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AIDS from chimps? Not!
By Reid Furniss
Editor's note: World AIDS Day is Wednesday, Dec. 1. Some U.S. researchers and media are trying to prove that the AIDS epidemic started with contamination of humans by African monkey retroviruses. Their effort fascinates me, because it reveals so much about the inflammatory nature of HIV politics, and the eagerness of some to find easy answers. Thus a recent Reuters story rushed to judgment with this statement: "A chimpanzee named Marilyn has helped confirm that the AIDS virus first passed into people from chimps, researchers said." Recent cross-over from monkeys, if verified, would reinforce the belief that AIDS is a new thing. Religious righters who believe AIDS is punishment of today's sexual sins by a wrathful God would feel vindicated. Those who blame big drug companies for the epidemic point to the possibility of defective polio vaccines used in Central Africa in the 1950s. One journalist fumed about vaccine researchers "accidentally starting AIDS in humans (which they almost certainly did.)" Conspiracy mavens believe that HIV is the Frankenstein child of biological-weapons research. Some gay men believe they were deliberately infected with this new HIV. Another theory is that white racist shadow powers are using HIV to depopulate black Africa and seize its vast lands and resources. True -- some animal diseases do leap to humans. In the livestock business, people knew for a long time that bovine brucellosis can jump to humans, in a form called Malta fever. Trichinosis in pork, psittacosis in birds, encephalitis in horses -- these are other examples. The spooky possibility that transmittable spongiforms from animals (sheep scrapie and mad-cow disease) are now eating away at human brains, in the form of something new called CJD, is being investigated. In overpopulated Africa, malnourished people are so hungry for meat protein that they often eat any kind of animal they can catch. So handling of fresh carcasses, or consumption of undercooked meat, can open the door to inter-species infection. Example: Lassa fever, a lethal viral disease, which leaped to humans from its natural reservoir in wild rats. The rats are hunted and eaten by Nigerian villagers as an essential part of their diet. The monkey theory sounds plausible -- till you start looking at it closely. Yes, Africans may carry a strain of HIV traceable to their native monkeys. But HIV genes are volatile, shooting off mutations like fireworks, making it hard for researchers to see a clear history of world development. The strain that is found in millions of people in Africa only recently showed up in the U.S. So the monkey faction have a shaky case when they try to link African primates with HIV in the U.S. In fact, another science -- population genetics -- suggests that some HIV strains have been around for thousands of years. Long-time history of a disease in a given population leaves indelible genetic footprints, in the form of susceptibilities or immunities that can be inherited as specific alleles, or genes. For example, wildlife workers know that wildcats can contract feline distemper from domestic cats. But wildcats have to be vaccinated with killed-virus vaccine because they have no inherited immunity to distemper. Live-virus vaccine usually kills a wildcat. Whereas domestic cats have mutated some degree of distemper immunity over thousands of years, so it is safe to give them live-virus vaccine. Human gene researchers are now fingering alleles that confer immunity (or susceptibility) to a specific human disease. In an important recent book, The History and Geography of Human Genes, three Italian researchers -- L. Luca Cavalli-Sforza, Paolo Manozzi and Alberto Piazza -- reveal the initial findings of three decades of groundbreaking global DNA research. Blood samples were collected in localities where the local gene pool might have remained relatively uncontaminated by outside contact. These were analyzed to determine frequency levels for 200 key markers, from blood type to the immune factor for malaria and its genetic link to sickle-cell anemia. When a population is challenged by a new disease, its DNA can mutate resistance to that disease. A single woman or man passing an altered gene copy to children is enough to start a mutation on its way. Thus, when Europe was first challenged by an imported Asian epidemic -- bubonic plague -- Europeans had no inherited immunity to plague. But after four centuries of devastating die-offs, from 1300 till 1700, the human genome of Europe responded with a mutant gene that protected its carriers from bubonic plague. Generation by generation, through a slow process called "drift," new genes that favor survival of a disease can spread through a population, at a rate that geneticists can calculate mathematically. Cavalli-Sforza and Bodmer (1971) calculated that a minimum of 2000 years is needed for a new mutation to reach 10 percent in a population of 50,000 people. But some markers -- like those related to malaria -- may reach higher frequencies than that, because they exist in large populations, following trade and immigration routes, for tens of thousands of years. In other words, if a marker has a high frequency level in a given population, the original mutation happened thousands of years ago. A low frequency suggests that the mutation is recent. Geneticists can look at a world map of gene frequencies for an allele, and pinpoint the spot where the original mutation may have occurred. If HIV first infected Europeans in the 1950s, one would scarcely expect to find a high gene frequency for HIV immunity in just three or four generations. So it's intriguing to see that, in 1996, two research teams announced they'd found a high frequency of immunity to HIV in a sampling of West Europeans. According to the Los Angeles Times, Dr. Nathan R. Landau reported that one out of 100 persons in the sample were genetically resistant to HIV infection. Both teams stated that not one in a similar sampling of Africans and Asians had the gene. These discoveries faded quickly from the news -- to be followed by another brief flurry in 1998, over a discovery that this gene may be genetically linked to the immune factor for bubonic plague. Why are politicians, activists and media yawning at these genetic discoveries? Because simple arithmetic shoots down the theory that HIV is a new thing. A high European frequency of HIV immunity means that Europeans have harbored that particular strain of HIV for hundreds of years -- possibly since medieval times, maybe longer. Europeans do not eat monkey meat. So, for Europe at least, there goes the monkey theory. And the conspiracy theory. And the racist theory. And the wrath-of-God theory. There's no doubt that many Africans are dying of something, and that many of them test positive for HIV. Perhaps we are seeing the initial die-off in a part of the world with no previous exposure to a particular new disease, or a particular strain of it, at any rate. 20th-century Africans may be dying of a new viral infection the way Europeans died of bubonic plague in the 14th century. But Africans may have their own "genetic drift" going, and their own new allele which will confer immunity to their own HIV strain. Genetics, and its relationship to epidemic disease, touches into many politically sensitive areas -- from new vaccines to immigration policy. Clearly there are many nagging questions. If European HIV immunity is linked to bubonic-plague exposure, what about Africa, which was never ravaged by plague like Eurasia was? Africans have been eating monkeys for eons, so why did this alleged cross-contamination wait till the 1950s to happen? Bubonic plague has been endemic in Asia for centuries. If Asians have developed immunities to bubonic plague, would they also have genetic protection against HIV? Or does it all boil down to which strain of HIV you've been exposed to? And how useful will an HIV vaccine be if it doesn't nullify all, or most, of the HIV strains found everywhere in the world? Personally, as I look at the emerging evidence, I think it's dangerous to rush to judgment, and assume that "AIDS came from chimps."
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Archived Months:
September
1998 |
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