The number of U.S. syphilis cases rose for the fourth straight year in 2004, fueled by increases among men, while the gonorrhea disease rate reached a historic low, federal health researchers said on Tuesday.

The annual report on sexually transmitted diseases (STD) by the U.S. Centers for Disease Control and Prevention also showed a rise in chlamydia cases, but the increase is believed to reflect better screening and not a rise in new infections.

The rate of primary and Secondary syphilis, the earliest and most infectious stages of the disease, rose 8 percent to 2.7 cases per 100,000 people last year, the fourth straight year of increase since an all-time low in 2000, the CDC said.

In 2003, the rate was 2.5 cases per 100,000 persons.

Though Syphilis is rarer in the United States than gonorrhea and chlamydia, health officials said its rise was occurring primarily among men who have sex with men. Syphilis can cause insanity and death if left untreated.

A separate CDC analysis estimated men with male sex partners accounted for 64 percent of early-stage syphilis cases in 2004, up from just 5 percent in 1999. Cities with large gay populations had some of the highest rises in Syphilis cases, topped by San Francisco.

“We do have good evidence that men are engaging in higher risk behaviors,” said Ronald Valdiserri, acting director of the CDC’s HIV, STD and Tuberculosis prevention programs.

At the same time, the data showed syphilis declined among groups that had been hardest hit in the past. For women and blacks, syphilis rates fell 55 percent and 37 percent, respectively, from 1999 to 2004, the CDC said.

The nation’s gonorrhea rate fell to 113.5 cases per 100,000 people last year, down 1.5 percent from 2003 and the lowest level on record since reporting began in 1941.

But officials said gonorrhea was under-reported, and noted that higher rates among U.S. ethnic minorities and rising resistance to antibiotics were hurdles to treatment and prevention. If left untreated, gonorrhea can cause infertility in men and a serious infection of the reproductive tract in women.

The CDC said chlamydia, a bacterial infection that can cause infertility in women, rose 5.9 percent to 319.6 per 100,000 people between 2003 and 2004. The agency said the rise likely reflects broader screening and better diagnostic tests rather than an increase in new infections.

STDs, which are spread through vaginal, oral and anal sex, result in medical costs of $13 billion annually and can increase the risk of contracting HIV, the virus that causes AIDS, the CDC noted.