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Sunday, July 13, 2014
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Published: Sunday, 4/13/2003

Pigs and birds are part of China's SARS cycle

What's cookin' in southern China? Forget spring rolls, sweet and sour pork, and other Cantonese dishes in a region famous for cuisine.

Infectious disease experts recognize southern China as the mixing bowl for new influenza viruses. In the 20th century, they killed more people than all the wars combined.

Severe acute respiratory syndrome (SARS), the mysterious form of pneumonia now causing world-wide concern, is the newest virus with apparent roots in the area. World Health Organization (WHO) officials believe that SARS originated last November in Guangdong province, southernmost on China's mainland.

Most new influenza viruses do the same with less fanfare, and quietly kill 20,000 to 30,000 Americans in a typical year. These viruses have a slightly different genetic bar code than their predecessors. It gives them an edge in eluding the immune system's defenses, and they make people very sick.

These annual shifts in the flu virus's genetics are the reason why high-risk people need a new influenza shot each year. Drug companies usually must write a new vaccine recipe each year to match genetic changes in the flu virus.

And they underlie global flu epidemics, like the 1918-19 pandemic that hit 200 million people and killed 20 million, including 500,000 in the United States.

What makes southern China the epicenter for flu epidemics?

Close contact between pigs, birds, and people, which occurs all around the region.

Millions of small-scale farmers raise ducks, chickens, and pigs on the same land. In some places, ducks and pigs occupy the first floor of houses, with the people living above. Live birds and pigs are sold direct to consumers in tiny village markets and sprawling urban bazaars.

Ducks are the reservoir for avian, or bird, flu viruses. Their virus-laden droppings go onto the soil and into farm ponds. Pigs snort in the soil for food and wallow in the mud, inhaling and eating avian flu viruses.

Pigs then become the flu “mixing bowl,” combining the human flu viruses they already harbor from contact with people with avian viruses. The genes of human and bird flu viruses get shuffled like decks of playing cards.

The genetic shuffle is a bad deal for the public.

It usually results in “genetic drift,” a slight change in the flu virus's ability sneak past humans' immune defenses and infect people. The result is just a typical flu season.

Periodically, however, humanity gets dealt a bad hand. Major shifts occur in flu viruses' genetic make-up, leading to the sudden emergence of strains to which most humans have no immunity.

During the last 250 years, 10-20 flu pandemics like the 1918-19 episode have swept the globe.

Experts believe that flu pandemics will continue so long as Chinese farmers live in close quarters with ducks and pigs.

Change will be slow. About 900 million people in China - almost 1 in every 7 people in the world - have frequent contact with live pigs and ducks, according to one estimate.



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