Study Criticizes Swine-flu Follow-up

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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, according to a study released Thursday by researchers in Hong Kong.

No dangerous new strain has emerged, said several experts who saw the study. But 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 flu division of the Centers for Disease Control and Prevention. We have to stay vigilant.

While there is a long global history of testing humans in many countries to see how human flu strains shift each year, there are major gaps in testing pigs, Dr. Bridges said.

That is important because pigs can catch both human and bird flus. Flus easily swap their eight genes and any new combination ...
... might be able to spread among pigs and eventually reach another human.

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.

The message from our paper is not an inevitable disaster around the corner, but the need for continued vigilance, Malik Peiris, a flu expert at the University of Hong Kong and one of the study's authors, said in an e-mail message.

Among the globe-circulating flus that pigs could, in theory, catch are six swine flus, several human seasonal ones and at least two avian ones. The latter include the feared 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, and an H9N2, which has been found in about a dozen humans but caused only mild disease so far.

Last year's pandemic was originally dubbed a swine flu because 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 that we have to be very careful, said Dr. Peter Palese, a flu researcher at the Mount Sinai School of Medicine. On the other hand, I can argue that it hasn't happened yet. It's not clear that any of these new swine viruses have the potential to go into humans and cause problems.