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H1N1 (Swine Flu): Reassortment of Pandemic H1N1/2009 Virus in Swine in China Reported In New Study

June 18, 2010

One of the big learnings of the 2009 pandemic was the critical need for ongoing monitoring of animal populations worldwide to detect changes in the influenza virus.  After all, it is now suspected that a version of the H1N1 2009 pandemic strain had circulated worldwide in commercial swine for over ten years!  A study by Hong Kong researchers just released in the journal of Science reveals some disappointing news regarding surveillance of H1N1 virus in pigs in China.

The study has concluded that there is too little genetic surveillance of last year’s human pandemic flu, which has gone on to infect pigs in China and is readily mixing with other flus there – this evolution is called reassortment.

To date, no dangerous new strain has emerged, however in January the researchers found a new strain with one of the pandemic flu’s surface proteins — the outer spikes and knobs it uses to attach to cells. That was a reminder of how easily another swine strain capable of spreading among people could emerge.  “Just because we’ve just had a pandemic does not mean we’ve decreased our chances of having another,” said Dr. Carolyn B. Bridges, an epidemiologist in the influenza division of the CDC, “We have to stay vigilant.”

For years now, the WHO has tested humans in many countries to see how human flu strains shift each year.  There are however “major gaps” in testing swine.  This testing is critical as pigs can catch both human and bird influenza. Influenza can easily exchange their eight genes and any new combination might be able to spread among pigs and eventually reach humans.

Pigs in the giant hog-raising barns of the United States and Western Europe are tested regularly, but the millions of pigs on small farms and in big operations in Asia and Latin America seldom are.  Commercial hog operations do constant testing so they can formulate new vaccines and snuff out flu outbreaks. Flu seldom kills pigs, but it makes them lose weight, which cuts into profits.

In the new study, published online in the journal Science, the Hong Kong researchers sequenced viruses they found by regularly swabbing pigs’ snouts at the territory’s largest slaughterhouse, which gets pigs from all over southern China. That testing, supported by a United States government grant, has gone on for 12 years.

Malik Peiris, a flu expert at the University of Hong Kong and a lead researcher noted, “The message from our paper is not an inevitable disaster around the corner, but the need for continued vigilance.”

The fear is that a more dangerous flu could develop in these “pig mixing vessels” and create havoc. The one that is most feared is the H5N1, which has killed 60 percent of the 500 people known to have caught it since 2003 but thus far almost never spreads from person to person.

Last year’s pandemic was originally dubbed a swine flu because the five of the eight genes in its makeup had been seen before in American or Eurasian pigs during the previous 10 years, though never in the exact combination that was making people sick in Mexico.  It has not been found in any stored samples from people or pigs, so where it came from remains a mystery.  It has now reached 200 countries and is still infecting more people every day, though most cases are mild to moderate. It is now clear that it is also circulating freely in pigs in China and sometimes mixing genes with at least two other long-known swine flus.

The implication of this study is quite clear.  We must be vigilant in monitoring illness in the swine population around the world in order to be able to respond rapidly to an emerging disease.

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