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In 2007, 140 laboratory-confirmed cases of human West Nile virus (WNV) infection occurred in Kern County, California. These cases were preceded by a sharp rise in the incidence of WNV infection among Culex pipiens quinquefasciatus mosquitoes and then among birds. However, the outbreak was unanticipated based on surveillance data, including temperature, rainfall, and trends from previous years.
An aerial survey of the affected area revealed a large number of green or neglected swimming pools. In one representative photograph, 17% of visible swimming pools and hot tubs were green, indicating invasive algal blooms. Such blooms provide favorable breeding conditions that are rapidly exploited by urban mosquitoes.
Most of the neglected pools were p…