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Equine Health Resource

West Nile Virus, WNV

Synonyms: West Nile Fever or Disease, West Nile Neuroinvasive Disease, Near Eastern Equine Encephalitis, Lordige

West Nile virus is a disease that only affects a few mammal species, primarily horses and humans. It can cause encephalitis (inflammation of the brain) or meningitis (inflammation of the lining of the brain and spinal cord).

This disease is spread by birds and transmitted to horses by mosquitos that have fed on an infected bird. Symptoms include lethargy, incoordination, paralysis and weakness.

WNV was first detected in the United States in 1999. Since then, thousands of horses have been infected. Vaccination has effectively reduced the incidence of this disease, but it is now endemic across the United States.

WNV is a reportable disease, meaning that if a horse has or is suspected of having this disease, vets are required to report it to agricultural authorities (usually the State Veterinarian). These authorities may investigate the case as part of a larger effort to monitor equine health and coordinate with other States and the USDA APHIS in preventing the spread of illness or disease on a national and international level.

WNV is a zoonotic disease, meaning that it affects both another species and man. Infected horses cannot directly pass this disease to other horses, animals or humans and it is unlikely that a mosquito that feeds on an infected horse could pass this disease onto a human. However, a mosquito that feeds on an infected bird can pass the disease onto a human.

If you have been exposed to a horse with (or suspected of having) WNV you should contact your doctor because this virus may be active in your geographic region.

I Might ObserveRelated Observations

Depressed, Dull, Sick or Lethargic
Local Muscle Twitching
Noticeably Wobbly or Weak
Lip Quivering, Lip Flapping, Strange Movement of Lips
Fever, Rectal Temperature Greater than 101.5 (in Adult)
Cannot Seem to Get Up, Lying Down, Seems Aware
Head in Corner or Against Wall, Head Pressing
Lying Down & Paddling
Struggles to Rise, Gets up with Difficulty
Mosquitoes Bothering or Biting Horse
Teeth Grinding (in Adult)
Seizures or Convulsions (in Adult)
Not Eating, Loss of Appetite, Not Hungry
Vision Seems Poor, Running into Things or Objects
Drowsy, Seems to Fall Asleep on Feet, Could Even Collapse
Spasm or Tensing of Neck Muscle
Cannot Swallow, Difficulty Swallowing
Tongue Sticking or Hanging Out
Hind Limb Bows Outward at Hock When Walking
Sudden Collapse or Apparent Loss of Consciousness
Eyeballs Seem to be Pointing Different Directions
Eye is Making Abnormal Rapid & Jerky Movements
Change in Personality, Strange Behavior
Lameness, Severe, Cannot Support Weight on Limb
Hypersensitive to Touch, Generally
Knuckling Over or Rolling Over on a Fetlock
Urination, Straining or Difficulty
Hypersensitive to Touch on Back or Topline
Abdominal Pain, Colic Signs
Sick Horse Hyper-Reactive to Loud Sound or Sudden Movement
Stretching Forelimbs Far Forward & Dipping Back
Seems Dazed or Confused
Compulsive, Forceful Walking, Driving Forward when Led
Hypersensitive to Touch on Flanks & Belly
Resists Contact with Face, Ears or Poll, Head Shy
Circling, Pacing or Weaving in Stall
Agitated, Anxious, Nervous or Stressed
Muscle Tone Poor, Muscles Seem Soft
Face or Head Seems Swollen or Enlarged


Good facilities management to decrease the population of mosquitos is recommended, including the use of fans, screens, and mosquito repellant.

Eradicate all pools of low lying stagnant water by cleaning clogged gutters, replenishing water for all pets every three days (at minimum), and stock ornamental ponds with mosquito eating fish.

Likewise, dump, dispose of, or fill all containers (old tires, cans, wading pools, ground holes) that catch and harbor stagnant water. Citronella products are helpful but should be used in combination with other strategies. Keep horse in protected stalls during times of high mosquito activity.

WNV vaccine is recommended as a "core" vaccine by the American Association of Equine Practitioners (AAEP).

Frequency of vaccination depends on your geographic region, age and exposure, so talk to your vet about an appropriate vaccination program. Proper vaccination is a key to prevention.

Helpful Terms & Topics in HSVGWritten, Reviewed or Shared by Experts in Equine Health

Author: Doug Thal DVM Dipl. ABVP


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