West Nile Virus


About the Virus

Clinical Signs






Zoonotic Risk

Veterinarian's Role

Helpful Links


What are the clinical signs of West Nile encephalitis in horses?

West Nile Virus infection does not cause clinical signs in all horses. In horses that do become clinically ill, the virus infects the central nervous system and causes symptoms of encephalitis. Encephalitis is defined as “inflammation of the brain”.

Clinical signs of encephalitis in horses include loss of appetite and depression, in addition to any combination of the following signs:
  • Fever
  • Weakness or paralysis of hind limbs, forelimbs or isolated to a single limb
  • Muscle fasciculations
  • Muzzle twitching
  • Impaired vision
  • Ataxia (incoordination)
  • Weakness or paralysis of face and tongue
  • Head Pressing
  • Aimless wandering
  • Convulsions
  • Inability to swallow
  • Circling
  • Hyperexcitability
  • Dysphagia (quidding or esophageal choke)
  • Urinary Dysfunction
  • Coma
Please click HERE to see a video demonstrating an ataxic horse affected with WNV.

It is important to realize that WNV infection may also cause an acute, fatal neurologic disease. In addition to the acute form of the disease, there is the more classic representation with neurological signs. It is important to remember that not all infected horses show clinical disease.

In horses with clinical disease, research has shown that moderate to severe ataxia, weakness, and rear limb incoordination were the most consistent signs. Fever was noted as not being a consistent clinical sign of WNV infection, and veterinarians cannot rely on fever in diagnosing West Nile encephalitis.

Extensive research has shown that WN viral infection cannot be diagnosed on the basis of clinical signs alone.


What is the incubation period and how long do clinical signs last?

  • The incubation period for the virus is approximately 5-15 days in horses.

  • On average, the clinical signs persist for 2-7 days in horses.

West Nile Virus in Other Species
(A Brief Overview)

West Nile Virus has been found in many other species. It has been founds in
humans, horses, dogs, cats, squirrels, rabbits, bats, skunks, chipmunks, and reptiles.

Dogs & Cats

The majority of dogs and cats infected with WNV show no clinical signs. There is a published report of WNV isolated from a dog in south Africa, and from a sick kitten in New Jersey, but cases in these species are extremely rare.

If an animal is infected and shows clinical signs, they typically demonstrate weakness, fever, and muscle spasms.

As in horses, blood tests are required to confirm a diagnosis.

Dogs and cats have been experimentally infected with WNV and have showed no clinical signs to mild clinical signs, all of which recovered fully. Therefore, WNV infection is of little concern in dogs and cats in comparison to the equine species.

As previously mentioned, full recovery from WNV infection has been seen in dogs and cats showing clinical signs, and the therapy is not specific. Therapy is supportive and according to the clinical signs the animal presents with.

There is currently no vaccine available against for WNV for dogs and cats.


Sheep, Cattle & Pigs

WNV has also been found in sheep, cattle and pigs in Africa and Eurasia. As with dogs and cats, most infections in these species occur without clinical signs. Animals typically develop antibodies against the virus and are able to overcome the infection.


WNV has also been found in crocodiles, alligators and one turtle in central Florida.


State of Connecticut Government:Department of Agriculture

UC Davis School of Veterinary Medicine: Centerfor Companion Animal Health.

Community Health Administration: West Nile Virus.

Center for Disease Control: West Nile Virus.

Austgen LE, Bowen RA, Bunning ML, Davis BS, Mitchell CJ, Chang G-JJ. Experimental infection of cats and dogs with West Nile virus. Emerg Infect Dis [serial online] 2004 Jan [date cited]. Available from: URL: http://www.cdc.gov/ncidod/EID/vol10no1/02-0616.htm

Steinman A, Banet-Noach C, Tal S, Levi O, Simanov L, Perk S, et al. West Nile virus infection in crocodiles. Emerg Infect Dis [serial online] 2003 Jul [date cited]. Available from: URL: http://www.cdc.gov/ncidod/EID/vol9no7/02-0816.htm

Image References