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SARS _ Swine Flu - human environment interactions - evolution.berkeley.edu

SARS _ Swine Flu - human environment interactions - evolution.berkeley.edu

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evolution.berkeley.edu Tracking SARS back to its source January 2006 The previously unknown SARS virus generated widespread panic in 2002 and 2003 when the airborne germ caused 774 deaths and more than 8000 cases of illness. But where did this mystery virus come from? Scientists immediately suspected that it had jumped to humans from some other organism. In May of 2003, attention focused in on cat-like mammals called civets. Infected civets were discovered at a live animal market in southern China (where they are occasionally eaten). However, since further searches failed to turn up more tainted civets, scientists concluded that they were not the original source of SARS and continued their quest. Then in the fall of 2005, two teams of researchers independently discovered large reservoirs of a SARS-like virus in Chinese horseshoe bats. The bats now appear to be both culprit and victim in this mystery: they are the carriers of the SARS virus, but the virus is probably only passed to humans through intermediate hosts when bats are captured and brought to market. A civet A horseshoe bat Where's the evolution? How exactly did biologists conclude that bats, and not civets, were the original source of the SARS virus? Figuring out the answer required reconstructing the evolutionary history of the virus. Viruses evolve rapidly and constantly, changing within a lineage and splitting off to form new lineages. As they evolve, they accumulate small changes in the sequences of their genomes . Based on these genetic differences, biologists can reconstruct the evolutionary relationships of different viral strains, building an evolutionary tree that reveals which strains evolved from which strains and in what order they evolved. In this case, biologists collected samples of the SARS virus's genetic material, RNA , from different sources: infected humans, infected civets, and different species of infected horseshoe bat. The RNA was then copied, sequenced, and used to build a phylogeny , or evolutionary tree. http://evolution.berkeley.edu/evolibrary/print/printable_template.php?news_id=060101_batsars (1 of 3) [8/27/2010 11:10:28 AM]
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evolution.berkeley.edu The tree showed that civet and human SARS viruses are very similar to each other and, most importantly, that both are nested within a clade of bat viruses so the ancestor of the civet and human strains seems to have been a bat virus! Based on this evidence, biologists have come up with a plausible path of transmission: infected bats and uninfected civets came into contact at a market, the virus was transmitted to civets and then multiplied and evolved in civets (or other animals) in the public market, until eventually the virus hopped to humans. Interestingly, viruses seem to frequently make the jump from bats to human hosts. Bats appear to be the natural reservoirs for many human viruses, including the Ebola, Hendra, and Nipah viruses and now we can add SARS to that list. But what is it about bats that makes them such a common source of viruses? Well, biologists aren't sure, but it might have something
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