Syphilis cases are on the rise in the U.S. among gay and bisexual men according to a new CDC study released on Thursday, Reuters reports:

The increase in syphilis among gay men is a major public health concern, said researchers at the Centers for Disease Control and Prevention, because it indicates sexual behavior that could lead to an increase in HIV transmission.

The U.S. syphilis rate in 2013 was 5.3 cases per 100,000 people, more than twice the all-time low of 2.1 cases per 100,000 people in 2000, the CDC reported. The majority of patients with the disease, which is treatable, were men who had sex with other men.

Strategies that proved effective in lowering heterosexual syphilis rates have not worked as well among gay and bisexual men, said Gail Bolan, director of the CDC's Division of STD Prevention.

Syphilis can lead to blindness and stroke if left untreated.

The federal health agency recommended a prevention effort that includes increased screening for the disease and stresses the importance of using latex condoms, limiting sex partners and encouraging monogamous relationships with partners who do not have a sexually transmitted disease.

Syphilis has reached its highest level since 1995, the AP adds.

Top image: vintage WPA syphilis poster.