Dr. Charles Gardner (CBC)

Dozens of children at three summer camps in Ontario's Muskoka region have been sent home after an outbreak of swine flu, health officials said Wednesday.

While most of the 227 children affected by the virus have left the camps, children whose parents weren't available or children from out of the country are staying put, said Dr. Charles Gardner, medical officer of health for the Simcoe-Muskoka District Health Unit.

Officials sent e-mails and telephoned parents with children at the affected camps, which have about 1,275 registered campers.

Gardner said most of the cases are mild and that the children were sent home to make it easier for the camps to manage the remaining children.

"They've had coughs, sore throat, fever, general unwell or malaise," he said. "None have required hospitalization."

He said it's likely some campers had the H1N1 virus when they arrived and it quickly spread.

The outbreak has affected three camps in the Simcoe-Muskoka region, about 150 kilometres north of Toronto, but health officials aren't naming the camps, citing confidentiality rules as their reason.

However, the Olympia sports camp, just outside of Huntsville, reported on its website that it had one confirmed case of swine flu and other campers with respiratory infections.

The Toronto Star reported that Camp Ramah, a Jewish education camp attended by more than 450 young people in Utterson, near Bracebridge, was one of the two other camps affected.

Authorities at the camps are encouraging parents to keep their children at home if they spot any flu-like symptoms and have banned any inter-camp events such as competitions or exchanges, Gardner said.

He said it's unusual to see the spread of the flu continue into the summer, meaning there could be more outbreaks.

"We may very well see other camps with outbreaks of influenza as children who have the virus arrive at the camp," he said.

An Ontario Health Ministry spokesman said it is the responsibility of local health units to monitor cases at summer camps.

"We expected that this had the potential of happening," said David Jensen, who added the cases at the three camps are "mainly localized."

As of Wednesday, there have been 9,855 confirmed cases of swine flu across Canada. Among those, 902 patients have been hospitalized. Thirty-nine people who contracted the disease have died, according to the Public Health Agency of Canada.

The World Health Organization in early June declared the spread of the virus had reached pandemic level, the first time a global flu epidemic has occurred in the past 41 years. The organization raised its pandemic alert level to Phase 6, the highest on the scale.