Mosquitoes tested for West Nile virus

EUGENE, Ore. – Public health officials across Oregon are starting to trap and test mosquitoes, searching for any sign that the West Nile virus has resurfaced in the state.

The number of human cases of West Nile virus jumped to 73 last year in Oregon, including the state’s first death, up from eight cases in 2005. All but one of the cases occurred in eastern Oregon, including 55 in Malheur County, which borders Idaho.

Public health officials say based on the experience of other states, they expect the number of human cases to increase again this year as the disease continues its march over the Cascades into the state’s more populous counties.

The purpose of the testing is to determine the prevalence of the virus in the region’s mosquitoes, to guide mosquito control and public education.

The traps set for mosquitoes each have two cylinders, one filled with dry ice and the other with a battery-operated light and fan. The dry ice emits carbon dioxide, which attracts mosquitoes because it mimics a large mammal breathing. As the insects are drawn toward the carbon dioxide and the light, they are pulled in through the fan into a small mesh bag.

Researchers are expecting to get about 200 mosquitoes in each bag. In the lab, he uses a microscope to sort the mosquitoes by species, sex and site.

The three species that most commonly transmit West Nile virus in Oregon are Culex Pipiens, Culex Tarsalis and AE Vexans. In 2003, 36 percent of Culex Tarsalis tested in the United States were positive for West Nile virus, making it the No. 1 transmitter of the disease, said John Parrott, an Oregon State University Extension agent.

Compared with other public health threats, the risk of contracting West Nile virus is relatively low.

Last year in the United States, there were 4,269 confirmed human cases of West Nile virus, resulting in 1,459 cases of encephalitis or meningitis both severe diseases of the central nervous system and 177 deaths.