Health officials say the virus was detected in two individuals who were tested by Canadian Blood Services as they were donating blood last week.
One person showed mild symptoms, namely headache and fatigue, while the other was said to have no symptoms, said Dr. Margaret Fast, the province's acting chief provincial public health officer.
Both individuals are in their 40s and live in Winnipeg, but Fast said it's not known if they were exposed to West Nile virus in the city or elsewhere in southern Manitoba.
Health officials are also trying to determine when the two may have contracted the virus.
The last time West Nile virus was detected in Manitoba was in 2009, when two human cases were reported. There were no cases in 2010 and 2011, Fast said.
In 2007, more than 500 human cases of West Nile virus were diagnosed in Manitoba, and several deaths were connected to the illness. Thirty-eight cases were reported in the province in 2008.
Mosquito numbers rising
While some Manitobans have noticed fewer mosquitoes than usual so far this summer, Fast said Culex tarsalis mosquitoes, which carry the virus, have been detected in central and southern parts of the province.
"We do know that in southern Manitoba, the number of mosquitoes and the number of infected mosquitoes is increasing," she said.
"At the moment, we have 14 communities across southern Manitoba where we have identified … pools of mosquitoes that are infected with the virus."
The ongoing hot and humid weather in the province will likely result in mosquito numbers continuing to rise, she added.
Fast said most individuals who are exposed to West Nile virus show no symptoms, while approximately 20 per cent may show some flu-like symptoms such as headaches, muscle aches and pain.
A very small number of people who are infected could develop neurological symptoms that can cause paralysis, coma or — in rare cases — death.