Indiana's syphilis cases surged 70 percent in a single year, a state health official said Monday, urging health care providers to aggressively test patients for the sexually transmitted disease.

Indiana's primary, secondary and early latent syphilis cases rose from 297 in 2014 to 505 in 2015, the health department said.

Indiana isn't alone in seeing a spike in the bacterial disease that's spread by direct, skin-to-skin contact during unprotected sex. Several states have seen significant increases in syphilis cases during the last few years, and the national rate rose 15 percent between 2013 and 2014, said Brian Katzowitz, a spokesman for the Centers for Disease Control and Prevention's Division of STD Prevention.

"We've seen similar trends on a state by state basis, and California is dealing with similar issues as well, with monumental increases in syphilis, so I don't think Indiana is unique in that way," he said.

About 90 percent of the nation's syphilis cases are among men. The state agency did not release data Monday on which demographic groups had been affected by the syphilis surge, and agency officials did not return messages seeking additional information. Katzowitz said that while the biggest increases in cases of the disease had been among gay and bisexual men since about 2000, in recent years there have been significant increases of syphilis among heterosexual men and women.

The increase in syphilis among heterosexual women has been accompanied by an increase in babies being born with syphilis passed onto them by their mothers, he said.

Health officials aren't able to pinpoint a precise reason for the increase in syphilis. Gonorrhea and chlamydia rates are also up, Katzowitz said. But, he said, dating apps such as Grindr that facilitate anonymous sex are making it more difficult to track sexual contacts of people "so it's harder to stop the cycle of transmission." The erosion of community resources for contact tracing also contributes, he said.

Indiana's health department is "working closely with local health officials and health care providers to make sure patients are getting tested and receive treatment," Commissioner Jerome Adams said in a statement. He also encouraged physicians and others to do a better job of educating patients about syphilis, which is curable with appropriate antibiotics but potentially deadly if not treated.

Indiana's health department issued a similar advisory about syphilis testing last fall after preliminary data showed a 53 percent increase in syphilis cases from January 2015 to early October 2015.

Beth Meyerson, the co-director of the Rural Center for AIDS/STD Prevention at Indiana University, said that a recent study she conducted found that Indiana's community health centers weren't routinely screening their patients for syphilis, chlamydia, gonorrhea or HIV.

Physicians at those clinics often don't know the medical histories and personal backgrounds of the people they're serving and therefore don't arrange for STD testing, she said.

"I'm glad the state health department put out this advisory because clinicians really listen to that," Meyerson said.

A CDC data sheet shows that during 2014, Nevada, Louisiana, Georgia and California had the nation's highest syphilis rates, with all four states reporting more than 10 cases per 100,000 residents of the disease. Nevada had the highest rate, with nearly 13 cases per 100,000.