Friday, October 28, 2016

The Origins of AIDS


This is a draft of the first part of an abandoned two-part book review I wrote a while back about three books about the origins and spread of the HIV virus. They were Tinderbox: How the West Sparked the AIDS Epidemic and How the World Can Finally Overcome It, by Craig Timberg and Daniel Halperin; The Origins of AIDS, by Jacques Pepin; and The Chimp and the River: How AIDS Emerged from an African Forest, by David Quammen.

Early in the 20th century, in the southeastern corner of Cameroon in central Africa, a hunter killed and butchered a chimpanzee for meat, nicking himself in the process. That event, as trivial as a papercut, set in motion one of the greatest catastrophes in human history: The global AIDS pandemic, which has killed 30 million people to date and continues to infect two million more each year. (The only disasters that exceed it in scale are the Black Death of the 14th century, the 1918 Spanish flu, and World War II.) At each step in the spread of the disease entirely contingent circumstances came into play, many the result of well-intentioned humanitarian initiatives, that amplified the pandemic when it might have burned itself out. These books tell two stories: One is the story of the origin and global spread of the Human Immunodeficiency Virus (HIV); the second is the story of the dedication, ingenuity and sheer good luck of the scientists who figured it all out.
AIDS, like the flu or smallpox, is zoonotic, that is, it passes from animals to humans. Variants of the Simian Immunodeficiency Virus (SIV), from which HIV derives, have circulated for millennia among the primates inhabiting the tropical rainforests of west and central Africa. In fact, on at least at least eight separate occasions in the 20th century a far less transmissible strain of the virus, the HIV-2, passed to humans from a species of monkey called the sooty mangabey. The mangabey, memorably described by science writer David Quammen as looking like an “elderly chimney sweep of dapper tonsorial habits,” inhabits the canopies of coastal west Africa, some two thousand kilometers from where the vastly more dangerous strain of the virus, HIV-1, emerged.
HIV-1 has also infected humans on multiple occasions—at least four that we know of, three times via chimpanzee and once via gorilla. By far the most lethal variant is known as HIV-1 Group M--with “M” as in main. This is the strain responsible for 99 percent of AIDS deaths worldwide, and because its simian counterpart is concentrated among the chimpanzees of southeastern Cameroon, scientists believe it almost certainly originated there. Figuring that out took intrepid work. Chimpanzees are an endangered species: Killing or even tranquilizing them to secure blood samples is unacceptable. Scientists had to develop new ways of testing urine and faeces to determine if they contained SIV antibodies. Early on, a distinguished AIDS researcher wounded himself in the forest chasing down the animals’ excrement and died of malaria after he was medivac’ed to England.  (Incidentally, it was once thought that chimpanzees had evolved a tolerance for the virus, as the mangabeys have. In fact, research at Jane Goodall’s Gombe ape preserve would later show they sicken and die from the virus as humans do.)
HIV-1M in turn has spawned an array of subtypes that have become endemic to different regions of the world. That there are so many global variants of HIV is a function of the speed with which the virus reproduces and the frequency with which it errs in doing so. The virus evolves a million times faster than animals, meaning that in a mere decade it recapitulates the ten million years since humans, chimpanzees and gorillas last had a common ancestor.