ATLANTA (Reuters) - The number of sexually transmitted chlamydia infections reported in the United States rose more than 5% last year, the Centers for Disease Control and Prevention (CDC) said on Monday.

The increase is due largely to better screening and diagnosing of the sexually transmitted disease (STD), the CDC said.

Total cases of the disease numbered 877,478 in 2003, compared to 834,555 cases in 2002. The agency estimated there were an additional two million unreported or undiagnosed cases last year, many among young women.

CDC officials, however, said they doubt the jump in reported chlamydia cases indicates a resurgence of the disease.

"We have reason to believe that chlamydia is not increasing and that we are holding the fort for a disease that is widely spread through the population," said Dr. John Douglas, director of the CDC's division charged with preventing STDs.

"The sobering part is that there is still a whole lot of chlamydia out there," Douglas said. The CDC recommends sexually active women under the age of 26 and older women with new or multiple sex partners be screened annually.

In addition to warning about chlamydia, the agency noted that syphilis cases rose last year, the third consecutive annual increase. In 2003, there were 7,177 primary and secondary syphilis infections, the earlier stages of the disease, compared to 6,862 in 2002.

Researchers estimate that more than 60% of the 2003 syphilis caseload occurred among gay and bisexual men. The growth of the disease in that high-risk group is of particular concern because of its links to transmission of HIV.

Up to 70% of gay and bisexual men infected in recent syphilis outbreaks in the United States were HIV positive.

In contrast to the rise in chlamydia and syphilis, the incidence of gonorrhea fell 4.8% to 116.2 cases per 100,000 in 2003 from 122 cases per 100,000 in the previous year.

There were 335,104 gonorrhea infections reported in 2003, compared to 351,852 in 2002.

STDs cost the nation as much as $15.5 billion in direct medical costs each year. Almost half of these infections occur in teenagers and young adults between the ages of 15 and 24.

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