Word: flu
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Pity the poor pig. The otherwise estimable mammal has never had a very good rep - something about the mud, the snout, the oink. Now add the flu...
...swine flu outbreak that has sparked widespread fear - so much so that Egypt has ordered the slaughter of the country's 300,000 pigs, even though no cases have been reported there - is easy to pin on the eponymous animal from which it emerged, but the fact is, the current epidemic is little more than an accident of evolution. If pigs are to blame, so too are birds and humans. (See pictures of thermal scanners hunting for swine flu...
...more than 10 proteins assembled into a genome that's simple even by microbiological standards, but that bare-bones genome is unusually flexible, with snap-in, snap-out gene segments that allow easy mutation and exchange of information with other viruses. That's the reason we need a new flu vaccine every year: by the time one flu season has ended and the next one begins, the virus has changed so much, it can simply shake off last year's shot. Compare that with, say, polio; the vaccine was perfected in 1955 and hasn't had to change much since...
What keeps the flu relatively in check is that there simply aren't that many species that are susceptible to it - with humans, pigs and certain kinds of birds leading the list. "There are surface markers on the cells of some species that bind with sites on the flu virus," says Dr. Peter Daszak, an emerging-disease ecologist and president of the Wildlife Trust. "The influenza virus evolved along with pigs, and it did the same with a few other mammals and with birds." (Read "To Travel or Not to Travel? A Swine Flu Dilemma...
Birds are the natural reservoirs of the common flu strains that strike in winter - and those strains reassort themselves to hit humans particularly hard. But while humans are not susceptible to every strain of avian flu, pigs definitely are. When bird flu viruses replicate in pigs, they pick up the viral machinery that gives more selective flu strains the power to spread to other mammals, like us. That's what makes pigs such potent mixing bowls for flu. The roundabout bird-pig-human route may be less common than the straight bird-human jump, but it may be more problematic...