HIV’s Primate Precursor Is Very Old. Why Did It Jump To Humans So Recently?
HIV became an epidemic in the human population just in the 20th century. Its precursor found in primates, called simian immunodeficiency virus or SIV, could be not just hundreds of years old, but tens of thousands of years old, according to a study out in Science.
Preston Marx and colleagues studied the monkeys of Bioko, an island off West Africa that has been cut off from the mainland for 10,000 years. By studying the way SIV evolved in that isolated population, the team calculated that the virus is at least 32,000 years old, and possibly much, much older. Says Marx:
“The biology and geography of SIV is such that it goes from the Atlantic Ocean to the Indian Ocean all the way to the tip of Africa. … It would take many, many thousands of years to spread that far and couldn’t have happened in a couple of hundred years.” [AFP]
This brings back the big question: If SIV has been circulating in Africa’s primates for that long, (and humans have been butchering primates all along), then what was so different about the mid-20th century that allowed it to hop into humans and spread through the population as lethal HIV?
We know that drug users’ reuse of needles speeds the spread of HIV. Are needles to blame for SIV’s jump to humans?
Dr. Marx believes that the crucial event was the introduction into Africa of millions of inexpensive, mass-produced syringes in the 1950s. Campaigns to wipe out yaws, syphilis, malaria, smallpox and polio required syringes, and many were reused, often with official approval. Traditional healers adopted them for injecting their decoctions [infusions], and they became status symbols; a study in Uganda in the 1960s found that 80 percent of families owned one. [The New York Times]
Not so fast, says virologist Michael Worobey. For him, the key factor is the incredible growth of cities in the last century.
Before 1910—around when Worobey believes the virus emerged—no town in Central Africa had more than 10,000 people. That has changed incredibly over the past century as Africans have moved from the countryside into cities—Lagos, the capital of Nigeria, has a metro population of more than 15 million, and it’s growing ten times faster than New York. [TIME]
It’s possible that before that rampant urbanization, human hunters exposed to primate blood contracted the virus. However, without the interconnectedness of modern society, they probably died before they could spread the virus too far.
Either way, Worobey says, the age of SIV isn’t good news. It probably required a long time to evolve into a less virulent form, which means that the same may be true of HIV.
Image: Wikimedia Commons