(MCT) NEW YORK—Cases of chlamydia soared past 1 million last year in the United States, marking the first time the country has seen such a large incidence. The highest rates of increase were among young adolescent girls, federal health experts reported Tuesday.

Not only did chlamydia cases reach a grim milestone—1,030,911 cases nationwide—but researchers from the Centers for Disease Control and Prevention in Atlanta also said the infection accounted for the most cases ever reported for a sexually transmitted disease of any kind.

By comparison, 976,000 cases were reported in 2005.

Public health experts partially blamed faltering public health campaigns and a general failure among physicians to test patients for chlamydia as well as other bacterial infections that are transmitted sexually.

About 19 million sexually transmitted diseases, commonly called STDs, of all kinds were recorded last year, half occurring among people between the ages of 15 and 24.

The spread of chlamydia is of intense concern, public health officials said, because it is a silent infection with few obvious symptoms in its initial stages.

Chlamydia can lead to pelvic inflammatory disease and infertility. Scores of cases, experts said, are likely going undiagnosed.

“We have reason to believe that chlamydia is dramatically underreported,” said Dr. John Douglas, director of STD prevention at the CDC.

Though Douglas and his colleagues estimated about 348 cases of chlamydia per 100,000 people in the population, he said government figures are probably off the mark, and the actual number of new chlamydia cases last year was probably in the neighborhood of 2.8 million.

Based on the new data, CDC researchers who examined the number of sexually transmitted diseases other than HIV last year estimated the cost in 2006 for treating STDs was $15 billion.

Estimates were up slightly for syphilis and gonorrhea, and experts said there is evidence of a gonorrhea super-strain that repels several powerful antibiotics.

The increase in syphilis was largely seen among men who have sex with men. All told, 9,756 cases of the infection were recorded last year, an increase of 13.8 percent from 2005.

The number of gonorrhea cases reached 358,366 last year, up 5.5 percent. Incidence of the disease had begun to increase in 2005, after a 74 percent drop between 1975 and 1997.

With respect to chlamydia, rates were eight times higher for blacks than for whites, and 18 times higher for gonorrhea.

“I don’t think the message is that the forest is burning and that a huge epidemic is out of control,” said Dr. Stuart Berman, also of the CDC.

But he pointed out that STDs represent a hidden epidemic and that most people are unaware of the number of STDs circulating in the population.

© 2007 McClatchy-Tribune News