There's no reason for Canadians to change plans to travel to Mexico, Dr. Allison McGeer says. (CBC)

Canadians who have recently returned from Mexico should be on alert for flu-like symptoms that could be connected to a severe respiratory illness, federal health officials said Thursday in issuing a travel advisory.

A severe respiratory illness appears to have infected 137 people in south and central areas of Mexico, with cases concentrated in Mexico City and three other areas, including 20 deaths, the Public Health Agency of Canada said.

In the United States, health officials in Texas and California were scrambling this week to deal with a new strain of swine flu, which has been diagnosed in seven people.

The states share a border with Mexico not far from a town where two deaths were reported.

The U.S. cases are unusual, because it appears none of the patients had contact with pigs, and the virus is one that health officials have never seen before.

No cases of this swine flu have yet been found in Canada. The country's national laboratory in Winnipeg is analyzing 51 samples from Mexico to determine if any of them are swine flu and are linked to the U.S. cases. Results are expected in two days.

New virus raises spectre of pandemic

If the link is made, it may signal the emergence of a new pandemic virus, infection control specialist Dr. Michael Gardam, told CBC News in an interview Thursday night.

"It certainly is suspicious that you have a new strain of swine flu showing up just north of the Mexican border, and you have an outbreak of cases south in Mexico," he said.

"We already have the Americans talking about the H1N1 strain, which is a new strain they haven't seen before .… If you had evidence that this strain was also in Mexico, you would have further evidence that something new has shown up which has the ability to go person-to-person and has crossed borders as well."

However, there is no reason to panic, said Gardam.

"A pandemic virus basically means a virus that is going to affect a large proportion of the population. It doesn't mean they're going to get necessarily all that sick."

In Canada, the travel advisory includes the same advice given to all travellers — to get a flu shot and take precautions such as covering coughs and staying home when sick — but also adds locations in Mexico where cases of a severe respiratory illness have occurred.

The agency has also warned medical officials across the country to be on the lookout for patients with severe respiratory problems.

However, federal Health Minister Leona Aglukkaq also stressed that there is no reason for Canadians to panic.

"Canadians aren't threatened by this situation," Aglukkaq said, "but we need to work with our partners to monitor the situation carefully because it is a concern to us."

Public health officials in Ontario have monitored about 10 people who recently returned from Mexico, with the numbers changing by the hour.

"We haven't had any that have been directly tied," said Dr. David Williams, Ontario's acting chief medical officer of health. "We will continue to look, any case that has any remote possibility of connection, we want to see if we can connect those dots correctly."

Cause of illness unknown

"We are not sure where this is in Mexico, but it's not the tourist area," said Dr. Allison McGeer, director of infection control at Mount Sinai Hospital in Toronto.

The deaths were mostly men between the ages of 25 and 44, according to an advisory sent out by federal health authorities.

Hundreds of people have also reportedly been hospitalized and health-care workers have also been infected.

Officials said there are not yet any reports of the illness outside Mexico.

Alberto Lozano, spokesman for the Mexican Embassy in Ottawa, said doctors in his country are on top of the outbreak.

"This is pretty serious situation. But on the other hand, it is not an epidemic, so preventive measures have been taken to reduce the risk of contagion in Mexico."

The cause of the illness has not been confirmed. Mexican health officials said samples so far have tested positive for influenza A and B.

The working hypothesis is that the cases could be a late-season surge in seasonal influenza, said McGeer, noting most of the illnesses in Mexico have not been serious and there has been nothing new in the last five or six days.

People infected with the virus initially suffer flu-like symptoms that include:

• Fever.
• Cough.
• Sore throat.
• Muscle and joint pain.
• Shortness of breath.

The illness may elevate to a severe respiratory illness within about five days.

Testing samples

Anyone who has returned from Mexico within the last two weeks and is suffering similar symptoms is asked to contact a physician.

"Until somebody tells us we don't have to worry anymore, these are viruses we want to collect," said McGeer.

"What we do want to do is identify if there's some new virus, and the way to do that is for people to get a test if they get a respiratory illness, but it's only in the first few days after you've come back."

Mexican health authorities said that many of those diagnosed with the illness have had to be put on ventilators.

There's no reason for Canadians to change their plans to travel to Mexico, McGeer said.

Public health officials in Canada advise travellers to ensure their vaccinations are up to date before flying, to be careful about washing their hands, covering coughs and sneezes and staying home when feeling sick.

With files from The Canadian Press