Tests will determine whether the much-hyped case was caused by a dual infection.

Months after New York City public health officials announced the discovery of a so-called 'AIDS superbug,' scientists are exploring whether the unusual case is the result of a dual infection, in which a person contracts viral strains from two different people.

Detected in a homosexual man who used methamphetamine, the superbug rapidly progressed to full-blown AIDS and showed resistance to three antiretroviral drugs, features the officials said signaled an “extremely rare” and potentially treacherous strain. Scientists and activists have since criticized the announcement, saying it caused undue alarm before the case was fully understood.

Researchers at New York's Aaron Diamond AIDS Research Center (ADARC) in March reported that the individual had multiple sexual partners in a short period, combined with a dramatic spike in viral load in just two months (Lancet 365, 1031–1038; 2005).

We cannot rule out the possibility of dual infection. Marty Markowitz, ADARC

But those factors could be the result of a dual infection, says Geoffrey Gottlieb, an infectious disease expert at the University of Washington in Seattle. In a letter to The Lancet in June, Gottlieb suggested that the case warranted further investigation to rule out the possibility of dual infection (Lancet 365, 1923–1924; 2005). Two other letters in that issue also raised questions about the case.

Dual infections occur in less than 10% of new HIV cases and can result either from 'coinfections,' in which individuals contract separate viral strains before developing immune responses, or 'superinfections,' in which an HIV-positive person gets reinfected years after the first exposure. In both cases, infected individuals rapidly advance to AIDS.

In their response to the letters, ADARC researchers agreed with Gottlieb's suggestion (Lancet 365, 1924; 2005). “We cannot rule out the possibility of dual infection,” wrote Martin Markowitz, ADARC's clinical director.

There are no commercial tests available to detect dual infections, and the cost of testing could set a research lab back by up to $5,000. Depending on the number of samples available, it could take researchers up to two months to complete the tests. If the New York scientists began testing the superbug after the Lancet letters, they might not yet know the results, but “if the researchers delved into [testing] right from February,” Gottlieb says, “they'd know the results by now.”

The health department and Markowitz remain tight-lipped, declining repeated requests for test details or when results might be expected. For now, Markowitz says, “the epidemiology of this case is being carefully addressed.”

If the case turns out not to be the result of a dangerous new strain, it will weaken the accompanying public health warnings about safe sex and drug use, says Richard Jefferys, basic science and vaccine project director of the non-profit Treatment Action Group. “If there are issues around dual infection,” he says, “I hope that they quickly reveal the work that they've done and explain exactly what is going on.”