Although it is well established that HIV has its origins in non-human primates, the series of events that led to one particular strain sparking a global pandemic in humans has been unclear. Now, using sophisticated statistical techniques to analyze hundreds of genetic sequences of HIV, a team of researchers has retraced the virus’ footsteps and painted a detailed picture of how it initially started to creep through the population. Their findings confirm that Kinshasa, the capital of the Democratic Republic of the Congo (DRC), was the epicenter of the HIV pandemic, but for the first time the scientists have also mapped out what happened next. The study has been published in Science.

The closest relatives of HIV are the simian immunodeficiency viruses (SIVs) that naturally infect numerous different non-human primate species. These viruses were actually transmitted to humans at least 13 times, but only one of these events led to the HIV pandemic. That event gave rise to the group that contains the most prevalent strains of HIV—HIV-1 group M—which is responsible for approximately 90% of infections worldwide.

So what separated group M from other groups which contain less successful strains? One idea was that these viruses were better at evading the host's immune system, but this new investigation highlights the role of other crucial factors such as transport and social changes that were previously downplayed.

For the study, Oxford University and University of Leuven scientists analyzed the genomes of viruses from 814 infected individuals living in central Africa between 1959 and the late 1980s. They also included the earliest known HIV-1 infection, known as ZR59, which came from a male living in Kinshasa in 1959.

By comparing the sequences over the time, the scientists were able to determine viral mutation rates and thus the rate of evolution. From this, the researchers were able to extrapolate backward and generate a viral timeline, extending into pre-epidemic times.

Earlier work found that the common ancestor of group M emerged between 1884 and 1924, having jumped from a chimpanzee into a human living in south eastern Cameroon. This was likely from a hunter or butcher coming into contact with infected blood. The virus then probably circulated locally before making its way to Kinshasa around 1920.

From here, the virus began to rapidly spread thanks to the construction of a new railway that not only attracted large numbers of workers to the area but also allowed infected people to reach other major cities in central Africa. By the end of the 1940s, over one million people were travelling through Kinshasa on this railway each year, which helped establish secondary transmission regions.

During this time, another group of viruses, group O, was spreading in Cameroon at similar rates, but after the 1960s group M began to explode. The researchers think this was likely due to DRC achieving independence in 1960, an event that was coupled with huge social and cultural changes. The sex industry flourished and public health campaigns unfortunately led to the use of contaminated needles, allowing HIV to spread like wildfire. While the researchers can’t rule out the possibility that viral differences were key to the success of group M, they think that it’s more likely a case of “right place, right time.” 

[Via Oxford University, Science, New Scientist, National Geographic]