West Nile virus has hit Oregon’s horse population. The news comes as no surprise to experts.

”I’ve been expecting it all summer,” said Dr. Don Hansen, state veterinarian with the Animal Health and Identification Division of the Oregon Department of Agriculture.

Three horses in Eastern Oregon have tested positive for West Nile virus, the Department of Agriculture reported Thursday. One horse is dead and two are improving, according to Dr. Emilio DeBess, Oregon State Public Health veterinarian for the Department of Human Services. Test results are being confirmed by the National Veterinary Services Laboratory in Ames, Iowa. Additionally, results are pending from three other cases.

Veterinarians are urging horse owners to vaccinate their equines against the virus, which first hit the state on Aug. 13. An infected dead crow was found in Vale, which is in eastern Oregon, 12 miles from the Idaho border.

Until Aug. 13, Oregon was the only state in the continental U.S. that had no reported West Nile infections since the virus arrived stateside in 1999. Central Oregon horses may be spared from infection, thanks to dropping evening temperatures and the impending cooler fall weather. However, eastern Oregon isn’t far as a mosquito flies, so the virus could make its way here.

”It’s basically a matter of time,” said Dr. Wendy Krebs of Bend Equine Medical Center. ”We’re one of the last states to get it. Since 1999, it’s been working its way from the East to the West.”

Dr. Darrin Kelley of Redmond Veterinary Clinic said the real question is how widespread the infection will be among horses.

”In another month, the mosquitoes will be gone,” Kelley said. ”There’s a good chance it will hit this year, but probably not in big numbers.”

West Nile can be scary and destructive for horses – the mortality rate is between 30 and 40 percent for infected equines. However, experts say West Nile doesn’t have to be fatal, provided horseowners are proactive about taking preventive measures.

”Vaccinate, vaccinate, vaccinate,” DeBess said. ”They need to get their horses vaccinated right away. And if they vaccinated last year, they need the booster, as appropriate.”

The West Nile equine vaccine became available in November 2002. There is no human inoculation yet. The horse vaccine is actually two shots that must be administered two to three weeks apart. It takes approximately three to four weeks after the second shot for the vaccine to become fully effective, Krebs said. Additionally, horses that receive the vaccine need an annual booster, although some veterinarians recommend two annual boosters, spaced six months apart.

Krebs said her clinic recommends one booster annually, as Central Oregon has cold winter weather that mosquitoes can’t survive. However, in warmer climes such as Florida, Krebs said twice-yearly boosters are suggested because mosquitoes are year-round inhabitants.

Kelley said there is nothing harmful in boosting horses twice annually and recommends it if horses were boosted in early spring. Another booster in the summer can only help, he said.

”It’s not going to hurt the horses to give them more,” Kelley said. ”Is it completely necessary? Probably not. But it’s probably the safe thing to do. The only place it will hurt is in your pocketbook.”

Kelley said there were horseowners last year who felt that the vaccine might be causing problems for pregnant mares. However, he said there has been no clear evidence. Still, he does recommend that pregnant mares not receive the vaccine during their first trimester.

”A lot of people last year had mares abort or had problems with foals, and wanted to blame it on the vaccine,” Kelley said. ”As far as the research done, there’s not been shown to be any untoward side effects from giving the vaccine to pregnant mares.”

Kelley said that for many of the people making those claims, the only change they could point to in their horses’ care was the administration of the vaccine. Only scientific studies will be able to determine if that’s true, he said. Equine vaccines are not subject to the same rigorous testing that human drugs undergo, as the Food and Drug Administration requires.

”They’re quick to jump on the bandwagon and blame the new vaccine, which could be true,” Kelley said. ”But millions of horses have been vaccinated with the vaccine in the U.S. with relatively good success.”

Krebs said that horses that received their initial innoculation or booster this past winter should be vaccinated again immediately.

”If people had their horses vaccinated in January or February, it might be good to get another,” Krebs said. ”I’d recommend doing it. There’s very little downside. It’s only about $25 per vaccine, and it’s been a very safe vaccine in our experience.”

It’s important to note that horses vaccinated against other encephalitis diseases – eastern, western, and Venezuelan equine encephalitis – are not protected against West Nile. Horses must have the West Nile vaccine to guard against infection.

