Where it came from

Many species of primates harbor viruses that are closely related to HIV. These viruses, called SIVfor simian immunodeficiency virus, occur in 26 African primate species. Both major groups of HIV — HIV-1 and HIV-2 — arose when the simian virus transferred from primates to humans. Scientists have been able to pinpoint which primate-to-human transfers resulted in both HIV-1 and HIV-2:

^ HIV-1 jumped to humans from chimpanzees, which harbor SIVcpz. In at least three different events, SIVcpz jumped to humans, resulting in the three major groups of HIV-1. Each group represents an independent origin for the virus.

^ HIV-2 jumped to humans from sooty mangabeys, which harbor SIVsm.

At least half a dozen independent origins of HIV-2 from sooty mangabeys exist.

The naming conventions of the simian viruses indicate the species of primate that harbors the particular virus: SIVcpz stands for the chimpanzee simian virus, SIMsm for the sooty-mangabey simian virus, and so on. If your eyes tend to glaze over when you see what appears to be a random string of uppercase and lowercase letters, you may not have figured this convention out already.

When a virus goes from one species to another

Sometimes, a virus in one species transfers to another species. In the case of HIV, a simian virus transferred to a human host, where it mutated into HIV. The phenomenon is fascinating and terrifying. People's fears about the bird flu — that after it's in a human host, it could mutate into a human version that's transmittable from human to human — have in essence already been realized with HIV viruses.

When people had no idea where HIV came from, they assumed a single origin. But when scientists started sequencing a bunch of simian immunodeficiency viruses, they realized that HIV (both 1 and 2) had leaped from primates to humans multiple times. Although scientists don't understand all that's involved in such a transfer, you shouldn't be too surprised. Given that humans are not all that different from primates (refer to Chapter 16), if we're going to catch a virus, getting it from other species of primates is as good a place as any.

The evidence of primate-to-human transfer

How can scientists be so sure that the human immunodeficiency virus originated in simian populations? First, they know that humans can catch primate retroviruses. In at least one case, an animal handler acquired a simian immunodeficiency virus from a rhesus macaque (SIVmac), and cases of acquisition of other retroviruses have occurred.

Also, a strong correlation exists in Africa between the distribution of HIV types and the distribution of primates. The center of HIV-2 infection is the same part of Africa where sooty mangabeys live. And although HIV-1 has spread throughout Africa and the world, the origin of that virus appears to be the region inhabited by the subspecies of chimpanzees that harbors a related virus. These regions also provide ample opportunity for human-primate contact. Both chimps and sooty mangabeys are hunted for food, and young ones are occasionally kept as pets.

Other evidence includes

^ Gene sequencing: The HIV and SIV viruses have been sequenced and compared, and the human viruses are remarkably similar to the simian virus that's considered to be its parent.

^ An HIV phylogenetic tree: In reconstructing the tree (refer to Chapter 9), researchers discovered that instead of the HIV-1 strains appearing off to one side on their own little branch (which would indicate a single origin from one chimpanzee virus), the HIV strains are interspersed with the SIVcpz strains, indicating that HIV originated from three different simian strains.

The same is true for HIV-2. Phylogenetic analysis doesn't show all the HIV-2 strains on their own branch of the viral tree, but instead several branches pop out of the sooty mangabeys viral tree.

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