Oct 11, 2010
Hi Doc. What do you think about Tulane University's recent study that suggests SIV has been up to 75,000 years ago? How does data change what we thought we knew about HIV, its lethality, and its presence amongst human populations in the past? Thank you.
| Response from Dr. Frascino
I found the new information about simian immunodeficiency virus (SIV) fascinating. However, it does not alter what we know about any aspect of human immunodeficiency virus (HIV). HIV is a related but quite different virus from SIV. The epidemiology of HIV hasn't changed. The new information suggests humans may have been exposed to SIV for a much longer period of time in Africa prior to the time HIV evolved in humans (1800s to 1959). I'll reprint some information below related to the new research on simian immunodeficiency virus (SIV).
Study Dates HIV Ancestor to at Least 32,000 Years Ago September 28, 2010
New research suggests that the simian immunodeficiency virus (SIV) lineage was already circulating in monkeys and apes at least 32,000 years ago. During the millennia since, humans would have been exposed to it countless times as they butchered monkeys for food. However, only between the 1800s and 1959 did a human infection from a chimpanzee virus spread widely enough to eventually evolve into the present HIV epidemic.
Previously, it had been thought SIV was a much younger virus, perhaps only a few hundred years old. In the current study, researchers examined SIV in monkey species that had developed in isolation on Bioko, a land mass that was separated from West Africa some 10,000 years ago as sea levels rose.
Four species on Bioko had SIV, but each strain was genetically very distinct, suggesting they did not come from a recently imported monkey with SIV in the last few centuries. Each virus also was close to the strain infecting monkeys of the same four genuses on the mainland, suggesting the SIV strains existed before Bioko was cut off.
Given that 10,000-year window, recalculating SIV's "molecular clock" by how fast it mutates places the common ancestor to all SIV strains at between 32,000 and 78,000 years old. That ancestor virus may have existed for millions of years.
If HIV had been in humans before the 20th century, it would have arrived in the Americas via the slave trade, said Dr. Preston A. Marx, a virologist at the Tulane primate center and co-author of the study. The immediate ancestor to HIV came from chimpanzees, and SIV still causes illness and death in chimps, but not quickly, suggesting a relatively recent adaptation.
Marx believes the introduction into Africa of millions of inexpensive, mass-produced syringes in the 1950s allowed a human infection by chimpanzee virus to spread more widely. Campaigns to eradicate yaws, syphilis, malaria, smallpox, and polio required many syringes, and their re-use was often officially approved. The devices also became status symbols in non-medical settings.
However, University of Arizona virologist Michael Worobey, who co-authored the study, and University of Alabama virologist Dr. Beatrice Hahn suspect the growth of colonial cities helped spark the epidemic. Before 1910, no central African city had more than 10,000 people, and syringes were handmade, expensive and rare. Later urban migrations fueled sexual contacts and prostitution. The earliest confirmed HIV infection in a human to date was traced to blood drawn from a man in Kinshasa in 1959.
The full study, "Island Biogeography Reveals the Deep History of SIV," was published in Science (2010;329(5998):1487).
Tulane University researchers find ancient roots for SIV
Published: Thursday, September 16, 2010 - 13:31 in Biology & Nature The HIV-like virus that infects monkeys is thousands of years older than previously thought, according to a new study led by researchers from Tulane University. Simian immunodeficiency virus (SIV), which is the ancestor to the human immunodeficiency virus (HIV), is between 32,000 and 75,000 years old and may even be more than a million years old, according to genetic analysis of unique SIV strains found in monkeys on Bioko, an island off the coast of Africa.
The research, which appears in the Sept. 17 issue of the journal Science, calls into question previous DNA sequencing data that estimated the virus' age at only a few hundred years.
"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," said virologist Preston Marx of the Tulane National Primate Research Center who led the study in conjunction with Michael Worobey, evolutionary biologist at the University of Arizona.
Marx tested his theory that SIV had ancient origins by seeking out DNA samples from monkey populations that had been isolated for thousands of years. His team collected bush meat samples from monkeys on Bioko, a former peninsula that separated from mainland Africa after the Ice Age more than 10,000 years ago.
Researchers found four different strains of SIV that were highly genetically divergent from those found on the mainland. They compared DNA sequences of the viruses with the assumption that they were tracking how both evolved over 10,000 years. The computer modeling showed the rate of mutation to be much slower than previously thought, indicating that virus is between 32,000 and 75,000 years old to have evolved to its current state. These dates set a new minimum age for SIV, although it is likely to be even older, Marx says.
The research has implications for HIV. Simian immunodeficiency virus, unlike HIV, does not cause AIDS in most of its primate hosts. If it took thousands of years for SIV to evolve into a primarily non-lethal state, it would likely take a very long time for HIV to naturally follow the same trajectory, Marx says.
The study also raises a question about the origin of HIV. If humans have been exposed to SIV-infected monkeys for thousands of years, why did the HIV epidemic only begin in the 20th century?
"Something happened in the 20th century to change this relatively benign monkey virus into something that was much more potent and could start the epidemic. We don't know what that flashpoint was, but there had to be one," Marx says.
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