An 18-month-old Suffolk horse euthanized three weeks ago has tested positive for the deadly Eastern Equine Encephalitis disease.

The female mustang marked Virginia’s first confirmed case of the disease in a horse this year, said Elaine Lidholm, director of communication for the Virginia Department of Agriculture and Consumer Services.

Though most cases of the disease occur in horses that have never been vaccinated or that have not been vaccinated in a long time, the Suffolk horse received an initial vaccination and booster in the fall of 2009, as well as a booster in May 2010.

“Normally, it is a very effective vaccine,” Lidholm said, noting there could be a number of possibilities why the horse still became ill. The horse’s immune system may already have been weak, or the vaccine could have been mishandled before being given to the horse.

“The answer is, we don’t know,” Lidholm said.

The horse was euthanized because of severe weakness and the inability to coordinate voluntary muscle movements, according to Lidholm.

Most states are experiencing an unusually high number of cases of the virus, but this is the first one reported in Virginia this year. The state had eight cases last year, most of them in the Tidewater area, Lidholm said. Generally, the virus is transmitted by mosquitoes. It has a mortality rate of about 80 percent.

Area residents can help by destroying standing water breeding sites for mosquitoes. Horse owners should use insect repellants, vaccinate their horses and get them boosters at least once a year, and they should remove animals from mosquito-infested areas during peak biting times, typically dusk to dawn.