CHICAGO (Reuters) - US health officials are making headway in preventing common sexually transmitted diseases in the United States, but the effect is largely among whites, and blacks continue to have high infection rates, according to a report released on Monday.

The US Centers for Disease Control and Prevention estimates there are 19 million new cases of sexually transmitted disease, or STDs, each year, which cost the US healthcare system $16.4 billion a year.

According to the report, reported cases of gonorrhea reached a record low in 2009, with declines in all racial and ethnic groups.

Yet the gonorrhea rate among blacks is 20 times higher than among whites, and almost 10 times higher than Hispanics, the CDC researchers found.

More people are also being screened for chlamydia, one of the most widespread STDs in the United States, with screening rates rising from 25 percent in 2000 to 47 percent in 2009.

Blacks accounted for 48 percent of chlamydia cases in 2009, putting the rate of chlamydia among blacks at eight times higher than whites and three times higher than Hispanics.

And most young women are still not screened for the infection, which has mild or no symptoms but can silently damage a woman's fallopian tubes, uterus and surrounding tissues, causing chronic pelvic pain, infertility and other problems.

The CDC estimates there are 2.8 million chlamydia cases a year, more than twice the number actually reported.

The report also found that syphilis rates did not increase among women for the first time in five years.

And cases of congenital syphilis, in which a mother transmits the disease to her infant, did not increase for the first time in four years.

Since 2000, syphilis rates have been rising among men who have sex with men and last year accounted for 62 percent of all cases, up from just 4 percent in 2000.

The rate of syphilis is nine times higher among blacks than whites, and four times higher than Hispanics.

The CDC said the large racial disparities in STD rates are consistent with other studies and reflect a range of factors, including poverty, lack of access to health care and high rates of STDs in predominantly black neighbourhoods that increase a person's risk of infection.

Fewer than half of people who should be screened got the recommended screening tests for STDs, according to the report.

If undiagnosed and untreated, STDs increase a person's risk for infection by the human immunodeficiency virus or HIV, which causes AIDS.