A new study from B.C. researchers is proposing a biological reason behind the "astounding" increase in syphilis cases, saying high-risk sexual behaviour doesn't explain the outbreaks that are happening.

Syphilis outbreaks have been on the rise worldwide, including B.C. and many other locations in Canada — primarily among men who have sex with men, many of whom are taking antiretroviral drugs to treat HIV, the study states.

Past research has speculated new, highly effective HIV treatments had encouraged more high-risk sexual behaviour.

However, the study's lead author said that didn't make sense as a sole explanation,  because other sexually-transmitted disease such as chlamydia and gonorrhea weren't rising as quickly.

"That led me to believe there must be something else going on," said Michael Rekert, clinical professor in the University of B.C.'s School of Population and Public Health, in a news release.

The study, published this week in the journal Sexually Transmitted Infections, proposes another possible cause — that the HIV therapy itself may make patients more vulnerable to syphilis.

"What we are hypothesizing is there is an additional reason, besides behaviour, for why we are seeing this increase in syphilis cases," said Caroline Cameron, a University of Victoria microbiologist and co-author of the study.

In B.C. for example, from 2005 to 2014, syphilis cases rose 90.6 per cent, much more than chlamydia (39.9 per cent) and gonorrhea (63.8 per cent).

Biology vs. behaviour

The paper hasn't demonstrated HIV therapy has this effect on syphilis but examines the case for it.

One part of that is looking at the microbiology: whether there's a plausible biological mechanism for the HIV treatment, known as highly active anti-retroviral therapy, or HAART, to affect how the body responds to syphilis.

"What we're hypothesizing is it's altering the immune response ... in a way that is increasing susceptibility to the pathogen," said Cameron. "It warrants further study."

The authors are clear: HAART drugs are life-saving for people with HIV and shouldn't be stopped even if they make someone more vulnerable to syphilis.

But it's important to explore the connection, with laboratory tests and other work, said Cameron.

"It's important for people to know that there is this risk, if it's real," she said.

She says people at risk could be screened more regularly for sexually-transmitted infections or even have tailored therapies based on the impact on the person's immune system.

With files from the Canadian Press