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December 5, 2012

How animals spread diseases to us

IOWA CITY — "Everybody knows that pestilences have a way of recurring in the world; yet somehow we find it hard to believe in ones that crash down on our heads from a blue sky." — Albert Camus, "The Plague"

In May, 1997, a 3-year-old boy was admitted to an ICU in Hong Kong after suffering from influenza for a week. Two days later, the boy died of pneumonia. His case would have been merely a curiosity if it weren't for 17 more patients who came down with the illness months later. In all, six people died from a strain of influenza that had never been seen in humans before, dubbed H5N1.

However, H5N1 wasn't really "new." It had caused outbreaks in Scottish chickens in 1959 and British turkeys in 1991. It had killed geese in Guangdong, China, in 1996. But these bird outbreaks weren't considered important by physicians or researchers on human disease — this was an avian strain of influenza, and it was thought that humans had little to be concerned about.

That changed abruptly in 1997, when the human cases led to the destruction of 1.3 million chickens in Hong Kong to stop the outbreak. That strategy seemed to work in the short term, but H5N1 has since surfaced in more than two dozen countries and caused more than 600 human infections since 1997 — almost half of them fatal.

While scientists were closely following the movements of H5N1, another influenza virus — H1N1 — snuck up on us in 2009 and spread around the world in a matter of weeks. This was the first influenza pandemic of the 21st century, and like H5N1, it moved to humans from animals — in this case, from pigs.

When new infectious diseases are discovered, one of the first questions is "where did this come from?" More often than not, the answer is one of our animal friends — a kind of disease called a zoonosis. Studies have shown that about 75 percent of emerging infectious diseases (diseases that are newly discovered, are increasing in frequency or have moved into a new geographic area) are of animal origin, as are 60 percent of all known pathogens. Even diseases that have spread freely in the human population, such as tuberculosis, HIV, measles and smallpox, have their roots in infections carried by animals.

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