County health officials have declared a syphilis outbreak in Las Vegas after reported syphilis cases in the Sin City have more than doubled since 2012. That brings Nevada’s syphilis rate to the highest in the Western United States, according to the Associated Press.

There were 694 new cases of syphilis in 2015 in Clark County, which includes Las Vegas, the Las Vegas Review Journal reports.

Syphilis rates are rising across the country, as are the rates of other sexually transmitted diseases, according to recent data from the Centers for Disease Control and Prevention. It is unclear how Las Vegas’s current syphilis rate compares to that of the rest of the country.

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Syphilis is a sexually transmitted disease which can cause rashes, blindness, dementia, and eventually death, if left untreated, according to the CDC. Men who have sex with men make up the vast majority of syphilis cases in the US.

“We are considering this an outbreak because disease rates in our community are continuing to climb and we see no plateau at this point,” said Dr. Joe Iser, chief health officer of the Southern Nevada Health District, in a statement, as reported by the Review Journal.

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There are many possible reasons for the increase, AP reports. Websites and smartphone apps make anonymous sex easier. People may be using condoms less frequently. And more people are getting tested, so more cases are identified.

In 2014, the rate of primary and secondary syphilis in Nevada was 12.8 per 100,000 people, the highest in the country outside of Washington, D.C., according to CDC data.