* Syphilis, gonorrhea rates largely unchanged
* STDs hit the young, gay and bisexual men hardest
CHICAGO, Dec 13 (Reuters) - The number of Americans newly diagnosed with chlamydia continued to rise in 2011, likely due to increased screening and more sensitive tests, while rates of syphilis remained unchanged and rates of gonorrhea hovered at near-historic lows, U.S. health officials said on Thursday.
According to the U.S. Centers for Disease Control and Prevention's annual report on sexually transmitted diseases, which tracks cases of the three reportable STDs - chlamydia, gonorrhea, and syphilis - young people and gay and bisexual men continue to be most affected by STDs.
In 2011, gay and bisexual men made up nearly three-quarters, or 72 percent, of all cases of syphilis. Young people had the most cases of chlamydia and gonorrhea, the CDC report found.
If left untreated, gonorrhea and chlamydia cause infertility, and syphilis can lead to serious long-term complications, including brain, heart and organ damage.
People with any of these diseases are at greater risk for infection with human immunodeficiency virus, or HIV, the virus that causes AIDS.
Here are some details from the report: * Gonorrhea - Overall, rates of gonorrhea rose 4 percent to 321,849, the second consecutive year of increases. But the disease remains at historically low levels. * Chlamydia - Reported cases of chlamydia rose 8 percent to 1.4 million in 2011, compared with 2010, continuing the 20-year increase in diagnoses due to expanded screening efforts and more sensitive screening tests. * Syphilis - The overall number of syphilis cases held steady in 2011 at 13,970, after falling 1.6 percent in 2010. According to the CDC, the steady trend masks declining infections among women and increases among men, especially gay and bisexual men.