Syphilis, on the rise across the United States for more than a decade, surged in Philadelphia last year, a worrisome signal that yet-to-be reported national numbers might reflect a similar pattern.
Infections rose 36 percent in 2016, the Philadelphia Department of Public Health reported Wednesday, driven largely by men who have sex with men. National numbers lag a year behind, but other big cities are experiencing similar spikes, said Caroline Johnson, acting deputy commissioner, who oversees disease control. "It's going on all over," she said.
Just last week, the Centers for Disease Control and Prevention released an analysis showing that gay and bisexual men accounted for the majority of cases in 2015. Rates of primary and secondary syphilis — the early periods, when the infection is most likely to be noticeable as genital sores or a body rash and fever — were 106 times higher for these men compared with heterosexual men and 168 times higher compared with women, the researchers reported.
Possible reasons for the increases range from condoms falling out of favor as fear of HIV has declined to riskier sex practices. But the Philadelphia data provide a strong clue.
Two-thirds of the gay and bisexual men for whom the source of the infection was known used mobile apps that rely on GPS to locate other men interested in sex. They represented virtually the entire one-year increase.
"These apps present a challenge for identifying and treating sexual partners of syphilis cases because the interaction is often anonymous and cannot be retraced," the department reported.
Johnson recalled the early days of AIDS, when "anonymous sex in bathhouses was a huge problem." The practice fell out of favor as the risk became known. Mobile apps "appear to be the new version of that, " Johnson speculated.
"They have given the population a whole new way to transmit their diseases," said Judith A. O'Donnell, chief of infectious diseases at Penn Presbyterian Medical Center, who ran the city health department's STD control program in the 1990s.
"They could be coming from Bucks County, they could be coming from New Jersey," she said, and then they go back home "and spread it a little further."
The city health department reported 925 cases of syphilis in 2016, up from 682 the year before. More than 60 percent of the cases were among men who had sex with men; 10 percent were among women.
Syphilis is the most serious sexually transmitted disease after HIV, which is not the death sentence it once was but still requires lifelong therapy to keep in check. The genital ulcers that syphilis patients experience significantly increase the risk of both contracting and spreading HIV. Infected women can pass syphilis to their babies during delivery or in the womb, causing devastating defects, some fatal.
The bacterium that causes it, Treponema pallidum, also can lie dormant for decades, resulting in progressive disease in the brain. Some scholars have speculated that the madness and other symptoms exhibited by historic figures ranging from Adolf Hitler to King Henry VIII and Ludwig van Beethoven were linked to neurosyphilis.
The disease is easily treated with penicillin, and rates have plummeted nationally since the early 1940s, when the federal government also began requiring states to report it. Even with the current spike, rates today are exponentially lower.
Cities often have more infections than other areas. Philadelphia's rate of 20.5 per 100,000 residents in 2015, the most recent year that numbers are available nationally, was nearly triple the U.S. average, but Manhattan was 50 percent higher. Many cities in the South were far higher than that.