Toronto Public Health says it is aware of a "significant increase" in the number of infectious syphilis cases reported in the city last year and in the first six months of this year.
Dr. Rita Shahin, associate medical officer of health, said 394 cases were reported in Toronto up until the end of June 2016. A total of 690 cases were reported to public health officials last year, as compared to 590 cases in 2014.
Shahin said about 97 to 98 per cent of the cases in general are reported in men. Last year, 89 per cent of cases involved men who had sex with men. Forty-five per cent of all people infected with syphilis are also infected with HIV, she said.
"The last couple of years are the highest we've seen in a long time," Shahin said. "It's not something that's unique to Toronto. It's definitely happening in other Canadian jurisdictions and the U.S., as well."
Toronto public health officials have not pinpointed a "specific cause" for the increase, although no condom use, especially for oral sex, is considered one of the factors contributing to the rise in the number of cases.
"There's definitely some thought that there's condom fatigue, particularly in people who are HIV positive," she said.
Other factors behind the growing numbers may be an increase in meeting people online, an increase in anonymous sexual partners as well as drug and alcohol use.
As well, she said an HIV treatment known as pre-exposure prophylaxis might also be a factor. That treatment, a daily pill to prevent infection, is used on people who do not have HIV, but are considered at risk of contracting it.
She said it significantly reduces the risk of HIV transmission but may also reduce condom use.
Early symptoms of syphilis include sores or ulcers but Shahin said they are often hard to spot. Shahin urged frequent testing..
According to an online "fact sheet" prepared by Toronto public health, infectious syphilis is a sexually transmitted infection caused by bacteria.
It can be spread through unprotected vaginal, anal or oral sex. It is passed on through direct contact with a syphilis sore. The bacteria can make its way into the bloodstream through a tear in the skin or mucous membrane.
Syphilis can also be spread through sharing of needles when injecting drugs. Pregnant women can pass syphilis on to their developing fetuses.
Toronto public health said a blood test is used to diagnose syphilis. It said it can take between two to 12 weeks for the infection to appear in the blood. The blood test needed to diagnose syphilis is not a routine one.
Toronto public health said it believes it is important to treat syphilis as soon as possible to prevent complications and to prevent the infection from being transmitted to another person. It said antibiotics will cure syphilis.
If not treated, syphilis can severely damage the heart, brain, liver, bones and eyes.