Reported West Nile virus cases dwindle
Veterinarians urge horse owners to vaccinate animals
By MITCH LIES
State veterinarians in Oregon and Washington are stumped.
But their perplexity in this case is a good thing.
Don Hansen and Leonard Eldridge, state veterinarians of Oregon and Washington, can't definitively explain why they have yet to record a case of West Nile virus in horses in 2010.
After the 72 cases in Washington and five in Oregon in 2009, the veterinarians said recently they are pleasantly surprised, but have little explanation for the drop in reported cases.
"I would like to think it is because we've worked so hard, and the horse owners have worked so hard to keep horses vaccinated that we have a vaccinated population," Hansen said.
"So the mosquito population might be positive, but they're not finding susceptible animals to spread the virus to," Hansen said.
Hansen said entomologists have found the virus in mosquitos in Malheur, Umatilla and Morrow counties, but not in horses or humans.
Washington inspectors also have found infected mosquitos in several Eastern Washington counties, but again, no infected horses. A Grant County woman in her 70s tested positive for the virus, according to a Grant County Health District press release dated Nov. 23, marking the first known human infection in Washington in 2010.
"We're pretty sure there has got to be some positive horses out there," Eldridge said, "but either they don't have enough of a viral load to show clinical signs of the disease or people aren't bothering to have them tested."
In both Oregon and Washington, the state departments of agriculture for several years now have encouraged horse owners to get animals vaccinated.
"We've put out press releases and news bulletins recommending horse owners vaccinate and take precautions for mosquito control," Eldridge said.
"We would like to think that has helped keep it so we don't have as many cases this year," he said. "But the reality is, it's probably a combination of a lot of things."
The number of West Nile virus cases has fluctuated dramatically since it was first identified in U.S. horses in 1999. That year 25 horses in New York were reported infected.
The virus spread gradually east-to-west, first striking Western states in 2002, when two infected horses were reported in Washington and one in Idaho. That year, 15,257 horses were reported infected with the virus nationwide.
In 2003, only one horse was reported infected in the five westernmost states. But by 2004, it had erupted in California, which recorded 536 cases of West Nile virus in horses that year.
One trend has become clear over time: Despite the year-to-year fluctuations in the number of cases, the overall number is declining. Total nationwide cases dropped to 276 as of 2009. And just 27 horses reportedly were infected with the virus in the U.S. so far this year.
Hansen said it is possible far more horses are infected than are being reported, but he doubts that is the case.
"I don't think that's the case," Hansen said. "We've made it easy to report cases, and there are no negative consequences for reporting a positive. There is no quarantine possibility, for example."