Where did HIV come from?

This week, we’re marking 40 years since the identification of a virus responsible for an estimated 40 million deaths: HIV. Will Tingle has been looking back at the timeline…

Will - June 5th, 1981 - the CDC publishes a paper reporting a rare form of lung infection in five young, previously healthy gay men in Los Angeles. And for some reason, their immune system is no longer working properly either. All five will die. The same day, a dermatologist calls the CDC to report a cluster of cases of a rare and unusually aggressive cancer among gay men in New York and California.

Anthony Fauci - We were calling it 'gay related immunodeficiency' or 'the gay cancer'. Because of Kaposi's sarcoma.

Will - The mysterious disease spreads throughout the gay community. The fact that this disease destroys people's immune system leads to it being named 'acquired immune deficiency syndrome' or AIDS, but the underlying cause is still unclear.

TV Interview - I had noticed these bumps on my skin and they looked a little suspicious. Uh, they didn't hurt, but they were very strange and they were multiplying.

Will - By the end of 1982, the number of confirmed deaths in the US reaches over 1000 and it is spreading fast.

TV Interview - Well, I think it's definitely transmissible, uh, just how, I don't know. Is it through blood? Uh, is it through saliva? Is it through... I don't know.

News Report - Paris today. Shocking figures about a deadly disease. 90% of those infected don't realise they've got it.

Will - May 20th, 1983. Finally, a breakthrough. Dr. Françoise Barré-Sinoussi and her colleagues at the Pasteur Institute report the discovery of a retrovirus that could be the cause of AIDS. That retrovirus is HIV. Her theory is later proved right, and she will win a Nobel Prize for it in 2008. So we've had the name for HIV for the past 40 years, but the virus itself has existed far longer than that.

Chris - Indeed, so where did this virus originate? And how? Paul Sharp, geneticist at the University of Edinburgh has spent much of his career following this detective story. The clues point to a jump of a related virus called SIV carried by chimpanzees into humans, most probably in the environs of Kinshasa, the capital of the Democratic Republic of the Congo. But when did this happen, and how do we know?

Paul - It's probably at least a hundred years ago. What we think is that one virus initially emerged in humans. And from that, as it spread through the human population, it has accumulated mutations in its genome and those changes occur at a fairly steady rate. So given that we now understand how fast those changes occur, we can look at the difference between two strains and ask when their common ancestry existed. And so we would estimate that they started to diverge about a hundred years ago.

Chris - And geographically, where was all this going on?

Paul - Well, we can see today that the greatest diversity of strains of these viruses is in the area around Kinshasa. And that suggests that that's where the virus really first started to diversify into these different forms. But we also can trace where the virus likely jumped first into humans because we have looked at samples from other primates that have a virus very closely related to HIV1.

Chris - Ah, so your argument is that some other animal, probably a primate, had a virus very similar to HIV, which got into humans and became HIV, sort of similar to how a bat virus that's a coronavirus can jump the species barrier and cause COVID-19 in us.

Paul - That's right. So when HIV1 was first discovered and described, the only virus that was known that was closely related to that to human virus was one which is found in sheep. But very quickly, people started to discover more closely related viruses in non-human primates, in monkeys and in apes in Africa. And so by about 1990, we knew of several different monkey species, which had these viruses, the viruses in the non-human primates, called SIV for simian immunodeficiency virus. The virus in non-human primates that was most closely related to the human virus was the one found in chimpanzees.

Chris - So how did it get from the chimps into us?

Paul - We can't know because this is probably something that happened more than a hundred years ago. So we can only speculate about what is likely to have happened. We know that HIV can be transmitted amongst humans through blood, and we know that humans in West Central Africa where we think this happened, hunt chimpanzees. And so when you hunt a chimpanzee, you are likely to do some butchery immediately that you've killed in order to carry it away more easily. And at that point, there would be blood with the chimpanzee virus in it, which could easily spread to an open wound on the hunter. And so we think that it's most likely through the butchery of chimpanzees that this virus spread into humans.

Chris - Presumably these viruses have existed in the primate species for hundreds to thousands of years and people have been hunting them for hundreds to thousands of years. So why did this happen when it did?

Paul - What we think changed was humans. In the sense that in this part of West Central Africa where we think the transmission from a chimpanzee to a human first occurred, until about a hundred years ago there were no really big settlements. There were no cities as such in West Central Africa. And it's only a little over a hundred years ago that people started to gather together in large congregations in large cities. And what we suspect is that the virus which would've initially been transmitted to an individual in a rural area would've needed to get to a large city, a large collection of people, for it to actually really take off in initially an epidemic, which became the pandemic.

Chris - Can I draw you then on where specifically you think all this was going on?

Paul - Chimpanzees, the common chimpanzee, actually is divided into four different subspecies, and only two of those subspecies are infected with the virus that is the ancestor of HIV. So chimpanzee viruses from one specific area, which is in the very southeast corner of Cameroon. These were the viruses that were specifically close to HIV one. And it indicated to us that this was the part of the world where the jump to a human first taking place. That's a very isolated rural area. I mentioned that the virus probably had to spread to a larger city in order for it to really get going. So what we can see when we look at the map is that the part of Cameroon where the chimpanzee viruses are, is surrounded by rivers which all flow south and join into the Congo, which then of course flows past Kinshasa. So the virus has made its way through one more series of infected individuals to Kinshasa. There it has started to really spread among people, and then it has spread out from there. And what we see then is, in the genetic makeup of the virus, we can see different strains of the virus that have spread to different parts of Africa. And then having spread across Africa, they then spread out across the world. One of my colleagues has shown that the most likely route out of Africa initially was through the Caribbean, through Haiti and then from Haiti into the United States, from the United States over to Europe.

Chris - That's quite a journey, isn't it? So that there was a huge sort of iceberg here. This was drifting on for maybe a century, nearly a century before we realised that it was established in all these different populations, in all these different geographies.

Paul - I'm often asked how could it be possible that a virus as deadly as HIV could be present in the human population for 50 or more years without us knowing about it. And the answer is, I think, fairly straightforward. If you look at the idea of, say, an exponential spread of a virus from one or a few people being infected a little over a hundred years ago to the many millions that have been infected now. If you chart the, the rate of increase in numbers are infected individuals, you would speculate that only a few thousand people had been infected by about 1960. And those people would all be in Central Africa and they would be, when they got ill from AIDS, they would be suffering from a wide variety of different disorders. Because what the AIDS virus does obviously is destroy your immune system and make you susceptible to other infections, other bacterial or fungal infections. And so it doesn't surprise me at all that this virus could go under the radar for such a long period of time.