Though we usually think of the HIV and AIDS pandemic as starting in the 1980s, scientists have known for a long time that the virus is much older than that. Exactly where it started and how it managed to sweep across the world, however, has not been well understood.
Now, an international group of researchers believe they have pinpointed not just where and when the virus emerged in people, but the factors that helped it become a worldwide phenomenon that has infected 75 million people to date.
HIV is known to be a mutated form of a virus found in chimpanzees, called simian immunodeficiency virus (SIV). It is thought that the virus made the jump to humans via blood-to-blood transmission when humans were butchering bush meat.
The virus, however, is known to have done this on multiple occasions. In fact, the researchers behind the current study say that SIV made the jump to humans at least 13 times. For example, HIV-1 group O is known to affect tens of thousands of people in Cameroon. But only one of the cross-species jumps, HIV-1 group M, spread across the world.
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To figure out why that is, the researchers traced the genetic history of HIV-1 group M by analyzing all available samples of HIV-1 that had been collected between 1985 and 2010 using “the latest phyleographic techniques, which enable us to statistically estimate where a virus comes from.” Phyleography combines genealogy, geography, and history to study how the geographic distribution of individuals came about.
These methods are more accurate than previous attempts to find how the virus spread, according to the researchers: “Until now most studies have taken a piecemeal approach to HIV’s genetic history, looking at particular HIV genomes in particular locations.”
The researchers, using their broader approach, determined that HIV-1 group M emerged in or around 1920 in Kinsasha, a city in what is now in the Democratic Republic of the Congo. It’s worth noting that this is not far off in time or geography from previous estimates that suggested the first infection likely occurred in the first half of the 20th century in the southeast part of Cameroon.
What sets this study apart in many ways is its focus on how the virus spread so far from its roots.
Previous theories have suggested that the genetic makeup of HIV-1 group M made it stronger than other versions of the virus and allowed it to spread, but these researchers instead point to urban development and human behavior.
In the early part of the 20th century, Kinshasa was called Leopoldville and was part of the Belgian Congo. It had a thriving railway, which by the end of the 1940s transported more than a million people in and out of the city each year. The early spread of the disease has been traced along these railway lines.
Along with being one of the most accessible cities in central Africa, the availability of work in the city attracted large numbers of male laborers from surrounding areas and eventually meant that there were two men in the city for every one woman. This imbalance led to a roaring sex trade and a documented early history of sexually transmitted disease.
The researchers suggest that not only did the sex trade spread the virus, but the public health response to it—which probably included reusing needles for multiple patients—likely contributed to the spread.
They add that other social changes in the 1960s, around the time the country achieved its independence from Belgium, helped the virus “break out” from small groups of infected people into the wider population. Among those infected, for example, were immigrant workers from Haiti who then carried their infection back to their home country, which is one of the places that the modern AIDS pandemic was first recognized.
The researchers refer to this as a “perfect storm” and note that there was only a small period of time in which these factors persisted.
By the mid-1960s, for example, the railway system was not nearly as active as it had once been. Dr. Oliver Prybus, the lead author on this study, explained, “Our research suggests that following the original animal-to-human transmission of the virus, probably through the hunting or handling of bush meat, there was only a small window during the Belgian colonial era for this particular strain of HIV to emerge and spread into a pandemic.”