Canadians are asked to avoid going out at dusk

But in a small number of severe cases, the virus eventually makes its way into the brain, where it interferes with the functioning of the nervous system, leading to a coma and even death.

As mosquitoes are so plentiful, there is a limit to what authorities can do.

Government officials are trapping the insects to monitor the virus, and some jurisdictions are spraying insecticide.

'Avoid dusk'

Jill Bennet from the Essex county health unit, in south western Ontario, says they can only ask people to be vigilante by avoiding dusk, and eliminating likely breeding-grounds.

"We are asking individuals to look around their homes, and to see if there are any areas where there is stagnant water, eaves troughs or debris in eaves troughs, tyres in the backyards, screens in the homes that are in need of repair because of holes," says Ms Bennet.

The virus has been common in the Middle East since the 1930s, but the first North American case was only detected in 1990, in New York city.

The new Canadian findings means it has spread voraciously north and as far south as Florida.