There are fears a dramatic rise in syphilis across Auckland could mean an outbreak of HIV is on the way.
Reported cases of the sexually transmitted disease jumped 78 percent last year, a new study has found.
If left untreated, syphilis can cause sores, rashes and blindness. It can also pass from mothers to their unborn babies, even killing them.
While 92 percent of syphilis cases were gay men, there was a 280 percent spike in heterosexual cases - from just nine in 2014 to 35 in 2015, according to the Auckland Regional Sexual Health Service.
"Numbers of syphilis cases managed by the Auckland regional sexual health service are at their highest in recent decades," the study, published in the New Zealand Medical Journal on Friday, reports.
The number of reported cases grew each quarter, with almost half the year's total coming in the last three months.
"Syphilis can enhance both the transmission and acquisition of HIV, so early diagnosis and treatment of identified cases may reduce an individual's risk of acquiring HIV."
Congenital syphilis - syphilis in children contracted while they were still in the womb - is on the rise in the UK and US.
"There is a real concern this could happen in New Zealand, with the recent increased number of cases in heterosexuals... The increased numbers could be a precursor to a rise in congenital syphilis cases if this outbreak is not contained."
Medical specialists hope when it becomes mandatory to report syphilis, as is planned under the Health (Protection) Amendment Act 2016, cases will be picked up and treated earlier.