Pigs may seem cute, but from a virological perspective, they’re oinking time bombs. That’s because swine can become infected with both avian and human flu viruses, making them influenza mixing bowls. An avian virus can reassort with a human one inside the cells of a pig, mutating and potentially producing a new flu strain that humans have no protection against. That’s one way that influenza pandemics can ignite — and when flu pandemics are bad, they’re really bad.
Unfortunately, scientists have rarely been able to systematically track the spread and type of flu viruses in pigs — meaning that we’re often caught off guard when a new virus emerges to infect human beings, as happened during the H1N1/A flu pandemic of 2009. But there’s one place where researchers are keeping close tabs on the swine flu viruses: Hong Kong, a crowded, globalized city where pandemics have begun in the past.
Thanks to new research published in the May 25 Nature, scientists have a better idea than ever before about how flu viruses spread among pigs. A team of scientists from Hong Kong, Singapore and the U.S. looked at the epidemiology and genetics of swine flu viruses in Hong Kong from more than 650 samples taken from pigs over 12 years of direct surveillance, and along with more than three decades of supplemental data. They found that the range of swine flu viruses was much larger than scientists had believed, and that the growing genetic diversity of the viruses was likely due to cross-border import of pigs into southern China.
Here’s how it works: over the past 20 years, pig producers in southern China have imported swine from North America and Europe, in part to improve the breeding stock. (Southern China is home to the world’s biggest population of pigs, and pork is a mainstay in the country’s diet — in 2006, the Chinese ate more than 50 million metric tons of pork.) The imported pigs brought whole new families of flu virus with them, and those viruses mixed among the dense population of Chinese pigs. As a result, new strains have emerged, including the Eurasian avian-like H1N1 virus — which is particularly worrisome, because human beings have no immunity to that lineage.
As lead author Vijaykrishna Dhanasekaran, a professor at Duke-NUS Graduate Medical School in Singapore, said in a statement:
I think the risk of swine-to-human transmission has not increased greatly, but the diversity of swine viruses has increased as shown in our study. This means that the repertoire of viruses that humans are in contact with everyday has increased and this may lead to a higher likelihood of swine-to-human transmission, although the risk remains unquantified.
Just because we don’t have antibodies to a flu virus doesn’t mean it will be deadly — the ultimate death toll from the 2009 pandemic was relatively small — but the new strain may spread easily among human beings. The best way to protect ourselves is through vigilant surveillance of viruses in pigs, so that researchers can identify new viruses as they emerge and gear up for pandemic prevention. The 2009 flu pandemic may have even been light, but that doesn’t mean the next one will.