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.

In rare cases, about one in 150, the virus can cause severe illness, such as neurological symptoms that can cause paralysis, coma or — in rare cases — death.

To reduce the risk of being bitten by mosquitoes, health officials in British Columbia recommend:

  • Avoid being out from dusk to dawn when mosquitoes that carry West Nile virus tend to be active.
  • Wear protective clothing outside especially in the early evening and at dawn. Long-sleeve shirts or jackets and long pants that mosquitoes cannot bite through are recommended. Tucking pants into socks offers extra protection.
  • Avoid dark-coloured clothing that can attract mosquitoes.
  • Use mosquito netting for babies and toddlers in cribs and strollers.
  • Use mosquito repellents that are federally registered, such as those containing DEET or PMD (lemon-eucalyptus oil).

There were 110 human cases of West Nile virus in the country last year, according to the Public Health Agency of Canada's website.

With files from CBC's Karen Pauls