The swine flu virus that is smoldering in this country and triggering a full-blown outbreak in Mexico is one of a growing number of animal pathogens to jump the species barrier -- and may be the microbe that jumpstarts the first globe-circling pandemic of the 21st century, experts said Tuesday.
The virus is the second known triple-hybrid swine pathogen in history, a concoction of swine, human and bird genetic components to which people have no immunity. It is also the first triple-hybrid to make the leap from pigs to humans.
Unlike with avian flu, which people can get only through close contact with a sick bird or feces, this strain of swine flu has learned the trick of seasonal influenza and can be passed person-to-person easily.
Scientists are considering flies as one possible vector that may have transmitted the virus from pigs to people. Flies, experts said Tuesday, hover over vast lagoons that hold feces and other waste, byproducts of factory hog farming in Mexico.
"The possibility that flies were vectors is being investigated now," said Dr. Kristine Smith, a veterinarian with the Wildlife Conservation Society at the Bronx Zoo. "Whether that actually happened, no one knows yet."
And one of the people who survived an early, aggressive wave of infections at the beginning of the month apparently was a 4-year-old boy who lives near a Mexico factory farm with 1 million pigs on it, owned by a joint U.S.-Mexican company, global health officials say.
When people sustained these early infections, Mexican health officials were unaware that swine flu had started its run. The child's samples are being sent to the CDC and to the World Health Organization and will be able to confirm how early the sweep of H1N1 began.
The UN Food and Agriculture Organization is dispatching a team to the farm to determine the extent of the disease in animals. "At this stage it is ... not an animal crisis." said Joseph Domenech, UN chief veterinarian.
Smithfield Farms, the Virginia-based company that is co-owner of the factory farm in question, said in a statement Tuesday its herd and employees are free of the virus.
Dr. Michael Greger of the Humane Society of America said the first triple-hybrid swine virus was identified 11 years ago.
"Swine flu viruses were relatively stable until 1998 when the first triple-hybrid emerged on a factory farm in North Carolina," he said. "It was first detected in pigs in gestation crates. Animals like these are moved around a lot. They may start out in North Carolina but they were fattened in the Corn Belt and slaughtered in California."
In each state where pigs were transported that year, the rare triple-hybrid strain emerged. However, it remained in pigs, didn't cause serious disease, and didn't leap to people.
The new strain doesn't cause serious porcine disease, either. "Pigs catch the flu all of the time," Greger added, "they get a loud barking cough."
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