Scientists say they have so far been unable to figure out why a new drug-resistant strain of HIV identified in New York City progresses so quickly to full-blown AIDS or to match the virus to any previously identified strain.

Speaking at a medical conference here, the investigators provided an update of a case first reported two weeks ago involving a gay New York City man with rapidly progressing HIV infection resistant to almost all of the antiretroviral agents that are usually used to fight the disease.

While “additional investigations will reveal whether this is an isolated case or not,” the public health implications of this single patient should not be minimized, Dr. David Ho of the Aaron Diamond AIDS Research Center and his colleagues in New York reported during the 12th Annual Retroviral Conference.

However, they also stressed that as HIV prevention efforts are stepped up, “care must be taken to avoid punitive measures against the populations most vulnerable” to the disease.

The scientists said that the patient, who is in his late 40s, was infected no longer than 20 months ago, which was most likely only four months before he progressed to AIDS. The patient has a history of extensive unprotected sex and multiple partners, often while using crystal methamphetamine.

The patient’s “unique” strain of HIV has shown some sensitivity to the HIV drugs efurvitide (T-20) and efavirenz, the scientists said. But it is resistant to all three of the other classes of antiretroviral drugs used to fight HIV - protease inhibitors, nucleoside reverse transcriptase inhibitors and non-nucleoside transcriptase inhibitors.

The patient first sought medical help in December 2004, when he was suffering from an inflamed throat and fatigue. He tested positive for HIV at that time, although five tests performed between September 2000 and May 2003 had been negative. The patient recalled that he had suffered from a week of flu-like symptoms in early November about two weeks after he had unprotected anal intercourse with multiple partners.

On January 13, 2005, tests indicated that the patient’s disease had progressed to AIDS. Over the past three weeks, the patient lost four kilograms of body weight.

The presence of multidrug resistance, rapid progression to AIDS, the patient’s history of multiple high-risk sexual encounters, and crystal methamphetamine use prompted New York City health officials to issue an alert to the public and physicians, to begin to trace the patient’s sexual contacts and to call for increased efforts to prevent the spread of HIV.