This year’s wet spring weather provided optimal conditions for mosquito breeding and, thus, a large population of mosquitoes. Late summer coincides with peak mosquito season, which brings an increased risk of mosquito-borne diseases such as West Nile virus (WNV) and Eastern equine encephalitis (EEE) for humans, but also for horses.
“Owners are encouraged to talk to their veterinarian about having their horses vaccinated against these viruses,” said Delaware State Veterinarian Heather Hirst, DVM, MS. “It may take several weeks for a horse’s immune system to mount a response to the disease after the vaccine is given so owners should have their horses vaccinated as soon as possible.”
Both WNV and EEE are maintained in a cycle between mosquitoes and wild birds. Mosquitoes that feed on the infected wild birds might then bite humans or horses, infecting them with the viruses. Neither WNV nor EEE can be transmitted between horses or from horses to people.
Last week, the Delaware Division of Public Health Laboratory reported this year’s first finding of WNV in blood samples taken from the Delaware Department of Natural Resources and Environmental Control’s (DNREC) sentinel chickens that are monitored for mosquito-borne diseases. The samples are collected as part of a statewide surveillance program conducted by DNREC’s Mosquito Control Section.
Delaware has not had any cases of WNV or EEE in horses, humans, or wild birds so far in 2017. Illnesses were last seen in a Delaware horse with WNV in 2015 and two horses with EEE in 2013. However, several states south of Delaware have reported cases this year, including Virginia and South Carolina in July.
A viral disease, EEE affects the central nervous system and is transmitted to horses by infected mosquitoes. 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.
West Nile is also transmitted to horses via bites from infected mosquitoes. Clinical signs for WNV include flulike signs, where the horse seems mildly anorexic and depressed; fine and coarse muscle and skin fasciculations (twitching); hyperesthesia (hypersensitivity to touch and sound); 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%.
Horse owners should also consult their private practicing veterinarian to determine an appropriate disease prevention plan for their horses. Vaccines have proven to be a very effective prevention tool. Horses that have been vaccinated in past years will need an annual booster shot; in areas with a prolonged mosquito season, veterinarians might recommend two boosters per year—one in the spring and one in the fall. However, if an owner did not vaccinate their animal in previous years, the horse will need the two-shot vaccination series within a three- to six-week period.
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 using mosquito repellents.