BC Cases of West Nile Virus

Sept. 29, 2016, 9:24 a.m.

West Nile Virus (WNV) infection has been confirmed in 10 horses and 2 crows in BC. Nine of the equine cases are located in the south Kootenays, and 1 case is in the Fraser Valley. The distribution within the Kootenays is: 3 around Creston, 4 in the Cranbrook area, 1 each in the region of Castlegar and the south half of Kootenay Lake. All cases occurred on separate premises.

The 10 cases vary in age from 4 to over 20 years old and onset of clinical signs ranged from August 7 to Sept 5. Their clinical presentations varied and included lethargy, impaired mentation, ataxia, proprioceptive deficits, and muscle fasciculations. All cases had a positive IgM tests as determined at various labs including Cornell, IDEXX and Prairie Diagnostic Services. The combination of clinical signs and positive IgM titre meet the provincial and federal government’s confirmed WNV case definition.

One of Kootenays cases and the Fraser Valley case had arrived from the US and Alberta, respectively, within 10 days of the appearance of clinical signs. So WNV infection of these horses outside BC cannot be ruled out. Follow-up is occurring to determine the cases’ vaccination status and case sequelae. Data from a few cases indicates the animals were not current in their WNV vaccinations (and typically were never vaccinated for WNV) and the cases are recovering.

Kootenay practitioners report additional neurological equine cases with a high index of suspicion for WNV, but owners declined diagnostic testing. This year, equine cases of WNV disease have been diagnosed in the Washington state, northern Idaho (including the county that borders BC) and Taber, Alberta.

Two dead crows were collected on August 8th and 11th within 30 meters of each other in Cranbrook and submitted to the Animal Health Centre. No gross lesions were noted on necropsy and both tested strongly positive for WNV using PCR. Results for all cases were shared with BC public health officials.

WNV has not previously been diagnosed in horses in the Kootenays. The cluster of Kootenay cases confirms the risk of WNV. Horse owners and veterinarians are reminded that BC WNV cases typically occur in August or September. Appropriately timed vaccination, management of mosquito habitat (such as standing water) and avoiding mosquito exposure (staying inside at dusk and dawn, and use of insect repellant) are techniques to reduce the risk of WNV infection. There were no confirmed BC cases of WNV in the previous 2 years; an Okanagan horse was confirmed with WNV in 2013.

WNV is a notifiable disease in BC. Notifications can be made to Dr. Jane Pritchard, BC’s Chief Veterinary Officer. In a clinically ill horse, IgM titres are the preferred WNV test because a positive test result, in conjunction with appropriate clinical signs, is confirm- atory for a clinical case of WNV. In contrast, IgG or SN (serum neutralization) titres require a minimum 4- fold rise between paired sera because vaccination or historical natural infection can be reflected in the titres.

Equine practitioners are also invited to utilize the BC Equine Disease Surveillance Report available at http://www2.gov.bc.ca/gov/content/industry/

agriculture-seafood/animals-and-crops/animal-health/

reportable-notifiable-diseases/equine-disease-report.

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