OTTAWA -- The Public Health Agency of Canada says at least 30 Canadians who had surgery at a clinic in Tijuana, Mexico are at risk of potentially deadly infection.

But the agency isn't sure, because of the difficulty in responding to health alerts in other countries.

An investigation by the U.S. Centers for Disease Control last month found Canadians underwent weight-loss surgeries at the Grand View Hospital in Tijuana, just like American patients who became infected with an antibiotic-resistant strain of bacteria. Tijuana is right next to the U.S. border, close to San Diego.

"It may be that we never know the exact number of Canadians involved," said Dr. Howard Njoo, Canada's deputy chief public-health officer. "We're trying to ensure that all patients should be having communications from the treating facility to inform them of their potential risk so that they can do medical follow-ups."

The federal public-health agency issued a notice Wednesday recommending patients who went to the Grand View Hospital, or other Tijuana facilities, starting last August, seek medical help immediately if they're experiencing signs of infection, including fever, redness, or pus or swelling at the surgical site.

The bacteria involved is called pseudomonas aeruginosa, and it's a known hazard in medical settings, especially for patients who have had surgery.

The health agency also warns of risk of blood-borne infections including HIV, hepatitis B and hepatitis C due to poor equipment sterilization at Grand View. Other patients are advised to avoid the hospital until Mexican authorities give the all-clear.

Medical tourism presents a challenge to health authorities because Canadians are under no obligation to report when they've had treatments abroad, Njoo said.

"We can't demand or track any Canadian who decides to go overseas," said Njoo. "Anyone could be going abroad for tourism and if that includes going for an elective procedure in a medical facility that's not something that we would necessarily be aware of."

A 2017 survey by the International Society of Aesthetic Plastic Surgery found Canadians were the third-biggest market for plastic surgery outside their home country, behind only Americans and Spaniards.

According to Statistics Canada, overseas health-related spending by Canadians rose from $447 million in 2013 to $690 million in 2017.