A nasty stomach bug that sickens thousands of people in Canada each year is growing resistant to most of the medicines used to kill it and could begin to pose serious threat in this country, says a Canadian microbiologist.

A "superbug" strain of shigella is being blamed for an increasing number of illness outbreaks in Canada and the U.S.

Last week, the U.S. Centers for Disease Control and Prevention reported that the antibiotic-resistant strain had caused several clusters of illness that began when people picked up the bug while travelling to such places as India and the Dominican Republic, and then spread it to others back home.

In all, it said there had been 243 cases of infection with the superbug in the last year.

Dr. Vanessa Allen, Chief of Medical Microbiology for Public Health Ontario, says doctors have seen antibiotic-resistant shigella for some time, but the problem is growing.

"The resistance seems to be increasing both internationally, but also here in Ontario and in Canada over the last five years," she told CTVNews.ca.

Shigella may not be as well known to Canadian as salmonella or E. coli, but it too is a family of bacteria that can cause severe, bloody diarrhea, with the very young and the elderly most at risk of serious illness.

Like E. coli, shigella emits a toxin that kills and damages intestinal tissue, sometimes leading to complications such as intestinal perforations and sepsis. But what makes Shigella different is that it should always be treated with antibiotics, says Dr. Allen.

"With other bacteria like salmonella for example, you can let it go and run its course. But all the guidelines suggest you treat shigella because it's so much more severe," she said.

That's what makes the issue of antibiotic-resistance in shigella so concerning, she says.

Resistance to the main drug used to treat shigella infections -- ciprofloxacin (or Cipro) -- has been on the rise for some time now, affecting less than five per cent of cases back in 2010, to about 14 per cent of cases today. In some populations, including in men, resistance rates are as high as 24 per cent, she says.

"That means that one-quarter of the people you treat with that antibiotic may not get better. So that's quite concerning," she says.

What also makes shigella unique is the way it can spread. Like other foodborne illnesses, it's usually contracted by eating uncooked food or drinking water contaminated with Shigella-infected feces, or through contact with Shigella-contaminated surfaces. But Shigella can also be spread sexually when there is contact with even small amounts of infected feces.

"That's an unusual way for a diarrheal bacterial disease to spread," says Allen.

An outbreak of shigella infections in Montreal in 2011, for example, was traced to gay men having unprotected sex. All those infected in that outbreak had contracted Cipro-resistant Shigella.

Allen says Canadians can protect themselves from shigella by practicing safe sex with condoms, as well as washing their hands before and after handling food, particularly in countries where Shigella outbreaks are common. Health Canada's website offers several tips for avoiding infection.

But Allen says it's also important for Canadians to seek proper treatment if they develop severe diarrhea.

"What we need most is to be diligent about taking (stool) samples and testing them for shigella to make sure to use the right antibiotic. Because we now can't trust that using just any drug will work when we have these rates of resistance," she says.

This CDC report is also a reminder that several common bacterial illnesses are now growing antibiotic resistant.

"In gonorrhea, for example, we are getting close to not being able to treat it at all. So we need to preserve the utility of the drugs we have," she says.

If the serious global health issue of drug resistance isn't tackled soon, she says, doctors could soon have no way to treat once easily cured illnesses.

"We've been so lucky to be able to treat these infection over the last 70 years, and now the question we are asking is: will we still be able to do so in 20 years?"