for National Geographic News
Scientists now say that the simian immunodeficiency virus (SIV) in chimpanzees (Pan troglodytes), which is believed to have been transmitted to humans to become HIV-1the virus that causes AIDSdidn't start its life in chimps.
Instead, it was a product of separate viruses jumping from different monkey species into chimps, where they recombined to form a hybrid virus, according to a new study.
Researchers believe the chimpanzee virus is a hybrid of the SIVs naturally infecting two different monkeys, the red-capped mangabey (Cercocebus torquatus) and the greater spot-nosed monkey (Cercopithecus nictitans). Chimps eat monkeys, which is likely how they acquired the monkey viruses. The hybrid virus then spread through the chimpanzee species, and was later transmitted to humans to become HIV-1.
The study suggests striking parallels between SIV infection of chimps and HIV infection of humans. Just as chimps acquired viruses from two different sources, humans are infected by two distinct AIDS viruses: HIV-1 and the less virulent HIV-2, which humans acquired from sooty mangabey monkeys.
"Because of the similarity between chimpanzees and humans, any virus that successfully adapts to spreading among chimps would be a candidate for a further jump to humansa potential HIV-3," said Paul Sharp of the Institute of Genetics at University of Nottingham in England, who led the study.
The discovery also spotlights how many of our most virulent viruses were transmitted across species. The jumping of viruses from animals to humans occurs all the time. Diseases like SARS and monkeypox, which recently hit the northern United States, likely originated by cross-species transmissions.
"It seems this is happening more frequently because of two reasons," said Michael Lai, a virus expert at the University of Southern California, Los Angeles. "First, the detection of the virus is easier today because of advances in medical science. Second, international travel brings animals from far-away places to a new place."
From SIV to HIV
In 1999, the same team of scientists identified the origin of HIV-1 as being the transmission of a virus known as SIVcpz from chimpanzees to humans, but they didn't know how chimps acquired the virus in the first place.
SIVs are carried by many species of monkeys in Africa, but chimps are the only apes known to be naturally infected. While monkeys have been infected with SIVs for a long time, chimps apparently acquired the virus more recently.
The study shows that the SIVcpz strain arose in chimps through repeated transmission and recombination of SIVs from the red-capped mangabeys and greater spot-nosed monkeys. Chimps prey on both of these species and their ranges overlap in West and central Africa.
It is now widely accepted that humans contracted HIV from chimpanzees, probably by butchering them for bush meat. The new findings thus show that humans are not the only primate species to acquire two different immunodeficiency viruses by cross-species transmission.
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