West Nile Virus

What is West Nile Virus?

West Nile virus (WNV) is a mosquito-borne flavivirus. WNV primarily causes disease in birds, humans, and horses and is transmitted by many different species of mosquitoes. Since 1999, more than 27,600 U.S. horses have been confirmed with WNV neuro-invasive disease with an estimated average case fatality rate of 30–40%. During 2002 alone, over 15,000 horses were affected in the U.S. WNV is now considered endemic with yearly activity in the U.S. (with an average 300 cases per year), Canada, Mexico and the Caribbean.

Clinical Signs: 

  • Clinical Signs for West Nile Virus include the following: 
  • Depression and anorexia without fever when initially infected 
  • Mild low-grade fever (101.5-103.5°F or 38.6-39.7°C) in about 25% of affected horses 
  • 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 fasciculations of the face and neck muscles 
    •  Cranial nerve paralysis-- facial paralysis and weakness of the tongue are very common 
    • Head tilt, droopy lip, muzzle deviation o Weakness, ataxia, and dysmetria (incoordination) in one or all limbs 
    • Complete paralysis of one or more limbs 
    •  Colic 
    • Recumbency (inability to stand)
    •  Death


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

Treatment: There is no cure for West Nile Virus. Supportive care is administered in cases which show clinical signs.

Prevention: Keep all horses up to date on vaccinations. Initial vaccination is followed in 4 to 6 weeks with a booster; yearly revaccination is recommended after. 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. o 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


February 07, 2024
Charleston County, SC
Confirmed Case(s) - No Quarantine
November 20, 2023
Lexington County, SC
Confirmed Case(s) - No Quarantine
November 15, 2023
McIntosh County, OK
Confirmed Case(s) - No Quarantine