A horse in Central Point has died of West Nile virus, and four human cases have been identified in other Oregon counties.

State public health officials said a Lane County man who tested positive for the virus told them he believed he had been bitten by infected mosquitoes in Josephine County. A resident of Malheur County was probably infected by mosquitoes in Oregon, said Dr. Emilio DeBess, while residents of Benton and Marion counties were probably infected during visits to California.

Three of the human cases experienced fever and weakness, DeBess said, and one developed a rash. The Lane County man, who had no symptoms, was diagnosed when he gave blood.

The dead horse is the newest evidence of the virus’s presence in Southern Oregon. Birds in two of Jackson County’s four flocks of "sentinel chickens" also have tested positive for the virus, and mosquitoes collected from several areas of the county have been found to be carrying the virus, said Eugene Papineau, manager of Jackson County Vector Control.

Infected mosquitoes pick up the virus when they bite infected animals, typically birds. They can then transmit the virus to other animals they bite, including humans.

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Papineau said infected mosquitoes have been found in standing water near the Denman Wildlife Area and near the intersection of Agate and Nick Young roads. Mosquitoes collected from pools near Peninger and Upton roads also tested positive for the virus in preliminary tests performed locally, but those results have not yet been confirmed by state laboratories.

The infected horse deteriorated quickly in less than 48 hours and had to be euthanized, said Betty Taylor of Central Point, who owned the 16-year-old mare.

"It was my own stupidity," Taylor said. "I didn’t think it would happen to me and it did."

Taylor said the horse suddenly became lethargic and acted as though it was intoxicated. Sores developed on its tongue and inside its lips, and it began to bite its tongue.

"It was like nothing I’d ever seen," she said. "What it does to an animal is unbelievable, and it’s fast."

Taylor said she had vaccinated her other horse, but left the pregnant mare unvaccinated out of concerns that the vaccine might harm the developing fetus. Some of the most conservative recommendations for administering the vaccine caution against using it until after the mare has passed the 120th day of pregnancy to minimize any side effects.

Timmons said horse owners have to consider the clear risks of not vaccinating against the potential risks of vaccinating. With the virus clearly present in Southern Oregon, the choice is whether to risk losing a potential foal — or the foal and its mother, said Tom Timmons, a veterinarian at Rogue Valley Equine Hospital in Eagle Point.

Reports that have circulated on the Internet connecting the equine West Nile vaccine to problem pregnancies lack scientific credibility, Timmons said. He noted that bred mares complete a full- term pregnancy only about 60 percent of the time under normal circumstances. Some people who vaccinated their horses against West Nile virus and lost a foal have blamed the vaccine, but there has been no science to confirm their contentions.

"Vaccinating has just given them something to hang (their problems) on," he said.

Timmons noted that the equine West Nile vaccine was administered to 32 pregnant mares during clinical testing, and all of those mares delivered foals with no ill effects. There is no data that connects the vaccine to infertility problems in stallions.

Some Jackson County residents have almost certainly been infected with the virus, but have not become sick enough to seek treatment or get tested, said Gary Stevens, the county’s environmental health manager.

"The number of human cases is highly under-reported," he said.

Federal officials at the Centers for Disease Control and Prevention estimate that fewer than 1 percent of the people who are infected become seriously ill. The virus does not infect every mosquito. During an outbreak in Arizona in 2004, only about 15 percent of mosquitoes that were collected for testing carried the virus.

People can reduce their chances of being bitten by staying indoors during early morning and sunset hours when mosquitoes are most active, and by covering exposed skin with DEET-based insect repellents when they go outdoors.

Reach reporter Bill Kettler at 776-4492, or e-mail bkettler@mailtribune.com.