For horseowners who have yet to have their equines inoculated against West Nile, it’s not too late.

”It’s never too late,” Kelley said. ”All the mosquitoes will be gone by the time the horse is fully protected. However, if they haven’t been vaccinated, I would still do it. You could wait, but the safe thing is to give them the two (shots) this year and then the booster next year.”

Looking ahead to 2005, Krebs said she recommends giving the booster in the late spring. This ensures that the vaccine is fully effective in a horse’s system before the mosquitoes attack.

According to Animal and Plant Health Inspection Services, as of Aug. 12, Oregon and Washington had no cases of equine infection, while some bordering or nearby states had several: California, 62; Nevada, 21; Idaho, 1; Arizona, 50; and Wyoming, 26.

All told, the U.S. presently has 210 equine-infection cases. However, in 2002, there were more than 15,000 horses reported infected in 40 states.

”That tells us the vaccine is working or the virus itself is getting weaker, but I don’t know what the chances are of that,” Kelley said, adding that the equine infection rate ”has significantly gone down. It’s probably more likely because of the vaccine than the virus getting weaker.”

According to the Centers for Disease Control and Prevention, when a horse gets bitten by an infected mosquito, ”the West Nile virus multiplies in the horse’s blood system, crosses the blood-brain barrier, and infects the brain.” This results in inflammation of the brain, also known as encephalitis.

Infected horses, like infected humans, may exhibit no symptoms, which means their immune system fought off the infection.

”Many horses that are infected don’t show any signs,” Krebs said. ”If they have been infected and aren’t showing any sings, then they’re probably out of danger. They’ve fought it off.”

However, when infected horses do begin displaying symptoms, they can be chilling to see.

The early symptoms aren’t as shocking as those seen during the disease’s later stages, but they’re no less worrisome. Early symptoms may include a fever (normal temperature range for horses is 99 to 101 degrees); general malaise; and a lack of interest in eating and drinking.

As the disease progresses, the telling neurological symptoms appear. Horses may display weakness in their rear ends, perhaps difficulty in raising their tails. Also, their muscles, particularly those in the lips, may twitch. And they likely will have coordination problems – difficulty walking, stumbling, wobbliness. Infected horses may attempt to sit or lay down, or may fall down and be unable to get up. Their vision may also be impaired and they may be blinded.

Symptoms of West Nile mimic another equine ailment – Equine Protozoal Myelitis, or EPM. It, too, is a debilitating neurological disease that affects a horse’s brain, brainstem, spinal cord, or a combination of the three. It has many of the same symptoms as West Nile, but a key difference, Krebs said, is the muscle twitching that is more commonly associated with West Nile. The best way to determine what disease has hit your horse is to call a vet, who can conduct a blood test, Krebs added.

There is no cure for West Nile. All vets contacted for this story couldn’t emphasize enough the need for horses to be vaccinated and boostered regularly. Additionally, there are plenty of steps horseowners can take to give their horses an edge in the battle against infection. Ultimately, it’s a fight against mosquitoes.

A theory in some horse circles suggests that feeding a vitamin B supplement helps horses ward off mosquito attacks. Kelley said that from personal experience, he thinks it could be true. However, there is no scientific research to back up that claim, he added.

”It’s not going to guard against West Nile, at least as far as I know,” said Kelley, who added that since he was a youngster, he’s taken high doses of a vitamin B supplement before heading into the backcountry. He said he doesn’t know if it would work for horses, but said it worked for him.

”It really does help to keep the mosquitoes away from you,” Kelley said. ”I don’t have any evidence to prove it works, but I used to do it and it seemed to work.”

So what if, despite your best efforts, your horse contracts West Nile and begins to display symptoms? Call your vet immediately. There’s no cure for West Nile virus, but there’s plenty that can be done to make your horse comfortable while it fights off infection.

Krebs said anti-inflammatories, including Banamine and intraveneous DMSO, can be administered to help assuage the neurological effects. Also, IV fluids keep the horse hydrated. There are antiserums available, Krebs said, although they are very expensive.

”Most horses do recover fully, although they may have some minor ongoing deficits,” Krebs said. ”The rate of recovery depends on how severe the infection is in the horse.”

Dallas Finn can be reached at dfinn@bendbulletin.com.