CORVALLIS – As the number of cases of West Nile virus continues to increase in Oregon, a little-known state laboratory is helping keep it at bay.

Oregon State University’s Veterinary Diagnostic Laboratory is “an invaluable resource” in the effort to alert horse owners when the virus is present in an area, State Veterinarian Brad LeaMaster said.

“We use that as a reminder tool, and I know the local (veterinary) practitioners in these areas use that as a reminder to their clients (to get horses vaccinated),” LeaMaster said.

Technicians began finding the virus in mosquitoes in Oregon in July. On Aug. 22, West Nile was confirmed in a Klamath County horse, which later died.

Oregon Health Authority veterinarian and Assistant State Epidemiologist Emilio DeBess announced Aug. 28 that two humans, one in Coos County and one in Malheur County, had contracted the virus. It marked the first human infection in Oregon since 2009.

The virus was first detected in Oregon in 1999.

Each summer, technicians at the Corvallis lab test thousands of mosquitoes for the virus by inserting a liquid solution into small vials of crushed mosquitoes.

A special procedure called a polymerase chain reaction test confirms whether the virus is present.

In horses, lab technicians look for antibody responses in blood tests to confirm whether the disease is present.

“When an animal experiences infection, it has an antibody response in the blood to combat that,” said Rocky Baker, the lab’s virology supervisor. “That’s what we look for, evidence a horse is developing an immune response.”

When technicians find a positive result, the laboratory contacts DeBess and LeaMaster.

Mosquitoes pick up the virus by feeding on infected birds. They then pass it to humans and horses by biting them.

“Infected crows, ravens and other members of the corvid family are considered ‘reservoirs’ and will carry very high viral loads of West Nile,” Donna Mulrooney, supervisor of the molecular diagnostics section of the laboratory, said.

“Horses, on the other hand, are known as ‘dead-end’ hosts,” she said. “They don’t carry enough of a viral load to infect mosquitoes if bitten.”

Testing mosquitoes for the virus is time-consuming and requires attention to detail, said Mulrooney, who has worked at OSU for 24 years.

“You need to have a strong attention to detail,” she said, “because you’re putting exact and very small amounts of solution into tubes. You have to be very accurate.

“You also need a good science background to understand basic principles of DNA,” she said. “You learn to recognize if there is a problem, if your results are real or not.”

“We have stringent protocols that we follow and we include controls all along the way to monitor the proficiency of the test,” she said.

As of Aug. 30, 58 mosquito pools have tested positive for the virus in Oregon in 2012. That is high compared to three pools in 2011 and four in 2010, but far less than the 1,100 pools of mosquitoes that were found with the virus in 2006.

The season isn’t over and, by some accounts, is just reaching its peak.

Mulrooney said she’s concerned the virus is moving west from its concentration in Malheur County, site of 55 of the infected mosquito pools, and could continue to expand its range into mid- or late September.

Baker said West Nile activity in horses typically increases in late August and continues until the first frost. He expects to see more cases of West Nile in horses before the year is over.

LeaMaster said it’s not too late to get horses vaccinated or get an annual booster shot.

The horse that died in Klamath County had been vaccinated but hadn’t had its booster, LeaMaster said.

According to the Centers for Disease Control and Prevention, as of Aug. 28, 1,590 human cases of West Nile have been reported in 43 states and the District of Columbia. Deaths now number 66, nearly half of them in Texas.

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