Six-year-old Layla Vigh adjusts her respiratory mask at Pearson International Airport in Toronto after arriving from Mexico City on Monday. (Nathan Denette/Canadian Press)

The number of Canadians confirmed to have suffered mild cases of the human form of swine flu swelled to 13 on Tuesday, with new cases reported in Alberta, Ontario and British Columbia.

The new cases in Ontario, Alberta and British Columbia were anticipated, and do not change the federal government's course of action, Canada's chief public health officer, Dr. David Butler-Jones, told a teleconference on Tuesday.

"The more cases we see, the greater odds that some will be severe," Butler-Jones said.

Ontario's acting chief medical officer of health, Dr. David Williams, said four people in the country's largest province were confirmed as having the same strain of influenza that may have killed as many as 152 people in Mexico since mid-April.

The four cases, which were the first to be confirmed in Ontario since the disease spread outside Mexico, were all mild and involved people who had travelled to Mexico recently. Two were from the York region, north of Toronto, and the other two were from the Durham region, east of Toronto.

"It hasn't shown rapid movement from patient to patient or person to person," Williams told a news conference.

"Personally, I know we're going to see a lot more cases. It's only a matter of time," given the extent of travel to Mexico, he added.

Experts estimate that roughly 19,000 people travel between Canada and Mexico during an average week in April.

New cases in Alberta, B.C.

Earlier Tuesday, officials in Alberta said two males had come down with mild cases, one in Calgary and one in northern Alberta. Both recovered without needing to be hospitalized.

British Columbia officials also confirmed one new case of swine flu in an update posted on the website of the B.C. Centre for Disease Control, bringing that province's confirmed total caseload to three. Two cases involving men from the Lower Mainland had been confirmed on the weekend.

Alberta and B.C. are now able to do confirmatory tests, so samples from those provinces are no longer being sent to the National Microbiology Laboratory in Winnipeg, the lab's scientific director, Dr. Frank Plummer, said Tuesday.

In Nova Scotia, four students from a private school in the Windsor area are recuperating after a class trip to Mexico.

Researchers have learned that the incubation period of this strain appears to be two to seven days, which is typical of influenza, Plummer said.

There is also "reasonable evidence" from the Nova Scotia school cases of spread among people who did not travel to Mexico.

The person-to-person spread was in a closed setting, which differs from more widespread spread in the community.

Meanwhile, the number of unconfirmed cases that are being monitored in Prince Edward Island has dropped to five from eight. Seven of the original eight people reporting flu-like symptoms had recently returned from Mexico, while the eighth had been in contact with at least one of the travellers.

An official with P.E.I.'s Health Department who did not want to be named said three of the original eight cases do not fit the case definition for swine flu.

Customs officers, passengers wearing masks

Airline passengers arriving from Mexico will hear onboard information about swine flu and will receive health alerts from all airlines, said Butler-Jones.

Anyone who reports having a fever or cough or contact with someone showing those symptoms in the last seven days will be referred to a quarantine officer, who may advise against further travel or seeing a doctor in severe cases.

Canadians who returned to airports from Mexico this week were greeted by mask-wearing customs officers. Many passengers were also wearing masks.

On Monday, the Public Health Agency of Canada issued a travel warning advising Canadians to postpone any non-essential travel to Mexico until further notice.

The agency urged travellers to take precautionary measures such as getting a flu shot, frequently washing their hands, and covering coughs and sneezes.

With files from The Canadian Press