Mosquito-Borne Disease Prevention

Mosquitoes threaten your horses’ health

Mosquitoes are vectors of arthropod-borne viruses, also known as arboviruses. These viruses are transmitted between birds by mosquitoes, then spread across the continent as infected birds migrate. Most cases of arbovirus diseases occur from June through September, when mosquitoes are most active. In warmer climates mosquitoes may be active in the winter months as well, but mosquitoes are most infective in warm weather.

How arboviruses infect your horse

Horses are considered “dead end hosts” since evidence suggests that they don’t transmit the virus once infected.

Equine Arboviruses

  • West Nile Virus
  • Eastern Equine Encephalitis
  • Western Equine Encephalitis
  • Venezuelan Equine Encephalitis


Antibiotics are useless against arboviruses and no truly effective anti-viral drugs have been discovered to date. Vaccines may aid in the prevention of arboviruses. Vaccines are available for Eastern Equine Encephalitis, Western Equine Encephalitis, Venezuelan Equine Encephalitis and now West Nile Virus.


  • Equine arboviruses attack the brain and spinal cord. Infected horses develop mild to severe symptoms which may include:
  • Depression/apprehension
  • Loss of appetite
  • Facial tremors/muscle twitching
  • Weakness
  • Sensitivity to touch or sound
  • Impaired vision
  • Incoordination
  • Head pressing
  • Aimless wandering or circling
  • Convulsions
  • Narcolepsy
  • Inability to swallow
  • Paralysis
  • Coma


Depending on the arbovirus, 10 – 90% of infected horses die or are euthanized. Some horses that survive are left permanently disabled.

West Nile Virus

In August 1999, an outbreak of West Nile Virus (WNV) was found in birds, horses and humans in New York. Since then, the virus has spread throughout the U.S., following bird migration patterns.

WNV has been found in at least 30 mosquito species. In some species, it can be directly transmitted from the adult mosquito to her eggs. Mosquitoes born infected introduce the virus earlier each year.

In 2002, more than 15,000 equines in 40 states were diagnosed with cases of WNV. Since then, WNV has spread to the southwestern states with devastation. In 2004, California alone has reported 422 equine cases of WNV.

Equine Cases of West Nile Virus

Animal & Plant Health Inspection Service, US Department of Agriculture

There is no cure for WNV. Supportive treatment consistent with standard veterinary practices for horses infected with a viral agent is most often used, but recovery depends on the ability of the horse’s immune system to fight the virus.

Horses vaccinated against Eastern, Western and Venezuelan Equine Encephalitis are NOT protected against West Nile Virus. Vaccinations are now available for WNV. Contact your veterinarian for more information.

Eastern Equine Encephalitis

Eastern Equine Encephalitis (EEE) outbreaks happen most often during the summer and fall. An EEE vaccine is available for horses.

Western Equine Encephalitis

Western Equine Encephalitis (WEE) occurs in North, Central and South America, with most cases reported in states west of the Mississippi. A WEE vaccine is available for horses.

Venezuelan Equine Encephalitis

Venezuelan Equine Encephalitis (VEE) is an important veterinary problem in Central and South America that occasionally spreads to the United States. An outbreak that started in South America in 1969 reached Texas in 1971, killing over 200,000 horses. A VEE vaccine is available for horses.