A rare strain of chlamydia is spreading among gay and bisexual men in America causing an increase in risks of HIV, according to US health officials.

The Associated Press reported that the infection can increase chances of getting or spreading the AIDS virus.

Called LGV chlamydia, this sexually transmitted disease has already troubled Europe. Diagnoses confirmed by U.S. health officials still are low, just 27 since they warned a year ago that the strain was headed there.

But specialists say figures are incorrect because the illness is incredibly hard to diagnose, Few U.S. clinics and laboratories can test for it. Painful symptoms can be mistaken for other illnesses, such as irritable bowel syndrome. Becuae of this, it is feared people may silently hold and spread the disease.

Phillipe Chiliade, of the Whitman-Walker Clinic in Washington, told the Associated Press: “My feeling is that what we’re seeing now is still the tip of the iceberg,”

The Centres for Disease Control and Prevention already was counting an 8 percent increase in HIV among gay and bisexual men between 2003 and 2004, before LGV’s arrival was recognized.

“We are really concerned about this,” said Dr. Catherine McLean of CDC’s HIV and STD prevention programme.

Increasing the ability to test for LGV is “what’s really critically important,” she added. “The prevalence of the disease is probably quite a bit higher than the reported cases indicate, either here or in Europe, but we don’t yet know that.”

Three weeks of antibiotics effectively treats LGV. But patients have to know they’re at risk, and then find a test.

Chlamydia, caused by bacteria, is among the most common sexually transmitted diseases. As many as 3 million Americans a year may become infected with common strains, best known for causing infertility in women if left untreated.

This more virulent strain is called “lymphogranuloma venereum,” or LGV. It is rarely seen outside of Africa or Southeast Asia. The Netherlands announced an outbreak that reached over 100 cases in 2004. Cases also have surfaced in much of Western Europe and Britain. As with the U.S. cases, many also have HIV.

Symptoms differ from regular chlamydia: swollen lymph nodes in the groin; genital or rectal ulcers; and painful bowel movements and other gastrointestinal symptoms that may mimic inflammatory bowel disease. Such symptoms leave patients particularly susceptible to HIV infection if they also encounter that virus.