The Canadian flight attendant widely blamed for bringing HIV to the United States and triggering an epidemic that has killed nearly 700,000 people has been exonerated by science, more than 30 years after his death.
Instead, the researchers report that Dugas was one of thousands of people who were infected with the human immunodeficiency virus by the late 1970s, years before it was officially recognized by the medical community in 1981.
The genetic analysis also reveals the path taken by the most common strain of the virus after it traveled from the Caribbean to the United States. Upon arriving in New York City around 1970, it circulated and diversified for about five years before being dispersed across the country.
"There really is no question about the geographical direction of movement," said study leader Michael Worobey, an evolutionary biologist at the University of Arizona in Tucson.
The new evidence comes from two caches of serum samples taken from gay men in New York City in 1978 and 1979, and in San Francisco in 1978. (Serum is blood with the red and white blood cells removed.)
The men were participating in a study of hepatitis B, which was prevalent in the gay community at the time. They weren't tested for HIV because it wasn't known to exist back then. However, if any of them had it, there should be evidence in their blood in the form of antibodies, the proteins that our immune systems make to fight an infection.
To see if any of the men in the hepatitis B study had contracted HIV, a different group of researchers analyzed the New York City samples and found that, indeed, 6.6% of them contained HIV antibodies. Inspired by this work, Worobey and his team did a similar test for the West Coast samples and discovered HIV antibodies in 3.7% of them.
The presence of antibodies can show whether a person has contracted a certain virus, but they can't provide much detailed information about the virus itself, Worobey said. So his group's next step was to scour the serum samples for fragments of HIV RNA that may have been circulating in these men's bloodstreams.
It was a bit of a long shot. The samples were decades old, and RNA is generally too fragile to survive for long periods.
But the researchers were persistent. They selected 53 samples for attempted genetic sequencing, and they were eventually able to cobble together the full HIV genomes of eight of them — three from San Francisco and five from New York.
"It took a mixture of patience and insanity, but these old sequences are as good as a time machine," Worobey said.
Scientists track the evolution of a virus over time by tallying the number of substitutions, or mutations, in its genome.
To explain how this works, Worobey likes to conjure a cactus. (He lives in Arizona after all.)
Imagine you are staring at a saguaro with a tall central column and a couple of branches curving up like arms. If you have just a single time point — now — it's difficult to gauge the cactus' age.
However, if you could get a picture of the cactus from 10 years ago, you could calculate the growth rate for each of its branches over that time. Then you can make an educated guess about how much the cactus grows each year.
"That's what we do with HIV molecular clocks," Worobey said.
Geneticists measure how many mutations occur in the genome in a given time period. This is how they determined that HIV was first transmitted from a chimpanzee to a single human early in the 20th century in sub-Saharan Africa.
The HIV molecular clock also allows researchers to trace landmark moments in the evolution of the virus. After the initial transmission from chimp to human, the next one came when HIV began circulating in Kinshasa, the capital of the Democratic Republic of Congo. By 1960, viral samples from the city were tremendously diverse, suggesting HIV had been present in the area for a long time, Worobey said.
Around 1967, a branch of the virus known as HIV-1 group M subtype B took hold in the Caribbean and diversified in Haiti.
The new work makes clear that this strain first jumped from the Caribbean to New York City in 1970 or 1971.
By the late 1970s, when the eight serum samples were collected, the five viral genomes from New York already had a high degree of genetic diversity; the three from San Francisco did not. This indicates HIV probably was in New York much longer than it was in San Francisco.
"It appears that New York City was a key turning point," Worobey said. "New York City acts as a hub from which the virus moved to the West Coast — and eventually to Western Europe, Australia, Japan, South America and other places."
Dr. Beatrice Hahn, a professor of medicine at the University of Pennsylvania who studies the origin and evolution of immunodeficiency diseases, said she was impressed by the lengths the authors went to in order to find actual virus samples from so long ago.
Hahn, who was not involved in the research, noted that while many of the findings had been put forward before, the new study provides more convincing evidence because it was the first to isolate the complete, full-length virus genome from such early samples.
"It always helps to nail it," she said.
She added that the work does provide some new insights, including that the virus came to New York much earlier than previously recognized and that thousands of people were already infected by the time HIV was first recognized in the medical literature.
As part of this study, the authors assembled the complete HIV genome from a serum sample collected from Dugas in 1983, just one year before his death at the age of 31.
The genome was typical of HIV strains in the United States at the time, the researchers reported, and there was nothing to indicate that it had set off the epidemic here.
"In short, we found no evidence that Patient Zero was the first person infected by this lineage of HIV-1," they wrote.
So, how did this myth start?
It turns out Dugas was initially identified by the Centers for Disease Control and Prevention as case No. 057, which means he was likely the 57th patient they heard about who had been sickened by the mysterious illness.
A few years later, he participated in a study by the CDC in California that connected 40 men in 10 American cities who all had AIDS and were linked through sexual contact.
Dugas was placed near the center of this cluster, and identified as patient O for "Outside of California." But when investigators numbered the cases according to the date when each patient's symptoms started to appear, the letter O was mistaken for the number 0, and the Canadian flight attendant entered the literature with that dubious label.
In actuality, Dugas' symptoms were not reported until 1979 and 1980, considerably later than other cases in the cluster.
The medical community never released Dugas' name to the public, but journalist Randy Shilts identified him in his 1987 book "And the Band Played On" about the first five years of the American AIDS epidemic. (Shilts himself died of AIDS in 1994.)
Dugas told researchers that he averaged 250 sexual partners a year, and Shilts portrayed him as a vain and arrogant playboy who ignored doctors' orders to stop having sex to keep his infection from spreading. Media reviewers took the idea of a villainous Patient Zero and ran with it.
"Despite attempts at clarification and protest then and since, many still believe the story today," the study authors wrote.
While there are epidemiological benefits to knowing how an outbreak begins, the researchers say that doesn't mean anyone is at fault.
"Nobody should be blamed for the spread of a virus that nobody even knew about," Worobey said.
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5:10 p.m.: This article was updated with information about the death of Randy Shilts, the journalist who identified Gaetan Dugas as "Patient Zero."