Alberta hasn't seen a syphilis outbreak like this in decades, with a sudden and severe spike in infection rates across almost every part of the province — with one, notable exception.

Its largest city.

Somehow, Calgary and the surrounding area have not seen anywhere near the same increase in syphilis rates, which have alarmed public health officials in other regions of Alberta.

And researchers are still trying to figure out why.

"That is a critical question that we're looking into," Alberta's chief medical officer of health, Dr. Deena Hinshaw, said this week.

Consider just how much of an outlier Calgary has been.

One of these is not like the others

Alberta is divided into five broad health zones, four of which saw syphilis rates at least double — if not triple or quadruple — in 2018, compared with the year before.

In the Edmonton zone, the rate jumped 305 per cent. In the North zone, it was up 325 per cent. In the Central zone, it jumped 267 per cent, and the South zone saw an increase of 139 per cent.

Meanwhile, in the Calgary zone? Just seven per cent.

The difference between Calgary and the other zones was so stark, Hinshaw said health officials had to make sure there wasn't a problem with the data. They checked but found no mistakes in the calculations or the methodology.

"We haven't done anything differently in, say, Edmonton versus Calgary in terms of how we count numbers or how we do outreach," Hinshaw said.

"So I don't think there's anything in terms of the data collection that would explain the difference."

For a decade prior, rates of syphilis in the Calgary and Edmonton zones were pretty close.

But the 2018 surge saw Edmonton's rate shoot up, particularly among women. It now has nearly 19 times more syphilis cases, per 100,000 females, than Calgary.

So what's going on?

Social and sexual networks

Hinshaw said "there can be lots of different reasons" for a sudden spike in sexually transmitted infections.

While the exact causes of the recent syphilis outbreak aren't yet clear, she said it often boils down to the social and sexual "networks" of the people who have been infected.

"Ultimately, we know that when an infection gets into a network of people, it can spread quite quickly," she said. "And sometimes it's just a matter of which networks syphilis has gotten into — and which ones it hasn't."

She cautioned, however, that sexually active people in the Calgary area shouldn't be complacent.

While Calgary's syphilis rate didn't jump sharply in 2018, it has been on the rise, more slowly, for years.

Slow and steady rise

In the early 2000s, syphilis was rare in the city and surrounding area, with only about one diagnosis a year per 100,000 people.

But it's been making a comeback more recently.

In 2017, the Calgary zone actually had the second-highest syphilis rate in the province, at 11.8 cases per 100,000, behind Edmonton's rate of 17.6.

But the outbreak of 2018 saw Edmonton's rate surge to nearly 70 cases per 100,000, while the North jumped to 43.5 and the Central zone hit a rate of 18.5.

That put Calgary in a distant fourth of the five zones, with the South not far behind.

Use protection, get tested

The trouble with syphilis is that it's often hard to realize you have it — at least, at first.

"Syphilis is often referred to as 'the great imitator' because of the wide range of symptoms that infected people may experience," Health Canada warns. "These symptoms can easily be confused with those of other conditions."

The bacterial infection can be transmitted through genital, oral or anal sex.

Alberta Health advises "consistent and correct condom use" as one of the best ways to reduce your risk of infection, and regular testing is important, to catch syphilis — and other sexually transmitted infections (STIs) — early.

It recommends sexually active people get tested every three to six months if they have had a sex with a new partner, a partner with a known STI, or a previous STI diagnosis, themselves.

"People should be thinking about their sexual health as a part of their overall health," Hinshaw said.

Effects of untreated syphilis

Left untreated, Hinshaw said a syphilis infection can cause problems with the heart and the brain and lead to blindness in some cases.

A pregnant woman can also pass a syphilis infection on to her unborn child, and this can lead to birth defects and even the child's death. These cases were previously rare, but between 2014 and 2018, there were 22 cases of congenital syphilis, one of which resulted in a baby being stillborn, says Alberta Health.

In response to the recent outbreak, the provincial government has formed a "co-ordination committee" that is tasked specifically with addressing the recent rise in syphilis.

Over the next three months, the group plans to develop a strategy to boost public awareness of sexually transmitted infections and encourage more Albertans to get tested.

Hinshaw said they will also be looking at why Calgary seems to have been spared from the recent surge in syphilis rates elsewhere in Alberta.

"That's something that we're doing a lot of work to try to understand more, so we can understand how we can stop it where it already is — and prevent it from getting into networks where it isn't."