PINEDALE – West Nile virus is a serious mosquito-borne illness that is affecting horses across the state as warm, wet summer weather hangs around the region.

On Aug. 24, the Wyoming Livestock Board reported 14 laboratory-confirmed positive cases of West Nile virus (WNV) among horses in six Wyoming counties, with the highest number in Fremont County at seven infected equines.

Diagnostics are “pending on multiple cases” in the state, the Wyoming Livestock Board added.

Fortunately, there are no laboratory confirmed instances of WNV in horses in Sublette County at this time. Nonetheless, Pinedale veterinarian Dr. Tina Gehlhausen cautioned horse owners to be aware of the situation in the state and to consider vaccinating their equines or updating existing vaccines.

The horses contracting WNV in Wyoming either lacked the vaccine or were not current on their shots, the Wyoming Livestock Board noted.

What is West Nile virus?

As the name suggests, WNV emerged near the source of the Nile River in present-day Uganda in the 1930s, the National Institutes of Health (NIH) stated. The virus arrived in North America in 1999, with the first equine case reported in Canada in 2002, according to the NIH.

The disease is transmitted by mosquitos that feed on infected birds and then bite a horse, infecting it with the West Nile virus, explained Dr. Gehlhausen. Once a horse contracts WNV, it cannot spread the disease to other horses or humans, she added.

The particularly wet weather in the Rocky Mountain region this summer seems to have produced more mosquitos than usual, and the insects are still buzzing around in late August. In an Aug. 10 press release, the Wyoming Department of Health noted that mosquito pools across the state tested positive for WNV at a higher rate than recorded over the past 10 years.

WNV in horses presents with symptoms that are similar to other neurological diseases spread by mosquitos, like eastern and western equine encephalitis, so it is important to confirm WNV through a laboratory test administered by a veterinarian, said Dr. Gehlhausen.

Symptoms depend on the severity of the illness and can include muscle tremors, weakness in the hind end and the horse “presents down” (unable to stand up),  Dr. Gehlhausen continued.

Horses with mild cases tend to recover completely while animals with severe cases may never fully recover, said Dr. Gehlhausen.

There is no cure for WNV, and treatment consists of keeping a horse hydrated and reducing neurological inflammation, Dr. Gehlhausen explained.

If the illness becomes severe, horses may go down or experience impaired vision and convulsions, the Wyoming Livestock Board stated.

Severe WNV cases can lead to death, the Wyoming Livestock Board added. According to the Merck Veterinary Manual, the mortality rate for horses presenting with clinical WNV symptoms approaches 50 percent.

Both the Wyoming Livestock Board and Dr. Gehlhausen encouraged people to contact their veterinarian if any of the above symptoms are observed.

An ounce of prevention

Completely avoiding mosquitos, especially this summer, is impossible. There are ways to reduce contact with the insects, however, like removing old tires from corrals and pastures or covering water buckets where the bugs thrive, said Dr. Gehlhausen.

Dr. Gehlhausen stressed that vaccines are also an effective tool. Rutgers University and the New Jersey Agricultural Experiment Station cited an effectiveness rate for WNV equine vaccines of between 93 and 95 percent.

WNV vaccines are administered once followed by a booster in two to four weeks, Dr. Gehlhausen said. Horses then need an additional booster annually, she added. The yearly booster maintains and builds a horse’s immunity each season, Dr. Gehlhausen noted.

The WNV vaccine can cause soreness, although injection reactions are “very minimal,” Dr. Gehlhausen told the Roundup.

Dr. Gehlhausen recommended scheduling the shots or boosters in spring to ensure a high level of immunity before mosquitos emerge for the summer in Sublette County.

The WNV vaccine may be given alone or in an injection that contains antibodies for other insect-borne diseases like certain respiratory viruses, eastern and western equine encephalitis and tetanus, said Dr. Gehlhausen.

WNV booster shots are available for folks who might not know their horse’s vaccination history, Dr. Gehlhausen added.

For questions regarding WNV in horses and the vaccine, please reach out to your veterinarian.