A strain of HIV that progresses to full-blown AIDS within three years if left untreated has become “epidemic” among newly infected patients in Cuba who reported having unprotected sex with multiple partners, according to a study published this week by international researchers working with patients and doctors on the Caribbean island nation.

The strain of human immunodeficiency virus – a combination of three subtypes of the virus – progresses so fast, researchers at Belgium’s Catholic University of Leuven said they worry that patients infected with the mutated virus may not seek antiretroviral therapy until it’s too late.

The finding, published this week in the medical journal EBioMedicine, raises concerns among U.S. AIDS researchers who worry that mutated HIV viruses are more difficult to diagnose, might eventually become resistant to therapy and challenge vaccine efforts.

Hector Bolivar, a physician and infectious disease specialist with the University of Miami Miller School of Medicine, said the HIV research community has long known about the virus’ capacity to mutate.

Anne-Mieke Vandamme, a professor at the Belgian university, and a team of researchers reported that they traveled to Cuba after clinicians reported an increasing number of HIV infections that rapidly progressed to AIDS.

To conduct the study, researchers recruited patients at the Institute for Tropical Medicine Pedro Kouri in Havana who had tested negative for HIV less than three years before diagnosis and who had not received therapy.

Researchers reported studying the blood of 73 patients recently infected with HIV – 52 who had been diagnosed with AIDS, and 21 without AIDS – and then comparing the results with blood samples from 22 patients who had progressed to AIDS after living with HIV for more than three years.

None of the patients had received therapy for the virus.  But all the patients infected with the mutated strain of HIV developed AIDS within three years.