- Aerial insecticide was applied to two areas (shaded dark
gray) of Sacramento County, California, in 2005 to combat West Nile Virus.
A followup study found that the incidence of the disease in humans
declined in the treated areas, but did not drop significantly in untreated
areas. Click for larger image. Image courtesy of Emerging Infectious Diseases.
- The incidence of human West Nile virus cases can be significantly
reduced through largescale 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 pyrethrinbased insecticide,
EverGreen Crop Protection EC 606. 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 welldefined 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 SacramentoYolo 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 largescale 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.
-
- ~ Story by Michael Greenwood
-
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