The rapid rise of syphilis, including among young women, has prompted the health ministry to form a special research team to find ways of stopping the sexually transmitted disease.

According to hospital reports from across Japan, there were 4,259 syphilis cases from Jan. 1 to early December 2016, up 77 percent from the 2,412 cases logged in the same period in 2015, and up by more than seven-fold from a decade ago.

Most of the cases are believed to be caused by heterosexual intercourse, but cases of mother-to-child transmission, which is rare, are also on the rise.

The Health, Labor and Welfare Ministry is urging people who feel they might be infected to get tested immediately because the disease can be transmitted despite the absence of symptoms.

Factors including changes in sexual behavior in Japan’s youths and its adult entertainment business, as well as the surge in tourists from countries with high infection rates are often cited as causes, but the true cause of syphilis’ rapid spread over the past decade remains unclear.

When screening patients for syphilis, doctors ask whether they have recently had sex. They then file the reports in accordance with the laws governing infectious diseases, documenting age and gender but not more specific information, such as nationality or occupation.

Syphilis is especially rife in Tokyo’s Shinjuku Ward, the capital’s primary entertainment and nightlife hub. Local hospitals say the ward accounts for 40 percent of Tokyo’s syphilis cases and 20 percent of all cases across Japan.

To determine the major infection routes, Shinjuku Ward’s health care center is encouraging hospitals to ask all patients if they have worked in the adult entertainment business and where they come from.

The ministry’s special research team plans to find out how the disease is being spread, especially via heterosexual routes, by the end of March 2018 by working with hospitals in Tokyo.

“We intend to clarify which groups of people are at most risk and stop the spread of the infection by educating about how to prevent and cure the disease,” said team leader Makoto Onishi, a section chief the National Institute of Infectious Diseases who specializes in bacteria.

In its primary stage, syphilis presents only as minor lesions in affected areas, such as the genitals, but the symptoms often disappear naturally. In the secondary phase, around three months later, patients develop rashes, frequently on their palms or the soles of their feet. The symptoms then often disappear again.

Without medical treatment, the infection could cause inflammation across the body three years later, and possibly cause eventual damage to the brain and heart.

“Condom use is effective in preventing the disease, but it’s not enough. Everyone is at risk unless they have specific uninfected partners,” said Kunio Kitamura, an obstetrician and gynecologist who serves as the chief of the Japan Family Planning Association.

The disease can also be transmitted through oral sex, causing sores in the mouth, he said.