Worobey and his colleagues looked at subtype B. Though it is found mainly in North America and Europe, the strain is present in the most number of countries.
The researchers analyzed tissue samples from five Haitian AIDS patients collected in 1982 and 1983. All five had then recently immigrated to the U.S. and were among the first recognized victims of AIDS.
A family tree constructed from the HIV-1 genes of the five Haitians and subtype B gene sequences from 19 other countries place the Haitian virus at the root of all branches.
"This is strong evidence that HIV-1 subtype B arrived and began spreading in Haiti before it did elsewhere," Worobey said.
It is generally thought that the virus arrived with Haitian professionals returning from Zaire (now the Democratic Republic of the Congo) following a wave of nationalism there in the 1960s.
Using advanced statistical techniques, Worobey and his colleagues estimated that the subtype B strain reached Haiti sometime around 1966 and the United States around 1969.
"Until AIDS was initially recognized in 1981, the virus was cryptically [hiddenly] circulating in a sophisticated medical environment for the better part of 12 years," Worobey said.
Beatrice Hahn is a microbiologist at the University of Alabama in Birmingham who was not involved with the study.
"The paper is a nice piece of evolutionary sleuthing,'' she said. "It shows how chance events can shape a major epidemic and that one virus introduced under the right circumstances can create major havoc."
"The findings are significant," added Robert Garry, a microbiologist at Tulane University. They indicate "an important lineage of subtype B HIV was present in Haiti, which eventually spread elsewhere," he added.
But he is not fully convinced that a Haitian origin is the only explanation for subtype-B strains in the Americas, however.
It is quite likely that other B lineages appeared in the Americas prior to and in all likelihood independently of the Haitian lineage, he said.
"It is possible that HIV made many incursions into the United States. Most of these likely never spread or spread cryptically for a while and burned out," he added. "The one discussed in this paper appears to have been the bomb that actually went off."
Study leader Worobey, a former forest firefighter, likes to use a wildfire analogy.
"It is like a forest fire, it often produces sparks that fly out in front of a fire. Some of those sparks ... die out. But every once in a while one of those sparks ... can start a new wildfire. And that is what we are seeing in this case."
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