Health officials are looking for passengers, including nine Canadians, who were on a flight that landed in Montreal last week and was carrying a man infected with a drug-resistant form of tuberculosis.

The Atlanta man, who had been diagnosed with a drug-resistant form of TB, known as XDR-TB, is in isolation under a federal order, the U.S. Centers for Disease Control and Prevention (CDC) said Tuesday.

"The patient was ordered to be in isolation and is required to stay in isolation until the responsible public health officials deem that he is no longer infectious to others," Dr. Julie Gerberding, the centre's director, told a teleconference with reporters.

State public health authorities had advised the man against travelling, but it is not clear whether he knew his strain of TB was drug resistant when he left the U.S.

When in Italy, he was advised by the CDC not to fly back to North America on a public flight. 

"In this case, the patient had compelling personal reasons for travelling and made the decision to go ahead," Gerberding said.

As tests came in, health officials discovered he had a rare, more dangerous form of tuberculosis.

On May 12, the patient left his home in Atlanta and boarded Air France Flight 385 for Paris that was arriving May 13.

By May 24, he was in Prague, and took Czech Airlines Flight 0104 to Montreal, where he landed that afternoon and rented a car to drive to the U.S.

American authorities caught up with him in New York and flew him on a special CDC flight from New York back to Atlanta.

Testing advised for people on flights

Health officials say some of the people aboard the two flights should now be tested for exposure to tuberculosis.

"During these two long flights, the patient may have been a source of infection to the passengers," said Gerberding. "The passengers most likely to be at risk would be the passengers who were seated in seats immediately close to the patient."

"Canadians should not be alarmed by this situation," said Dr. Howard Njoo of the Public Health Agency of Canada. "In this particular situation, the risk of transmission aboard the aircraft is very low; however, we are taking this seriously."

In deciding to issue the rare isolation order, Gerberding said the agency considered:

• The man's ability to transmit the disease.
• The seriousness of the resistant organism.
• The length of the flights.
• The chance other passengers may have a higher risk because of reduced immunity.
• Differences in the ability to diagnose and treat XDR-TB worldwide.

Testing challenges

Health officials strongly recommend that crew members and passengers seated next to the infected man, as well as two rows ahead and behind him, get skin tests for TB. They should be retested in several weeks to make sure they aren't incubating the infection.

The CDC is working with the airlines to check seat numbers, which will then be released to federal health authorities in other countries, who will assess and track cases.

"I think the good news is that the odds are nobody has probably become infected from this exposure, but you can't tell for sure," said Dr. Michael Gardam, director of the tuberculosis clinic at Toronto Western Hospital.

"The challenge is going to be that even when you skin test people on that plane, a number of them may already be positive from some other exposure. The final challenge is there is no known preventive therapy for this kind of XDR-TB, so you can't offer people medication to try to decrease their risk of getting active TB later on."

In Canada, there have been two reported cases of XDR-TB, one in 2003 and the other in 2006, both in Ontario. The U.S. had 49 reported cases between 1993 and 2006, the CDC said.

Health officials estimate about half of all XDR-TB cases are fatal. By definition, XDR-TB resists more than the first- and second-line drugs normally used to treat infection.

The last time CDC staff recalled using their federal authority to quarantine a person was in 1963 for smallpox.

Anyone in Canada with questions about TB or this particular case can contact the Public Health Agency of Canada through Health Canada's toll-free number at 1-866-225-0709.

With files from the Canadian Press and the Associated Press