ORONTO, Sept. 1 — The West Nile virus, a mosquito-borne illness that appeared for the first time around metropolitan New York in 1999, has now surfaced in Canada, health authorities say.

The arrival of the virus was seen by officials here as inevitable, given its prevalence in the American Northeast and Great Lakes states, said Gord Haugh, a spokesman for Ontario's health minister, Tony Clement. All cases so far — none reported involving humans — have been found in Ontario, with about half in the greater Toronto area, Canada's largest metropolitan region.

"We've obviously been watching for it," Mr. Haugh said. "Birds and mosquitoes don't really respect borders."

Canadian health officials, hoping that cooler weather will end the problem soon, have begun planning for next spring, when suspect ponds or other bodies of water where mosquitoes breed will be sprayed with larvicides.

The presence of the West Nile virus in Canada was first confirmed on Aug. 22 in a dead crow found in Windsor, Ontario, just across the United States border from Detroit. At that time, Michigan and Indiana were reporting their first cases there.

On Friday, officials said that a dead crow found in Wisconsin had tested positive for the virus.

Since then, Ontario has found that 29 dead birds, all crows and blue jays, were confirmed to be carrying the virus, which can cause flu-like symptoms in humans. Serious consequences, including very occasionally death, occur in only about 1 percent of people infected, health officials say.

But older people, those with immune deficiencies and those with diabetes are considered the most vulnerable. Health experts say that perhaps only a quarter of cases are reported or identified because most are only low-level infections that do not cause concern.