Environmental Health Starts Testing For West Nile Virus

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West Nile Virus

Lane County Environmental Health will begin its summer mosquito testing for the West Nile virus.

The mosquito-borne virus first reached the United States in 1999 and began its march westward, reaching Oregon in 2004. Lane County has been testing mosquitoes for the virus since 2004.

West Nile virus is a disease found in birds that is transmitted to other birds and mammals via the mosquito.

The Oregon Department of Human Services reported 73 human cases of West Nile virus in Oregon last year. Most of these were east of the Cascades in Malheur and Baker Counties, but Dr. Emilio DeBess of the Oregon Department of Human Services predicts that the virus will continue to move west into the more populous areas of the state.

The first positive test of this season was found in a bird in Multnomah County. Lane County found one bird infected with the virus near the Santa Clara area of Eugene in 2006. Public Health officials agree that continuing surveillance efforts and taking steps towards prevention are vital in containing the virus.

"Planning ahead is not an option, it's a requirement," Jon Conlon of the American Mosquito Control Association said about controlling West Nile.

Certified Master Gardener at the Oregon State University Extension Service John Parrott will head Lane County's surveillance efforts by beginning trapping of mosquitoes on July 19 and testing on July 20. Parrott will continue this cycle on a weekly basis until the beginning of September. The purpose of testing is to determine the prevalence of the virus in the area's mosquito population in order to guide mosquito control and public education.

Parrott sets two cylindrical traps in each of three sites around the county. The traps are set in areas where birds roost because this is what attracts the mosquitoes. The three species responsible for the majority of West Nile transmission are night-feeders. Birds and other mammals are more accessible to the mosquitoes while they are still and sleeping.

The traps consist of two cylinders, one filled with dry ice and the other with a battery-operated light and fan. The mosquitoes are attracted to the CO2 from the dry ice because it mimics a large mammal breathing. As the mosquitoes are drawn towards the CO2 and the light they are pulled in through the fan to a small mesh bag.


Parrott collects the traps the following day and takes them to the lab. Each trap averages 200 mosquitoes, Parrott says. In the lab Parrott sorts the mosquitoes according to species, sex and site using a microscope.

The three species that are the most common West Nile transmitters in Oregon are Culex Pipiens, Culex Tarsalis and AE Vexans. In 2003, The Centers for Disease Control reported 36 percent of the Culex Tarsalis tested in the United States were positive for West Nile, making it the number one transmitter.

Parrott needs between 30 and 50 females of each species from each site in order to run a test. The majority of the mosquitoes caught in the traps are females.

"The females are the large blood feasters because they need the energy for mating and reproduction," Parrott said.

Lane County has never had a mosquito test positive for West Nile, even though we know there are positive mosquitoes because we have had birds test positive, according to Parrott.

"I can tell you there are positive mosquitoes in Lane County right now, but I can't prove it with a test," Parrott said. "Lane County is so large, that the number of sites we are able to test is a very small sample of what is out there."

Parrott said surveillance is an important aspect of West Nile virus prevention. Public Health officials predict that this year will be a big one for West Nile. Parrott said that personal responsibility to control West Nile is important. Below are some ways that you can limit your risk.


* Look around your home and property and eliminate sources of standing water that can support mosquito breeding, for example gutters, birdbaths and old tires.

* Because mosquitoes are most active at dawn and dusk, avoid playing or working outside at these times or wear repellent and/or clothing that covers your skin.

* When outdoors, wear long pants, long sleeve shirts and other protective clothing.

* Wear an effective insect repellant. So far, DEET has been found to work the best.