Vaccinate Texas Horses for EEE, WNV

Vaccinate Texas Horses for EEE, WNV

Officials are urging vaccination following high disease rates in Texas horses last year.

Photo: Kevin Thompson/The Horse

In 2015, the Texas A&M Veterinary Medical Diagnostic Laboratory (TVMDL) confirmed 38 cases of West Nile virus (WNV) and six cases of Eastern equine encephalitis (EEE) in Texas horses.

The positive cases were found from the panhandle to Dallas, to south Texas and the Houston area. The southeastern region of the state had the most positive cases.

The common factor in each case was confirmation from the owner or veterinarian that the horse had not been vaccinated against these common neurologic diseases. As such, Terry S. Hensley, MS, DVM, TVMDL assistant director and Texas A&M AgriLife Extension Service veterinarian, recommends that all Texas horses should be vaccinated annually because of the state's history with WNV and EEE.

"Horse owners may want to save a few dollars by not vaccinating their horse for some of the diseases they see as uncommon to their home area," he said. "But the wide geographic range of cases in 2015 dispels the belief that only horses in areas of the Texas with higher rainfall are susceptible to these mosquito-borne diseases."

While WNV, EEE, and Western equine encephalitis (WEE) can cause similar neurologic signs, other diseases such as equine herpesvirus-1 (EHV-1), can also result in a clinical presentation. Therefore, Hensley recommends horse owners work closely with their veterinarian to determine the cause of their horse's illness.

"Vaccinating for all three mosquito-borne diseases—West Nile, EEE, and WEE—is important for all horses, as well as mules and donkeys," Hensley said. "The AAEP (American Association of Equine Practitioners) lists these three diseases, and rabies and tetanus, as diseases for which the majority of horses be vaccinated.

"If the horse travels to shows, competitions, breeding farms, boarding stables, etc., they are at increased risk of exposure to infectious diseases and horse owners need to work closely with their veterinarian to design an appropriate vaccination program,” Hensley continued. “A proper vaccination program is one of the most cost effective preventative health measures that an owner can do."

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%.

The clinical signs of both EEE and WNV can be consistent with other important neurologic diseases such as rabies and EHV-1; therefore it is important to work with your veterinarian to get an accurate diagnosis through laboratory testing.

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