The Oklahoma Department of Agriculture, Food, and Forestry is encouraging horse owners to take precautions and vaccinate their animals to protect against the West Nile virus (WNV) and Eastern equine encephalitis (EEE).

In the recent past, there have been more than 40 cases per year in Oklahoma where horses have been positively diagnosed as infected with mosquito-carried diseases. The bird population serves as the reservoir for the viruses, which are then spread by mosquitoes to horses and humans.

The mosquitoes most likely to transmit WNV and EEE lay eggs in small pools of standing water. Adult mosquitoes can hatch in 10 days during the warmest months of the summer. Mosquitoes can become infected with both WNV and EEE after feeding on an infected host; such as birds carrying the virus. Within 10 to 14 days, the mosquito can transmit the virus to both humans and horses.

“Signs of West Nile virus include weakness, fever, incoordination, seizures, blindness, and difficulty getting up,” said Assistant State Veterinarian Michael Herrin, DVM “There are several vaccines available, and we are encouraging horse owners to visit with their veterinarians and determine the vaccination protocol that will best fit their operations.”

The mortality rate for horses with WNV can be as high as 30-40%.

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.

Horse owners can reduce the risk of both EEE and WNV by eliminating places where mosquitoes breed such as standing water, by not letting water stagnate in birdbaths or water tanks, and turning over wheelbarrows when not in use. Any containers that are needed for constant use, such as water troughs, should be emptied and flushed at least twice each week in order to remove the possible mosquito eggs. When possible, owners should reduce horses’ outdoor exposure at dawn and dusk, the times of day when mosquitoes that transmit the West Nile virus are most active.