Response to Central Nervous System Signs in Horses in West Nile Virus Endemic Areas

Clinical signs of WNV in horses could be indistinguishable from those seen in rabies. Thus, any horses showing neurological signs suggestive of and clinically indistinguishable from rabies have to be reported to the Canadian Food Inspection Agency (CFIA) for investigation.

The following provides a brief description of the epidemiology and pathogenesis of WNV in horses and, based on this, the course of action the CFIA will take to fulfill its mandate of protecting human and animal health.

Epidemiology and Pathogenesis of West Nile Virus in Horses

Natural WNV infections in horses have been reported in Europe, Africa, the Middle East and recently in North and South America. It appears that WNV affects horses of all ages, breeds and sexes. Its occurrence in North America is seasonal, and coincides with the presence of the mosquito vector. Most cases of WNV in horses are reported from mid-August to late October. It is estimated that between 10 to 40% of horses in endemic areas can be infected with WNV, but only 8% of these will manifest clinical signs of the disease. The WNV incubation period is usually from 5 to 15 days with a low level of transient viremia < 10^2.5 plaque-forming unit per millilitre (PFU/ml) of serum (range 10^1.0 to 10^3.0) developing one to two days post infection. Four to eight days following the infection, the WNV is no longer detectable in the blood of infected horses. Neurological signs may become apparent from 5 to 22 days post infection and most horses are usually not viremic at that stage. The clinical signs of WNV in order of their frequency include: ataxia, weakness of limbs, recumbency, muscle fasciculation, fever, paralyzed or drooping lip, twitching face or muzzle, teeth grinding, blindness, and traumatic lesions of the forelimbs and head due to compulsive movement. WNV in horses does not result in any gross pathological lesions and the virus can only be isolated from the brain and spinal cord of clinically ill horses. Approximately 60 to 70% of horses with clinical signs may fully recover.

Considering the sporadic occurrence of WNV-associated diseases in horses, the development of low-magnitude and short-duration viremia, as well as limited amount of antigen detected in CNS tissue, horses are considered an incidental and dead-end host of WNV. Consequently, horses do not play a significant role in the epidemiology of WNV and do not pose a risk to humans. Slaughter of clinically healthy horses under normal circumstances does not constitute any WNV health hazard to inspection staff or plant employees.

Field Investigation

All horses showing neurological signs indistinguishable from rabies must be reported to the CFIA. A CFIA Veterinarian will investigate those cases to rule out rabies as per internal procedures. After rabies has been ruled out (horses surviving seven days post the onset of neurological signs), the horse will be released from quarantine and it will be up to the owner and the attending private veterinarian to pursue eastern equine encephalomyelitis/western equine encephalomyelitis (EEE/WEE) and WNV diagnosis if they wish to do so.

Once they have recovered from clinical signs of WNV, horses may be sent to slaughter without any further restrictions.

Registered Establishment Investigation

All horses showing CNS neurological signs on ante mortem inspection will be placed in the subject pen and will be dealt with as per internal procedures. Also, if a reportable disease is suspected, the veterinarian- in-charge will follow the internal guidelines for dealing with other reportable diseases in registered slaughter establishments.

As in the field investigation situations, a CFIA Veterinarian will investigate these cases to rule out rabies. If for humane reasons the animal must be condemned immediately, blood samples can be drawn for EEE/WEE and WNV testing prior to euthanasia. Brain tissue will be collected for rabies diagnosis. Appropriate tissue can be collected as a courtesy and, along with the blood samples and held by the District Veterinarian should the owner wish to pursue a diagnosis of EEE/WEE or WNV.

Note: WNV is an immediately notifiable disease under the Health of Animals Regulations. All persons involved in the handling of these animals should follow occupational health and safety standards for handling slaughter animal species, as recommended in the document West Nile Virus - Protect Yourself! by Public Health Agency of Canada.