Numbers Tell West Nile Virus Story

Dr. Tasha Epp always knew it was a question of when not if a number of Saskatchewan horses would become infected with West Nile virus this summer. But when the first trickle of clinical cases came in August, even Epp was caught off guard by the virus spread.

What surprised me was how rapidly the numbers increased. The number of reported clinical cases rose quite quickly after early August, then things peaked during the last week of August when we had 28 confirmed cases.

Epp is very familiar with this years WNV statistics because of two investigations that she is conducting as part of her Masters program in veterinary epidemiology. Other WNV studies have been done in the U.S., but because of differences in climate, environment, and bird and mosquito populations, Epp believes its important to understand how the virus plays out in Western Canada.

We wanted to get a picture of what are the most common risk factors for the disease in Saskatchewan and what impact the environment has on the rate of infection. That will give us a better idea of what areas are more at risk and what management practices are the most effective for preventing infection, explains Epp.

For her case control study, veterinarians across Saskatchewan connected Epp with clients whose horses had tested positive for WNV. Once owners agreed to participate, Epp interviewed them by telephone about their horses environment and their management practices for mosquito control.

Other than clusters of infected horses around Regina and Saskatoon, the WNV cases were spread out across the province. Most infected horses were kept on pasture without any mosquito control, and the majority werent vaccinated. About half of the horses that tested positive for WNV and showed clinical signs of the disease were euthanized. If the horses went down right away, they didnt tend to make it. But if they just started out wobbly, then owners and their veterinarians have been trying to treat them, says Epp.

Before the mosquito season peaked in mid-August, Epp collected blood samples from 200 randomly selected horses across the province during a nine-day, 4,600-kilometre road trip. Twenty clinics each gave names of three or four equine clients, then we randomly picked one herd to bleed 10 horses, explains Epp. Some were show horses, some were equine ranching mares, some were working ranch horses, so it gave us a wide range of ages, breeds and answers to our questionnaire.

Epp returned to take more blood samples from the same 200 horses in October, and once laboratory technicians have a chance to complete all of the random sampling surveys blood analyses, the researcher will be able to gauge the spread of WNV infection across Saskatchewan before and after the peak season for mosquitoes.

This will also give us a perspective of how many horses actually get infected with the virus in total, how many are asymptomatic, and how many Numbers Tell West Nile Virus Story show clinical signs of the disease, explains Epp. We have those statistics for humans, but we still dont have a percentage for horses.

Results from this seasons studies wont answer all of the questions that western Canadians have about WNV. But its a start, and if researchers can secure funding for similar studies in the next few years, Epp says a bank of data specific to Western Canada will take some of the guesswork out of dealing with the virus for veterinarians as well as owners.