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A ProMED-mail post
ProMED-mail is a program of the
International Society for Infectious Diseases
Date: Tue 23 Jun 2009
Source: New York Times [edited]
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 1st 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 2009. A carpenter who worked on
the farm after visiting Mexico had been thought to have infected the
herd. But in mid-June 2009, 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 7 of
the new flu's 8 genome sequences, was noted in an article in Nature
magazine on 11 Jun 2009, 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 1st isolated in late April 2009 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 2009, 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 30 Apr 2009 and
blood samples stored since January 2009. But since the human outbre!
ak in Veracruz is believed to have started in February 2009, many
veterinary experts said testing pig snouts for live virus in April
2009 proved nothing. Any pig sick in February 2009 would have long
since recovered and, since hogs are usually slaughtered at 6 months
old, many of those alive in early February 2009 would be bacon by
April 2009. 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 2009, it would
probably still be there at the end of April 2009."
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.
[Byline: Donal G. McNeil Jr.]
--
Communicated by:
ProMED-mail Rapporteur Mary Marshall
[This analysis is highly speculative, but it does identify some of
the difficulties inherent in attempts to determine the origin of the
novel 2009 (swine-origin) A (H1N1) influenza virus, and it provides a
review of some current attempts to determine the role of this virus
in disease in pigs globally. - Mod.CP]
[see also:
Influenza A (H1N1) - worldwide (73): case count, epidemiology 20090622.2288
Influenza A (H1N1) - worldwide (72): case count, epidemiology 20090619.2261
Influenza A (H1N1) - worldwide (71): comments on 1918 virus (02) 20090619.2262
Influenza A (H1N1) - worldwide (70): risk factors 20090619.2260
Influenza A (H1N1) - worldwide (69): other viral infections 20090618.2254
Influenza A (H1N1) - worldwide (68): southern hemisphere 20090618.2253
Influenza A (H1N1) - worldwide (67): comments on 1918 virus 20090618.2251
Influenza A (H1N1) - worldwide (66): new strain, sequence analysis
20090617.2235
Influenza A (H1N1) - worldwide (65): antivirals in pregnancy 20090616.2224
Influenza A (H1N1) - worldwide (64): case count, pandemic 20090616.2221
Influenza A (H1N1) - worldwide (63): case count, pandemic 20090611.2166
Influenza A (H1N1) - worldwide (62): Egypt, Lebanon 20090611.2150
Influenza A (H1N1) - worldwide (61): PCR test 20090610.2139
Influenza A (H1N1) - worldwide (62): Egypt, Lebanon 20090611.2150
Influenza A (H1N1) - worldwide (60): Egypt (Cairo) 20090608.2117
Influenza A (H1N1) - worldwide (59): Worldwide 20090608.2117
Influenza A (H1N1) - worldwide (58): USA, Africa 20090607.2109
Influenza A (H1N1) - worldwide (57): Brazil, USA 20090605.2090
Influenza A (H1N1) - worldwide (56): case counts 20090605.2089
Influenza A (H1N1) - worldwide (55) 20090603.2056
Influenza A (H1N1) - worldwide (54): dynamics 20090601.2038
Influenza A (H1N1) - worldwide (53): case counts 20090531.2025
Influenza A (H1N1) - worldwide (52): seasonal vaccine 20090530.2010
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Influenza A (H1N1) - worldwide (25): case counts 20090513.1785
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....................................................cp/msp/dk
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