horse vaccines

A five-milligram mosquito can slay a 1,000-pound horse in a matter of days. All it needs is the right virus.

But with your help, the horse can survive the attack. All you need is the right vaccine—at the right time.

Now’s the time.

“Last year in South Carolina, we had 10 cases of West Nile virus (WNV) in horses and another nine of Eastern equine encephalitis (EEE),” said Sean Eastman, DVM, director of field services for the Animal Health Programs branch of Clemson University Livestock-Poultry Health, in Columbia, South Carolina.

“These diseases have a very high mortality rate in exposed, unvaccinated horses—between 30 and 40 percent for West Nile and 90 percent for EEE,” Eastman said. “With the emergence of mosquitoes and the appearance of these viruses in nearby states, proper vaccination for horses is essential.”

West Nile and EEE are transmitted to horses by mosquitoes, often the black-tailed mosquito, Culiseta melanura, which is a scourge from Maine to Mexico. Reducing mosquito populations around the farm is a good first step, but only vaccination can prevent the disease from developing once a tiny infected assassin buzzes into the barn.

Livestock-Poultry Health, a regulatory arm of Clemson’s Public Service and Agriculture unit, recommends at least annual vaccinations for both Eastern and Western equine encephalitis, WNV, and rabies in consultation with the owner’s veterinarian. A diagnosis or clinical signs suggesting any of these diseases are required to be reported to the state veterinarian’s office within 48 hours.

“These diseases can quickly get out of hand if we don’t vaccinate horses,” said Boyd Parr, DVM, state veterinarian and Livestock-Poultry Health director. “The best defense is for owners to maintain current equine vaccinations for their horses.”

Although tetanus is not among the required “reportable diseases,” Parr and Eastman highly recommend that horses be vaccinated for tetanus at the same time.

“The first thing they showed us in veterinary school was a video of people who had contracted rabies,” Eastman said. “I guarantee you, if you’d ever seen that video you would have your horse vaccinated immediately. And EEE is a quickly progressing disease, but it’s preventable with a vaccine. It’s not a hard thing to do.”

South Carolina has made progress in combatting these common equine diseases. In 2013, the state led the nation with 49 cases of EEE in horses, 48 of which died. The numbers have come down gradually, but the state isn’t out of the woods yet.

“When I was a commercial veterinarian I would ask my customers, would you like to pay $40 now or several thousand to treat your horse later and hope the treatment works,” said Eastman, who now directs Livestock-Poultry Health animal health inspectors who routinely visit sale barns across the state. “It honestly may come down to a choice between a dead horse or spending $40 to keep him safe.”

Clinical signs of EEE include moderate to high fever, depression, lack of appetite, cranial nerve deficits (facial paralysis, tongue weakness, difficulty swallowing), behavioral changes (aggression, self-mutilation, or drowsiness), gait abnormalities, or severe central nervous system signs, such as head-pressing, circling, blindness, and seizures. The course of EEE can be swift, with death occurring two to three days after onset of clinical signs despite intensive care; fatality rates reach 75-80% among horses. Horses that survive might have long-lasting impairments and neurologic problems.

Clinical signs for WNV include flulike signs, where the horse seems mildly anorexic and depressed; fine and coarse muscle and skin fasciculation; hyperesthesia; changes in mentation (mentality), when horses look like they are daydreaming or “just not with it”; occasional somnolence (drowsiness); propulsive walking (driving or pushing forward, often without control); and “spinal” signs, including asymmetrical weakness. Some horses show asymmetrical or symmetrical ataxia. Equine mortality rate can be as high as 30-40%.

In addition to vaccinations, horse owners also need to reduce the mosquito populations and their possible breeding areas. Recommendations include removing stagnant water sources, keeping animals inside during the bugs’ feeding times, which are typically early in the morning and evening, and applying mosquito repellents approved for equine use.

Any livestock in South Carolina, including horses, that display neurologic signs, such as stumbling, circling, head-pressing, depression, or apprehension, must be reported to the state veterinarian at 803/788-2260 within 48 hours, according to state law.