West Nile Virus in Manitoba in 2017

On September 1^st, 2017 the first human case of West Nile Virus (WNV) infection of the year in Manitoba was reported in a male youth in the Southern Santé Sud Health Region. Exposure in this case is believed to have occurred the week of July 30^th.

Manitoba Health, Seniors and Active Living (MHSAL) performs annual West Nile Virus surveillance in mosquitoes, people and horses (through reports from Manitoba Agriculture, Food and Rural Development) from mid-May to mid-September. Their aim is to provide an estimate of the risk of exposure.

West Nile Virus surveillance in mosquitoes in Manitoba is being performed via trapping and testing. The Culex tarsalis species of mosquito is most often involved in West Nile Virus transmission, thus specific trap / catch monitoring of this species is also being performed. In 2017 to date, there have been 40 West Nile Virus positive mosquito collections in Manitoba with positive mosquitoes found in all four southern health regions. In 2016, there were 38 West Nile Virus positive mosquito collections and 20 confirmed human West Nile Virus cases. The first West Nile Virus positive mosquitoes of 2017 in Manitoba were trapped the week of July 1st and Culex tarsalis numbers were at their highest the week of July 30^th to August 5^th.

Winnipeg’s Assiniboine Park had its first confirmed case of West Nile Virus in 2017 in a captive Great Grey Owl housed at the Zoo that died on August 8^th. This owl had a course of observed clinical illness that lasted less than 24 hours. The 4 year old female owl had been vaccinated for West Nile Virus with three, 1 ml doses of West Nile Virus Innovator vaccine (Zoetis) at ~3 week intervals beginning at 6 weeks of age followed by 1 ml of WNV Innovator vaccine in the spring at 1 year of age and 0.5 mls of the same vaccine annually, in the spring, subsequently. On necropsy she had a bright green liver with acute, multifocal, fibrinoid necrosis of her spleen, liver, kidney and brain. Her kidney was PCR positive for West Nile Virus. None of the other captive owls, or other captive birds, within the Zoo have developed clinical signs or illness associated with West Nile Virus in 2017 thus far. Birds in the Zoo are vaccinated for West Nile Virus based on species susceptibility.

Three wild young corvids (two ravens and one crow) were observed to show signs of severe lethargy, death or neurological signs resulting in euthanasia from August 14^th to 16^th, 2017 in Assiniboine Park as well. Results are pending, but these cases are also suspicious for West Nile Virus infection based on their clinical presentation and the fact that corvids are so susceptible to this disease.

West Nile Virus surveillance is not currently occurring in birds in Manitoba but positive wild corvids continue to be identified annually through the veterinary staff at Assiniboine Park and the Veterinary Laboratory at Manitoba Agriculture, Food and Rural Development at the University of Manitoba, along with the Western/Northern Region of the CWHC. The overlap of West Nile Virus continues to be evident each year in mosquitoes, birds, horses and people in the province. MHSAL encourages people to avoid being bitten by mosquitoes in Manitoba as exposure is predicted to continue to occur throughout southern Manitoba annually during the mosquito season.

http://www.cbc.ca/news/canada/manitoba/west-nile-virus-manitoba-mosquito-1.4273380

http://www.gov.mb.ca/health/wnv/stats.html

Contributed by Charlene Berkvens and Chris Enright, Veterinarians from Assiniboine Park Conservancy, Winnipeg, Manitoba