Horse owners should have their animals vaccinated against the West Nile virus, North Dakota State University Extension Service veterinarian Charlie Stoltenow advises.

Five cases of West Nile virus have been diagnosed in the past month: three horse fatalities in North Dakota, three geese fatalities in South Dakota and one positive equine serum sample in a horse with signs of central nervous system disease in Minnesota, according to Neil Dyer, director of the NDSU Veterinary Diagnostic Laboratory.

“It’s clear that the disease is still in the state and will continue to be a threat in coming years, and there is no treatment for horses that contract the disease,” Stoltenow says. “Vaccinating your horses is their best bet for protection. We still have six weeks or more left of the mosquito season.”

The West Nile virus is spread by the bite of an infected mosquito. However, even in areas where the disease is known to exist, less than 1 percent of mosquitoes are infected, and less than 1 percent of the people bitten by an infected mosquito become infected.

Historically, West Nile virus seems to recur around the world on about a 10-year cycle. West Nile first appeared in North Dakota in 2002.

Stoltenow also recommends vaccinating animals against Eastern equine encephalitis and Western equine encephalitis if horses aren’t already protected.

The first time horses are vaccinated for West Nile virus, they need two doses of vaccine separated by a period of three to six weeks. A booster dose is needed every year. Horses aren’t fully protected until three to four weeks after the second dose.

In horses, the disease can cause acute, fatal neurologic disease affecting the brain and spinal cord. However, clinical disease does not always occur. Horses may show signs of stumbling, weakness and rear limb incoordination. Mildly affected horses can recover in two to seven days. Horses that survive appear to recover fully.

The disease can be diagnosed through a blood sample.

“Rabies always should be considered when horses show signs of nervous system disease,” Stoltenow cautions. “Approach those animals cautiously and take measures to protect yourself from exposure.”

In addition to having horses vaccinated, horse owners should take steps to limit their horses’ exposure to mosquitoes, Stoltenow says. Those steps include:

• Removing water-holding containers
• Cleaning clogged roof gutters
• Turning over plastic wading pools or wheel barrows
• Changing water in birdbaths, ornamental pools and stock tanks frequently
• Adding screens to stables and using insect repellants containing DEET
• Keeping animals inside during sunset and dawn, which are peak mosquito-feeding hours
• Avoiding swamps and heavily forested areas where mosquitoes are more numerous
• Running fans in stables to disrupt mosquitoes

NDSU Agriculture Communication - Aug. 2, 2012