LAS VEGAS — Nevada is experiencing the highest rate of syphilis in the Western U.S. following an outbreak in Las Vegas.

It’s part of a national spike in cases tied to increased testing, a rise in anonymous sex via social media and less consistent use of condoms, health officials say.

Social media’s link to syphilis among gay men, which account for a majority of diagnosed cases, has led health officials to take their educational outreach directly to the websites and apps, in some cases creating profiles or buying advertisements.

Syphilis is a sexually transmitted disease that’s been around at least since the Roman times, said Dr. Tony Fredrick, the Southern Nevada Health District’s medical epidemiologist.

It’s never really gone away – it just comes in waves. It’s detected by blood testing, which means it’s not a part of the “bundle” of STDs found through urine screening.

Caused by the bacterium Treponema pallidum, syphilis spreads through skin-to-skin sexual contact when there’s a sore or lesion, typically in the genital or anal areas or mouth.

Symptoms aren’t always apparent and can progress for years, even decades, without treatment. In early stages, it’s highly treatable with penicillin.

Clark County health officials declared an outbreak in Las Vegas last week after noting a 128 percent increase in reported syphilis cases since 2012 – with 615 of the 694 cases involving men diagnosed last year. This makes Nevada’s rate of syphilis the highest in the Western U.S.

There has been an uptick in other parts of Nevada, too, but that could be tied to a population increase, the state health department said. Meanwhile, youth in the state are having sex at younger ages and using condoms inconsistently or improperly.

Syphilis outbreaks have appeared in pockets of the U.S. in recent years, including in Kansas, Pennsylvania and Hawaii.

The latest available data, from 2014, showed a 15 percent increase in cases overall, the Centers for Disease Control and Prevention said. A majority of cases involve men having sex with men.

There’s been an increase in other STDs. The most common, chlamydia, has risen to record levels.

Health providers and officials have warned about the prevalence of anonymous sex through social media, particularly with an increase in the use of smartphone apps.

Young people dependent on their iPhones can find quick, easy access to hookups, said Elizabeth Adelman, a senior disease investigator for the health department in Las Vegas. Not meeting in person first can make it harder to negotiate condom use, she said.

Las Vegas officials have been working to connect with various websites and apps. Adelman said they sought permission for a passive presence on platforms such as the gay website Adam4Adam, so users can reach out to them for information. Other popular apps that officials are looking at include Tinder and Grindr.

Tinder and Grindr couldn’t be reached for comment.