By Will Dunham

WASHINGTON, Jan 13 (Reuters) - U.S. syphilis rates rose for a seventh year in 2007, driven by gay and bisexual men, while chlamydia reached record numbers and gonorrhea remained at alarming levels -- especially among blacks, health officials said on Tuesday.

Blacks make up 12 percent of the U.S. population, but account for about 70 percent of gonorrhea cases and almost half of chlamydia and syphilis cases, the Centers for Disease Control and Prevention said.

Black women ages 15 to 19 have the highest rates of chlamydia and gonorrhea, and gonorrhea rates for blacks overall were 19 times higher than for whites, the CDC said.

Dr. John Douglas, who heads the CDC's division of sexually transmitted disease, or STD, prevention, said overall syphilis, chlamydia and gonorrhea rates are unacceptably high. Cases of these three STDs are reported by U.S. states to the CDC.

In 2007, 1.1 million U.S. cases of chlamydia were reported, up from about 1 million in 2006 and the most ever, and the rate rose by 7.5 percent from the prior year, the CDC said in a report. Douglas said the figures may reflect that more people are being diagnosed rather than a rise in infections.

In addition, more than 350,000 cases of gonorrhea were reported in 2007, essentially unchanged from 2006, the CDC said. Gonorrhea rates fell dramatically from the mid-1970s through the mid-1990s, with little progress since.

Chlamydia and gonorrhea are easily diagnosed and treated, but frequently have no symptoms and remain undetected.

Untreated, chlamydia and gonorrhea -- both bacterial infections -- can cause pelvic inflammatory disease and infertility in women. The two infections also can cause ectopic pregnancy, chronic pelvic pain and other health problems.

"Of all the causes of infertility, this is probably the most preventable -- since these infections can be prevented, diagnosed and treated," Douglas said in a telephone interview.

In men, gonorrhea can cause a painful condition of the ducts attached to the testicles that cause cause infertility. Gonorrhea also can spread to the blood or joints and can be life threatening. Chlamydia complications among men are rare.

Douglas said to avoid STDs, teens can delay the beginning of sexual activity, people can limit the number of sexual partners and use condoms. "Condoms have risk-reduction value for every sexually transmitted condition," Douglas said.

Syphilis is less common than the others, with 11,466 cases reported in 2007. Rates rose 15 percent from 2006. Syphilis rates dropped by 90 percent in the 1990s to a record low level in 2000, and officials thought it might disappear as a public health threat before its resurgence this decade.

Syphilis has increased each year since 2000 -- its rate is up 81 percent -- with gay and bisexual men representing 65 percent of cases, the CDC said.

Douglas said many cases are occurring in HIV-positive men who are choosing other HIV-positive men as sexual partners.

"Within that relationship, they are less concerned about the transmission of other conditions. They're not using condoms. They believe that their partner already has got the worst they can get -- they've got an HIV infection," he said.

When all STDs are considered, including human papillomavirus (HPV or genital wart virus) and herpes simplex viruses, almost 19 million new infections occur each year, with nearly half among those ages 15 to 24, the CDC said. (Editing by Maggie Fox and Doina Chiacu)

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