WASHINGTON (Reuters) - The U.S. syphilis rate rose for the seventh straight year in 2007, driven by a continued surge in cases among homosexual and bisexual men, the U.S. Centers for Disease Control and Prevention said on Wednesday.

Since 2000, when the national syphilis rate sank to a low of 2.1 per 100,000 people after a decade of progress in the 1990s, the rate has soared by 76 percent, the CDC reported.

Homosexual and bisexual men accounted for 64 percent of syphilis cases in 2007, up from about 5 percent in 1999.

CDC officials expressed concern not only because the recent increases in this bacterial sexually transmitted disease follows years of declines, but also because syphilis can elevate a person’s risk of being infected with the AIDS virus and the odds of giving it to someone else.

They also called rises among women and blacks troubling.

The overall national rate of syphilis rose by 12 percent in 2007 from 2006, reaching 3.7 cases per 100,000 people, based on preliminary CDC data released at a meeting in Chicago.

The rate for men was 6.4 per 100,000, a 14 percent rise from 2006.

The number of syphilis cases nationwide jumped to 11,181 in 2007 from 9,756 in 2006, with men accounting for six times as many cases as women. Rates for men and women had been roughly equivalent a decade ago.

Syphilis hit the black community very hard, with rates six times higher for men and 13 times higher for women than among whites, the CDC said. The rate for black men, 21.5 cases per 100,000, has risen 99 percent since 2003.

Syphilis rates have been surging in homosexual and bisexual men in the past decade, particularly among those who are highly sexually active with multiple sex partners.

RISKY BEHAVIOR

“Having multiple sex partners and other high-risk behaviors like not using condoms do put you at higher risk for HIV and syphilis,” CDC epidemiologist Dr. Hillard Weinstock said in a telephone interview.

“Syphilis can increase the likelihood of HIV transmission two to fivefold. And CDC recommends that sexually active men who have sex with men get tested for syphilis, HIV and other STDs at least annually,” Weinstock added.

“It is imperative that we make STD screening and treatment a central part of the medical care for gay and bisexual men,” while also finding ways to avoid these infections including HIV in the first place, said Dr. Kevin Fenton, who heads the CDC’s STD, AIDS, tuberculosis and viral hepatitis prevention effort.

Weinstock said despite the increases of this decade, syphilis rates remain lower than in the past.

After reaching 50,000 cases and a rate of 20.3 cases per 100,000 people in 1990 -- the highest rate since 1949 -- public health efforts helped drive down the rate to 2.1 per 100,000 people in 2000.

“We are concerned that the increases that we’re seeing now could continue,” Weinstock said.

Syphilis is passed person-to-person through direct contact with a syphilis sore. Transmission of the organism occurs during vaginal, anal or oral sex. Pregnant women with syphilis can pass it to their babies.