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

posted by Horse Owner Today    |   August 19, 2011 07:36


With numerous cases of the West Nile Virus (WNV) reported in horses in several Canadian provinces in the last couple of years, the question has arisen as to how to deal with situations where horses with central nervous system (CNS) neurological signs are reported from the field or at registered slaughter establishments. 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 < 102.5 plaque-forming unit per milliliter (PFU/ml) of serum (range 101.0 to 103.0) 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.