Aerial Spraying Effectively Reduces Incidence of West Nile Virus in Humans

The incidence of human West Nile virus cases can be significantly reduced through large–scale aerial spraying that targets adult mosquitoes, according to research by the Yale School of Public Health and the California Department of Public Health.

Ryan M. Carney, an M.P.H./M.B.A. student at Yale and the project’s lead researcher, examined infection rates in humans before and after planes applied an insecticide over two areas of Sacramento County, California. The infection rate of people within the treated areas decreased significantly after spraying, compared to that within areas of the county that were not treated.

West Nile virus is transmitted to humans through the bite of an infected female mosquito and can lead to severe fever, encephalitis, paralysis and even death. The disease spread throughout all 58 counties of California in 2004, and Sacramento County was the area hardest hit in the United States in 2005. The disease has appeared in all of the lower 48 states, with varying levels of intensity.

Over a period of several nights in the summer of 2005, two regions of the county measuring hundreds of square kilometers each were subjected to aerial spraying with the pyrethrin–based insecticide, EverGreen Crop Protection EC 60–6. It was the first time in state history that aerial insecticides had been applied over a large urban setting and that results were available from such well–defined application areas.

The two target areas had a combined population of 560,407 people. Prior to treatment, there were 48 documented cases of human infection from West Nile virus. The infection level fell to seven people following treatment with the insecticide and to zero postincubation (14 days after treatment). In contrast, the surrounding untreated areas (which had a combined population of 518,566 people) had 41 documented cases prior to treatment and 35 cases after spraying was completed in the treated areas. The researchers concluded that the risk of infection in the untreated areas was approximately six times higher than it was in the treated areas after spraying.

“Aerial [spraying] is generally the most effective manner when the density of adult mosquito populations needs to be quickly reduced,” said Carney. “This study provides the epidemiological framework for making more informed decisions and more rigorous evaluations of such treatments.”

The Sacramento–Yolo Mosquito and Vector Control District and California health officials decided to spray the two areas amidst a growing public health crisis in 2005 that had already resulted in several deaths statewide. Still, the decision was controversial as some people in the targeted areas were concerned about possible side effects from such a large–scale application. Some people in the untargeted areas, meanwhile, were upset that they were not being afforded the same protection as residents just a few miles away. The same insecticide is used on crops and has been approved by the U.S. Environmental Protection Agency.

Carney noted that if the decision is made to use aerial spraying, it is more effective to do it early in the West Nile virus transmission season, at the onset of amplification within mosquito and bird populations, rather than after human infection has set in. The disease generally peaks in July and August.

Details of the research were published in the May issue of Emerging Infectious Diseases.