Western Equine Encephalitis

What is Western Equine Encephalitis?  

Cases of Western Equine Encephalitis have principally occurred in the western US and very infrequently in parts of the Midwest. While large equine outbreaks have occurred historically, no equine cases have been reported in the last two decades. The most important mosquito vector of this disease is Culex tarsalis.

Clinical Signs: 

  • Depression and anorexia without fever when initially infected 
  • Moderate to high fever 102.5-104.5°F (39.17-40.28°C) 
  • Lack of appetite • Lethargy/drowsiness 
  • Neurologic signs- Onset of neurologic disease is frequently sudden and progressive 
    • Periods of hyperexcitability, apprehension and/or drowsiness o Fine tremors and fasiculations of the face and neck muscles
    • Convulsions o Cranial nerve paralysis-- facial paralysis and weakness of the tongue are very common 
    • Head tilt, droopy lip, muzzle deviation 
    • Weakness, ataxia, and dysmetria (incoordination) in one or all limbs 
    • Complete paralysis of one or more limbs 
    • Colic 
    • Recumbency (inability to stand) 
    • Death

Diagnosis: Diagnosis is made by a veterinarian by measuring antibody titers in serum (a component of whole blood), using an ELISA (enzyme-linked immunosorbent assay) or, less commonly, with PCR on CSF (cerebrospinal fluid).

Treatment: There is no cure for Western Equine Encephalitis. Supportive care is administered in horses which show clinical signs.


Keep all horses up to date on vaccinations. Initial vaccination is followed in 4 to 6 weeks with a booster; yearly revaccination is recommended. More frequent boosters (i.e. twice yearly) are recommended in areas with year-round mosquito seasons and in endemic areas. Practice vector management on all properties where horses are kept: 

  • Use insect repellents frequently; re-apply after rain. 
  • Keep horses in at night when possible and apply insect repellant. 
  • Eliminate or minimize standing water. 
  • Stock tanks or ponds with mosquito-feeding fish.
  • Eliminate brush piles, gutters, old tires and litter. 
  • Remove all equipment in which standing water can collect.