By NANCY VORIS
Indiana Correspondent

INDIANAPOLIS, Ind. — The reports of mosquito-borne viruses in horses have diminished greatly in the past few years, but horse owners should continue preventative practices, such as vigilance in decreasing mosquito breeding sites and using vaccines properly.

Indiana has no cases of West Nile virus (WNV) in people or equines so far this year, but reports of mosquitoes infected with the virus have been found in 18 counties, according to the Centers for Disease Control and Prevention.

The virus is most commonly spread by the Culex mosquito, a type that prefers dry weather. The cycle of heat, rain and dry weather usually brings an increase in WNV, as mosquitoes reproduce in stagnant water.

“The mosquito responsible for West Nile is more prevalent in times of drought,” said Dr. Timothy Bartlett, DVM, equine director at the Indiana State Board of Animal Health. “As the season progresses, the virus becomes more concentrated (in mosquito populations).”

Prevention is critical. Horses should initially receive a series of two shots, along with a booster shot each year for protection.

“While we know that an annual booster is very important to maximize protection, we also know that for those horses that have not been vaccinated, some benefit can be gained from getting at least the first dose,” Bartlett said.

“If a horse is not currently vaccinated, the owner should waste no time in getting it done. It could mean the difference between life and death for the animal.”

First diagnosed in Indiana in 2001, the WNV has sickened or killed hundreds of horses statewide. It peaked in 2002, when 722 cases were reported. Numbers dropped significantly after vaccines were introduced and people became diligent in eliminating mosquito breeding sites.

In 2007 only six cases were reported in Indiana, and there were no reported cases in 2008 and 2009.

Mosquitoes spread the disease by feeding on infected birds, then biting horses or people. An infected horse will show clinical signs typical of encephalitis: listlessness, stumbling and lack of coordination, weakness of limbs and partial paralysis or death.

While no cure for West Nile exists, horses diagnosed and treated early are more likely to survive the infection.

Another mosquito-borne virus is eastern equine encephalitis (EEE), which is more severe than WNV and usually results in death. Infected animals become comatose, so it is often called “sleeping sickness.” Neither EEE nor WNV can be transmitted from infected horses to other horses or humans.

Property owners are encouraged to work to eliminate potential mosquito breeding sites, including sources of standing water such as puddles, buckets, old tires, gutters and birdbaths. Horse owners should keep water in stock tanks fresh.