TUESDAY, May 13, 2003 (HealthDayNews) -- European scientists are speculating that HIV-2, one of the viruses that causes AIDS, may have crossed over to humans as early as 1940.
The same researchers, writing this week in the Proceedings of the National Academy of Sciences, say the virus may have become epidemic as a result of war conditions in Guinea-Bissau in the 1950s and 1960s.
Other experts don't believe that the data should induce a sea change in our thinking.
"The dates are very iffy," asserts Ernest Drucker, a professor of epidemiology and social medicine at Montefiore Medical Center and Albert Einstein College of Medicine in New York City.
The oldest evidence of HIV-2 in an actual patient dates from the mid-1960s, while the oldest actual example of HIV-1 is from 1959 in what is now the Democratic Republic of Congo, he adds.
"All we really know is that we have HIV-2 in hand from actual patients from the '60s," Drucker says. "That's the oldest example of HIV-2."
The scope of HIV-2 is limited primarily to West Africa and to parts of the world that were once under the control of Portugal, says Dr. David Markovitz, a professor of internal medicine and infectious diseases at the University of Michigan Medical School in Ann Arbor. In West Africa, the virus infects about 1 percent of the population. In the United States, by contrast, fewer than 100 people are known to be infected with HIV-2.
"The thing about HIV-2 that's very striking is that it's so similar and yet so different from HIV-1," Markovitz says. The majority of AIDS cases are caused by HIV-1.
Genetically, the two strains are only about 40 percent similar, yet they produce the same proteins. They also produce vastly different effects in the people who are infected. HIV-2 is harder to transmit than HIV-1 and, once transmitted, generally has a slower rate of infection.
Scientists believe that both HIV-1 and HIV-2 crossed over at some point from primates into humans. The big scientific questions of the day are when and how did this happen.
For this analysis, the study authors compared genetic sequences from historic and modern-day HIV-2 samples to SIV-SM, a predecessor of HIV-2 that is found in sooty mangabeys, a type of primate.
They then estimated the number of mutations separating the two viruses to try to estimate when the virus made the jump.
According to the study authors, two types of HIV-2 that eventually became epidemic, HIV-2A and HIV-2B, crossed over into humans in about 1940 and 1945, respectively.
The viruses, they say, did not reach epidemic proportions in Guinea-Bissau until much later, probably between 1955 and 1970. The nation's war of independence lasted from 1963 to 1974, leading the authors to hypothesize that the war itself -- perhaps partly because of a jump in the use of dirty needles -- may have pushed HIV-2 to epidemic status.
HIV-2 can be transmitted via needles, but the rate of transmission is probably going to be lower than that of HIV-1, Markovitz says. "It would require more blood," he says.
To Drucker, however, the exact date that SIV made the transition to HIV is less interesting than the fact that there are two different viruses that made the jump and that they come from two different monkeys. HIV-1 originated with chimps and HIV-2 with sooty mangabeys, two animals that live a thousand miles apart and have nothing to do with each other.
"Most people think HIV-2 is a branch of HIV-1, but it's not. It's a different virus," Drucker says. "Why did two of these viruses cross over into humans in a very short period of time?"
Drucker is working on the theory that the introduction of injections into Africa in the 1950s allowed the simian version of the virus to mutate until it was able to infect humans.
"Not just two but maybe five different monkey viruses crossed over into humans in a small period of time. We had hundreds of centuries before it didn't cross over," Drucker says. "That makes a compelling case that happened in Africa in mid century that led to this multiple species crossover." (HIV-1 and HIV-2 are the only strains that attained epidemic proportions. The others were not spread within the human population.)
As Drucker sees it, the timing that really makes sense is the introduction of massive-scale injections (such as those for penicillin) into Africa after World War II and the use of transfusions. "They didn't use clean needles," he says.
While it's plausible that injections also played a role in the spread of HIV-2 in Guinea-Bissau and other parts of West Africa, the dates don't entirely line up, Drucker says. "Everything points to more recent dates," he says.