The origin of the Aids pandemic has been traced to the 1920s in the city of Kinshasa, in what is now the Democratic Republic of Congo, scientists say.
Assisted by rail transport and the sex trade, the virus that causes Aids then spread across the continent and eventually the world, infecting some 75 million people and killing 36m of them.
An international team of researchers reconstructed the genetic history of the HIV-1 group “M” pandemic, and found the common ancestor of group M is “highly likely” to have emerged in Kinshasa around 1920.
While various strains of human immunodeficiency virus (HIV) have jumped from primates and apes to humans at least 13 times, only one such transmission event has led to a human pandemic.
HIV is a mutated version of a chimpanzee virus which probably made the species-jump through contact with infected blood while handling bush meat.
The virus made the jump on multiple occasions. One event led to HIV-1 subgroup O which affects tens of thousands in Cameroon. Yet only one cross-species jump, HIV-1 subgroup M, went on to infect millions of people across every country in the world.
Experts say the human pandemic emerged because it was aided by “a perfect storm of factors, including urban growth, strong railway links during Belgian colonial rule, and changes to the sex trade, [that] combined to see HIV emerge from Kinshasa and spread across the globe” between the 1920s and 1950s.
A key factor was the use of trains as transport, which helped bring the virus from isolated pockets of people into the larger city, Kinshasa, which was among the best connected of all central African cities, says the study in the journal Science.
“Data from colonial archives tells us that by the end of the 1940s, over one million people were travelling through Kinshasa on the railways each year,” said Nuno Faria of Oxford University’s Department of Zoology, first author of the paper.
Looking at genetic data, scientists could see that HIV spread across what is now the Democratic Republic of the Congo (formerly the Belgian Congo and Zaire) to other major cities, by the early 1950s.
These regional hubs were connected to southern and eastern African countries.
Social changes, including sex workers who took on a large number of clients, helped the spread in the 60s, while “public health initiatives against other diseases that led to the unsafe use of needles may have contributed to turning HIV into a full-blown epidemic,” the study said.
That was in reference to campaigns to treat sexually transmitted diseases which may have used dirty needles.
HIV was first identified in 1981, and ballooned until anti-retroviral drugs were created.
Professor Jonathan Ball, of Nottingham University, said: “Perhaps the most contentious suggestion is the spread had more to do with conditions being right than with these viruses being better adapted for transmission and growth.”