June 24, 2009
In New Theory, Swine Flu Started in Asia, Not Mexico
By DONALD G. McNEIL Jr.
Contrary to the popular assumption that the new swine flu pandemic arose on factory farms in Mexico, federal agriculture officials now believe that it most likely emerged in pigs in Asia, but then traveled to North America in a human.
But they emphasized that there was no way to prove their theory and only sketchy data underpinning it.
There is no evidence that this new virus, which combines Eurasian and North American genes, has ever circulated in North American pigs, while there is tantalizing evidence that a closely related “sister virus” has circulated in Asia.
American breeding pigs, possibly carrying North American swine flu, are frequently exported to Asia, where the flu could have combined with Asian strains. But because of disease quarantines that make it hard to import Asian pigs, experts said, it is unlikely that a pig brought the new strain back West.
“The most likely scenario is that it came over in the mammalian species that moves most freely around the world,” said Dr. Amy L. Vincent, a swine flu specialist at the Agriculture Department’s laboratory in Ames, Iowa, referring, of course, to people.
The first person to carry the flu to North America from Asia, assuming that is what happened, has never been found and never will be, because people stop carrying the virus when they get better.
Moreover, the officials said, the chances of proving their theory are diminishing as the virus infects more people globally. It has now reached more than 90 countries, according to the World Health Organization. Since some of those people will inevitably spread it to pigs, its history will become impossible to trace.
“To tell whether a pig is newly infected by a human or had the virus before the human epidemic began really can’t be done,” said Dr. Kelly M. Lager, another Agriculture Department swine disease expert.
The highly unusual virus — which includes genetic bits of North American human, avian and swine flus and Eurasian swine flu — has not been detected in any pigs except those in a single herd in Canada that was found infected in late April.
A carpenter who worked on the farm after visiting Mexico had been thought to have infected the herd. But in mid-June, Canadian health agencies said he was not to blame. The whole herd was culled, and the virus has not been found elsewhere in Canada, as it would have been if it were endemic, since American and Canadian laboratories test thousands of flu samples to help the pork industry develop vaccines.
But a sample taken from a pig in Hong Kong in 2004 was recently found to have a virus nearly matching the new flu. That flu, which had seven of the new flu’s eight genome sequences, was noted in an article in Nature magazine on June 11, which called it a “sister virus.”
Scientists tracking the virus’s lineage have complained that there is far too little global surveillance of flu in swine. Public databases have 10 times as many human and avian flu sequences as they do porcine ones, said Dr. Michael W. Shaw, a scientist in the flu division of the Centers for Disease Control and Prevention, and there are far fewer pig flu sequences from Asia than from North America and Europe, and virtually none from South America or Africa. “Something could have been going on there for a long time and we wouldn’t know,” Dr. Shaw said.
But national veterinary officials said they knew of no close relatives of the new virus in the large private North American databases, either. That makes it most likely, they said, that it has been circulating in Asia.
The new virus was first isolated in late April by American and Canadian laboratories from samples taken from people with flu in Mexico, Southern California and Texas. Soon the earliest known human case was traced to a 5-year-old boy in La Gloria, Mexico, a rural town in Veracruz.
Because that area is home to hog-fattening operations with thousands of pigs in crowded barns near lagoons of manure, opponents of factory farming were quick to blame the industry.
In May, the Mexican government said it had tested pigs on the Veracruz farms and found them free of the virus. Smithfield Foods, an owner of the farms, and the National Pork Producers Council, the industry’s lobbying arm, were quick to publicize that announcement.
But outside veterinary experts still disagree on whether those tests proved anything.
According to Smithfield, Mexican government veterinarians tested snout swabs taken on April 30 and blood samples stored since January.
But since the human outbreak in Veracruz is believed to have started in February, many veterinary experts said testing pig snouts for live virus in April proved nothing. Any pig sick in February would have long since recovered and, since hogs are usually slaughtered at 6 months old, many of those alive in early February would be bacon by April.
But Dr. Greg Stevenson, an expert in swine diagnostics at Iowa State University, said that since flu could persist in a large herd for months, “if it had been there in February, it would probably still be there at the end of April.”
The blood tests — in which scientists look for antibodies formed in response to a previous infection — present a different set of problems. Antibodies are much harder to tell apart from one another than viruses are.
A pig that had the new H1N1 flu would come up positive on an antibody test. But so would a pig that had the regular H1N1 swine flu that has circulated since 1930, or even a pig that had been vaccinated against the earlier H1N1 flu — and all the Smithfield pigs routinely get flu shots.
The company said vaccinated pigs could be distinguished from previously ill pigs because illness produced more antibodies.
But outside experts were skeptical. An antibody test specific enough to identify only the new flu strain “would take months to develop, at a minimum, and would require considerable R & D expertise and technology,” said Dr. Christopher W. Olsen, a swine flu expert at the University of Wisconsin’s veterinary medical school.
The governor of Veracruz has asked the National Autonomous University of Mexico to do its own investigation of industrial hog farming in his state; the work is expected to take months. Carlos Arias, the biochemist leading the team, said he hoped to test all the swab and tissue samples stored by the farms and the national veterinary laboratory.
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