A dead crow found in the Phoenix area is the first confirmed evidence this summer that the West Nile virus is still in Jackson County.
Scientists in Corvallis verified Thursday that the bird was infected with the virus, which was detected in birds a month earlier last summer. That could mean relatively fewer cases of the virus will occur in horses and humans before cold weather signals the end of mosquito season, said Eugene Papineau, manager of Jackson County Vector Control.
"We're keeping our fingers crossed that it won't show up in any large amount and affect people and horses," Papineau said.
In 2005, the first infected bird was identified on July 26. The virus was found in vector control's "test flocks" of chickens on Aug. 22, and a horse tested positive Sept. 1. The first human case was identified Sept. 22.
As of Thursday, there had been no other confirmed cases of West Nile virus in Jackson County birds, horses or humans.
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The virus affects horses more severely, killing about one animal among every three that are infected. A vaccine is available for horses, but it must be administered in two stages to be effective.
Papineau said the virus is much more widespread this summer in Eastern Oregon and southwestern Idaho. There have been humans and horses infected in Union, Baker, Harney and Malheur counties in Oregon, and more transmit it to other birds, horses or humans. About 80 percent of people who are infected have no symptoms. Only about one person in every 150 experiences more serious symptoms, which can occasionally be life-threatening.
The virus affects horses more severely, killing about one animal among every three that are infected. A vaccine is available for horses, but it must be administered in two stages to be effective.
Papineau said the virus is much more widespread this summer in Eastern Oregon and southwestern Idaho. There have been humans and horses infected in Union, Baker, Harney and Malheur counties in Oregon, and more than 500 human cases in southwestern Idaho.
He said there have been only about one-tenth as many citizen reports of dead birds around Jackson County this summer as there were in 2005.
Birds die in greater numbers when the virus first appears in an area because they have no natural immunity. The virus was identified in Africa, and first appeared in North America in 1999 in New York. It spread across the continent and was first detected in Oregon in August 2004.
Papineau said fewer local mosquitoes have tested positive for the virus this summer than last year, too.
Jackson County Vector Control's efforts to control mosquito populations are probably helping to hold down the spread of the virus locally, said Dr. Emelio DeBess, a public health veterinarian for the Oregon Department of Human Services.
DeBess noted that there are no vector control districts in eastern Oregon and southwestern Idaho. Without a vector control district to keep track of mosquito populations and chemically treat places that are heavily infested, the virus has become widespread in those areas, DeBess said.
Papineau said two mosquito species that frequently carry the virus to humans will stop feeding on blood in the next few weeks and look for hiding places to survive the winter.
"One dead bird this late in the year is nothing to panic about," he said, "but it's an indication that the virus is still here."
Reach reporter Bill Kettler at 776-4492 or e-mail:bkettler@mailtribune.com.
