Untreated syphilis also increases the chance of being infected with HIV.
In a written statement, Lakes District Health Board told the Rotorua Daily Post syphilis was making a resurgence around the world.
"In the past, the high-risk groups have been gay and bisexual men and transgender females, but in the last couple of years the proportion of new cases of syphilis in the heterosexual population is steadily increasing."
Lakes District Health Board, which covers both Rotorua and Taupō, had one case of congenital syphilis last year, meaning the infection was transmitted from mother to child in the womb.
Lakes District Health Board map. Image/Supplied
Infected babies who aren't treated can have seizures, brain damage, blindness, development delay or die.
Nationally, congenital syphilis caused two stillbirths last year, but the Lakes District Health Board case was not one of them.
The District Health Board said syphilis used to be more prevalent in the older age group but "these days the vast majority of cases are being diagnosed in under 35-year-olds".
"[This is] The same group which has a higher prevalence of other STIs including gonorrhoea and chlamydia."
Lakes District Health Board sexual health specialist Dr Teena Mathew said this was "worrying".
"It is known that increased rates of infectious syphilis in women of childbearing age group will lead to new cases of congenital syphilis in the community."
Dr Mathew said the 16 cases in 2017 were just "the tip of the iceberg".
"Syphilis is an infection that can have no symptoms or the symptoms can be so mild or transient that many people ignore them."
Most of the 16 cases presented with significant symptoms including neurological and eye involvement.
"Some sexual contacts of the symptomatic cases also had infectious syphilis without any symptoms. Some women were diagnosed in pregnancy through routine antenatal blood tests as syphilis is routinely included in these tests."
In its statement, Lakes District Health Board said it reported high rates of STIs like chlamydia and gonorrhoea, and now syphilis was also "in the mix".
"Unlike most cases of chlamydia and gonorrhoea, a third of syphilis cases can have serious complications if left untreated."
Testing (with a swab or urine) for chlamydia has improved the pickup rates for chlamydia in the Lakes District Health Board area, but this does not test for syphilis.
The health board said, "syphilis is easily diagnosed by a blood test in the vast majority of the cases and this needs to be specifically requested by a health professional".
"People having a sexual health check need to remember that a swab or urine test does not test for syphilis and if you are only having these tests – then this is an incomplete sexual health screen. Syphilis in its early uncomplicated stages is easily treated with antibiotics and is curable."
Syphilis in the Lakes DHB area 2014 - no syphilis cases 2015 - five cases 2016 - 13 cases 2017 – 16 infectious syphilis, including one congenital syphilis 2018 – 9 cases so far
