Health officials are warning people to take precautions against mosquito bites after three new cases of West Nile virus were identified.

The illness is spread from birds to people through the bite of infected mosquitoes, especially Culex tarsalis mosquitoes. Blood transfusions and organ transplants can be other sources of exposure.

Two human cases have been confirmed in Winnipeg and one in Windsor, Ont. All of the cases were mild and were discovered by Canadian Blood Services after blood donations.

In Ontario, health officials warned on Wednesday that there has been an increase in mosquitoes infected with the virus.

Entomologist Kateryn Rochon of the University of Manitoba said people should protect themselves.

"It is quite difficult to get people to realize there is a risk when you don't really feel the mosquitoes biting you," Rochon said.

"The ones that pester us and annoy us are not actually the most competent vectors. But the Culex mosquitoes that transmit the virus are on the rise."

Fogging to begin in Manitoba

Manitoba health officials have ordered mosquito fogging, using the chemical malathion, in the south-central city of Portage la Prairie, where the number of infected mosquitoes is on the rise.

Fogging trucks are expected to start driving around Portage on Thursday, provided that weather conditions are not too hot or windy.

Meanwhile, the City of Winnipeg says crews will continue to apply larvicide to parts of the city where Culex tarsalis mosquitoes could grow.

The province says there is no way to predict if fogging will be needed in Winnipeg this summer.

Symptoms vary

Tom Huffman is believed to be the first human case of West Nile virus in Manitoba seven years ago.

"Not something we should fool with because it can kill ya," Huffman said.

He started out with a headache and feeling tired and then became delirious after his temperature rose to 40 C. Huffman ended up in hospital for a week and was off work for more than three months.

He still has problems with this left eye.

Most individuals who are exposed to West Nile virus show no symptoms, while about 20 per cent may show flu-like symptoms such as headaches, muscle aches and pain, health officials in British Columbia said.