamwera arbovirus group; the other was an unidentified virus of the California arbovirus group from Aedes atlanticustormentor.