Horses and West Nile Virus


  • Horses are incidental or dead-hosts that do not generally allow transmission of the virus to its definitive host, therefore the pathogen cannot complete its development
  • Horses play an insignificant role on WNV epidemiology
  • There are only sporadic occurrences of WNV-associated diseases in horses
  • Low-magnitude and short-duration viremia
  • Limited amount of antigen detected in CNS tissues
  • No zoonotic risk to humans and clinically recovered horses can be slaughtered safely and without restrictions


  • Geographic distribution: Europe, Africa, the Middle East and recently in North and South America
  • Susceptibility: any ages, breeds and sexes
  • Risk factors: seasonal and correlates with mosquito vector (ie. mid-August to late October in North America)


  • Infection is much greater than disease
  • 5-15 day incubation period
  • 1-2 days post infection: low level of transient viremia
  • 4-8 days post infection: WNV is no longer detectable in the blood of infected horses
  • 5-22 days post infection: neurological signs likely apparent and most horses are usually not viremic at that stage
  • 60-70% of horses with clinical signs may fully recover

    Clinical Signs (in order of most to least frequent)

  • Ataxia
  • Weakness of limbs
  • Recumbency
  • Muscle fasciculation
  • Fever
  • Paralyzed or drooping lip
  • Twitching face or muzzle
  • Teeth grinding
  • Blindness
  • Traumatic lesions of forelimbs and head due to compulsive movement


  • No gross pathological lesions
  • Viral isolation in brain and spinal cord of clinically ill horses

    WNV vs. Rabies

  • Neurologically similar to rabies, need to report to CFIA!
  • CFIA Veterinarian needs to rule out rabies via monitoring and enforce a 7 day horse quarantine after appearance of neurological signs and subsequent survival
  • After horse released from quarantine it is the owner and/or private veterinarianís decision to pursue Eastern Equine Encephalomyelitis/Western Equine Encephalomyelitis (EEE/WEE) and WNV diagnosis
  • Once they have recovered from clinical signs of WNV, horses may be sent to slaughter without any further restrictions

    Citation: "Response to Central Nervous System Signs in Horses in West Nile Virus Endemic Areas." Government of Canada,Canadian Food Inspection Agency,Animal Health Directorate., 24 May 2011. Web. 25 Nov. 2013. [].