A genetic analysis of 25-year-old blood samples has outlined a new map of the AIDS virus' journey out of Africa, showing that today's most widespread subtype first emerged in Haiti in the 1960s and arrived in the United States a few years later.
The analysis fills in a gap in the history of the virus, whose migration has been known in only sketchy form from its origin in Africa in the 1930s to its first detection in Los Angeles in 1981.
Dr. Michael Gottlieb, an assistant clinical professor of medicine at UCLA and one of the discoverers of acquired immune deficiency syndrome, said the analysis placed the virus in the United States nearly a decade earlier than previously believed.
"It's pretty clear evidence for Haiti as a steppingstone," he said. "The suggestion that the infection was further below our radar than I'd previously suspected is kind of unnerving."
The analysis, published today in the Proceedings of the National Academy of Sciences, focused on a variety of HIV known as subtype B, the most prevalent form in most countries outside of Africa.
Michael Worobey, an evolutionary biologist at the University of Arizona and senior author of the study, analyzed five blood samples collected in 1982 and 1983 from Haitian AIDS patients in Miami.
The samples were held in frozen storage by the national Centers for Disease Control and Prevention.
Worobey and his colleagues looked at two viral genes and compared their sequences with viruses from around the world.
As a baseline, they used virus samples from Central Africa that are considered some of the earliest forms of the human immunodeficiency virus .
Because viruses constantly mutate, researchers could construct a rough timeline of development by measuring how much the genes in more recent samples had drifted away from their ancestral forms.
The team found that the Haitian samples were genetically the most closely related to the African virus, indicating that they were among the earliest to branch off.
Statistically, researchers found a 99.7% certainty that HIV subtype B originated in Haiti, Worobey said.
Worobey surmised that the virus was brought to Haiti by workers who had gone to the Democratic Republic of Congo, formerly known as Zaire, after it became independent in 1960. The virus appears to have been carried to the United States by Haitian immigrants between 1966 and 1972, according to the mutation timeline.
Researchers have debated for years whether the U.S. epidemic came directly from Africa or through Haiti.
They have also debated whether Americans exported the virus to Haiti via a tourist sex trade that flourished in the late 1970s and early '80s. Worobey said the latest study did "a good job of settling the debate. . . . This shows quite clearly that the data is really only consistent with a Haiti-first origin."
Dr. Beatrice H. Hahn, a virologist at the University of Alabama at Birmingham who was not involved in the study, said: "I think these calculations are as good as the currently available methods allow."
She cautioned against blaming Haitians or Central Africans for spreading the disease: "These viruses are fairly clever and they have to survive. They will find niches. . . . You realize chance events play a very important role."