2011 Mosquito-Borne Illness Season Heating Up

According to the U.S. Geological Survey, horses in two Florida counties have tested positive for Eastern equine encephalitis (EEE), and a horse in Georgia has tested positive for West Nile virus (WNV).

Area news websites indicate that one horse in Marion County (located in central Florida) and two horses in Holmes County (located in northern Florida) tested positive for the mosquito-borne disease EEE in late July.

More than 90 horses tested positive for EEE in Florida in 2010, the highest number since 2005 when more than 120 horses tested positive. Because of its mosquito-friendly climate, Florida has consistently been one of the top five states for numbers of confirmed EEE cases in recent years.

A viral disease, EEE affects the central nervous system and is transmitted to horses by infected mosquitoes. The fatality rate for EEE in horses is 75-95%. The course of EEE can be swift, with death occurring two to three days after onset of clinical signs despite intensive care. Horses that survive might have long-lasting impairments and neurologic problems.

Clinical signs for 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.

A report from a Georgia news website indicated that veterinarians confirmed a case of WNV in early July in a horse in Camden County (in the Southeastern part of the state).

In 2010, the USDA's National Animal Health Surveillance System reported 125 confirmed cases of WNV in 28 states. Clinical signs for WNV include flu-like signs, where the horse seems mildly anorexic and depressed; fine and coarse muscle and skin fasciculations (twitching); hyperesthesia, or 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 (incoordination on one or both sides, respectively). Equine mortality rate can be as high as 30-40%.

According to E.S. "Rusty" Ford, equine programs manager at the office of Kentucky State Veterinarian, vaccinating horses against EEE and WNV is crucial in trying to prevent them from contracting mosquito-borne illnesses.

Ford has been tracking WNV in Kentucky since 2001, when the disease was first recognized in the commonwealth, and has collected data that indicates the majority of horses have that contracted the disease over the past 10 years were not vaccinated against it. Additionally, Ford's data showed that the one horse that tested positive for EEE since 2001 was not vaccinated against the disease.

"The West Nile data I have tracked since first recognizing the disease in Kentucky gives overwhelming support of vaccine efficacy," he said.

Ford also suggests minimizing horses' exposure to mosquitoes can aid in disease prevention.

"Minimizing mosquito habitat is essential to mitigating the opportunity for horses to be potentially exposed," he said.

Other mosquito control strategies include:

  • Using topical fly sprays and fly sheets;
  • Keeping horses inside and under fans during peak mosquito hours; and
  • Limiting the amount of standing water around the property to reduce mosquito breeding areas.

About the Author

Erica Larson, News Editor

Erica Larson, news editor, holds a degree in journalism with an external specialty in equine science from Michigan State University in East Lansing. A Massachusetts native, she grew up in the saddle and has dabbled in a variety of disciplines including foxhunting, saddle seat, and mounted games. Currently, Erica competes in eventing with her OTTB, Dorado.

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