King County has seen a steep increase in syphilis over the past six years and officials are wondering if a ground-breaking HIV prevention treatment might be an unintended contributor.

According to King County Health data, the county had 444 reported syphilis infections in 2015 compared to 289 cases in 2010 — a 54 percent jump. The early count on the 2016 numbers indicate another 10 percent increase from last year. The jump prompted public health officials to wonder if PrEP, which is taken once daily in pill form to prevent HIV infection, might have actually led to decreased condom use because it was so effective at decreasing HIV infection rates.

“I think we can’t say with any certainly that this is due to PrEP or not,” said Dr. Lindley Barbee, medical director of the Public Health, Seattle & King County STD Clinic. “It certainly could be.”

PrEP, which stands for Pre-Exposure Prophylaxis, can stop the HIV virus’ ability to replicate once inside a body. This, in turn, keeps HIV-negative people from becoming infected. When used daily, the medication has decreased HIV infections everywhere it has been used. It began wide distribution in Seattle in 2014.

When it was developed, health experts nationally worried that it might have the unintended effect of decreasing HIV while increasing risky sexual behavior particularly among men who have sex with men. Health officials point to the spike in sexually transmitted disease cases after the birth control pill was introduced in the 1950s and the morning-after pill or “Plan B” pill in the 1990s. Behavioral sociologists refer to it as risk compensation, a theory that humans increase their willingness to engage in dangerous behavior when they feel safer.

“Many have likened PrEP — which prevents a lifetime unwanted consequence HIV, similar to preventing an unwanted pregnancy — as possibly heralding a new era or sexual revolution for men who have sex with men,” Barbee said.

However Tobi Hill-Meyers, communication director of Capitol Hill’s Gay City, said that socially transmitted infections were on the rise before PrEP came to Seattle in 2014.

“It’s important to remember (sexually transmitted infections) have been on the rise,” she said. “They were on the rise before PrEP and they’ve continued rising since PrEP. And just like Public Health said, we can’t be sure why.”

Barbee agreed. Sexually transmitted diseases largely have been on the rise in the county since 2001. Cases of syphilis have increased 334 percent in the past 15 years among men who have sex with men. It’s possible, Barbee said, that the recent increase isn’t related to PrEP.

“I don’t know that we have the data or could ever have the data to say cause-and-effect with what’s going on right now,” she said, adding that the numbers from a national study done by the Centers for Disease Control indicate the possibility of a link. But, she said, that could be because the PrEP protocol requires regular testing for syphilis and other STDs.

“It’s a little hard to tease out exactly (the cause of the increase), she said. “I think it’s very likely that’s what’s going on but I can’t say.”