Nothing succeeds like success, and horse owners have the chance to build on their success in 2018—providing they don’t rest on their laurels. South Carolina had one of the lowest incidences of Eastern equine encephalitis (EEE) in the Southeastern U.S. in 2018—just a single case in Chesterfield County—along with four cases of West Nile virus (WNV) in horses. North Carolina and Georgia each reported seven confirmed cases of EEE last year while Florida reported 66.
“We’d like to see our numbers reduced to zero this year,” said Boyd Parr, DVM, South Carolina state veterinarian and director of Clemson University Livestock Poultry Health, in Columbia, a state agency responsible for protecting animal and consumer health through disease control and meat and poultry product inspections.
Both EEE and WNV are viral diseases that attack horses and other equine hosts through the nervous system. Borne by mosquitoes, EEE is almost always fatal in unvaccinated horses.
“These diseases have a very high mortality rate in infected, unvaccinated horses—between 30 and 40 percent for West Nile and 90 percent for EEE,” said Sean Eastman, DVM, veterinarian and director of field services for the Livestock Poultry Health Animal Health Programs. “With the emergence of mosquitoes and the confirmation of EEE already this year in nearby states, continued vaccination for horses is essential.”
The recent success in South Carolina proves that point, the pair said. The Palmetto State led the nation in cases of the disease in 2013 with 49 EEE infected horses, all unvaccinated. Of those, 48 died. Over the next four years, the number of confirmed cases of EEE has shown a downward trend with nine in 2017 and only one in 2018. However, West Nile Virus has reappeared with 10 confirmed cases in 2017 and four in 2018.
“Since the 2013 case number peak, our office has stepped up efforts in cooperation with equine practitioners in South Carolina to increase awareness among horse owners of the risks associated with not vaccinating,” Parr said. “As a result, we think vaccination rates are up and that likely is a significant factor in the reduced the number of cases in 2018.”
Disease threats remain, though. Georgia and Florida each already have reported an EEE case this year. And other deadly diseases, including WNV and rabies, are a danger to equine livestock without vaccinations.
“Horse owners should check with their veterinarian to be sure their horses’ vaccinations are up to date,” Parr said. “This is the time of year that these diseases begin to appear. The best defense is to maintain current equine vaccinations for Eastern equine encephalitis, West Nile virus, and also rabies for your horses.”
Likewise, mosquito control is an important precaution. Both EEE and WNV are maintained in nature through a cycle involving the freshwater swamp mosquito, Culiseta melanura, commonly known as the black-tailed mosquito. Due to the warm, wet winter South Carolina experienced this past year, mosquitoes have been active all winter and the forecast for the summer is that more than a normal number of mosquitoes will be active.
Two to three days after becoming infected with the virus, a mosquito becomes capable of transmitting the virus. Infected mosquitoes can transmit the disease when they bite. Clinical signs in horses usually develop two to five days after exposure.
Any livestock, including horses, in South Carolina 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.
Clinical signs in horses include:
- Moderate to high fever;
- Lack of appetite;
- Cranial nerve deficits (facial paralysis, tongue weakness, difficulty swallowing);
- Behavioral changes (aggression, self-mutilation, or drowsiness);
- Gait abnormalities; and
- Severe central nervous system signs, such as head-pressing, circling, blindness, and seizures.
The disease can progress rapidly, with death occurring two to three days after onset of clinical signs despite intensive care in some cases. Fatality rates can reach 75-80%. Equids that survive might have long-lasting impairments and neurologic problems.
Not all infected horses show clinical signs, but those that do can exhibit:
- Flulike signs, where the horse seems mildly anorexic and depressed;
- Fine and coarse muscle and skin fasciculation;
- Hyperesthesia (hypersensitivity to touch and sound);
- Changes in mentation (mentality), when horses look like they’re daydreaming or “just not with it”;
- Occasional drowsiness;
- Propulsive walking (driving or pushing forward, often without control); and
- Spinal signs, including asymmetrical weakness; and
- Asymmetrical or symmetrical ataxia.
West Nile has no cure, however some horses can recover with supportive care.
Studies have shown that vaccines can be effective in preventing both EEE and WNV when administered properly and appropriately. Horses vaccinated in past years need an annual booster shot, but veterinarians might recommend two boosters annually—one in the spring and another in the fall—in areas with prolonged mosquito seasons. In contrast, previously unvaccinated horses require a two-shot vaccination series in a three- to six-week period. Full immunity takes several weeks to achieve.
In addition to vaccinations, owners should work to reduce the mosquito populations and possible breeding areas and horses’ exposure by:
- Removing stagnant water sources;
- Dumping, cleaning, and refilling water buckets and troughs regularly;
- Keeping animals inside during the bugs’ feeding times (typically early in the morning and evening); and
- Applying mosquito repellents approved for equine use.