Sometime in 2004, a pig was trucked with a herd of others to Sheung Shui Slaughterhouse, Hong Kong’s largest abattoir. It could have been shipped from any number of farms in China. The pig was penned in cramped conditions and later shoved onto a conveyor line. When it reached the end of the line it was electrocuted with a jolt to the head. A worker slit its throat. Before its carcass was hoisted up on a chain, scalded, and cleaned, scientists swabbed the pig’s mouth as part of a flu-monitoring program, run by Hong Kong University. The pig carried a seemingly harmless strain of influenza. The strain was genetically sequenced, baptized Sw/HK/915/04, entered into a database, and forgotten.
John Upton | NAUTILUS
Five years later a flu epidemic, which originated in pigs, raced around the globe through air travel, leaving a trail of nearly 300,000 people dead from the United States to Brazil, China to New Zealand, South Africa to Finland. Scientists determined the outbreak began in Mexico, not far from a huge pig breeding farm. Poring over databases, looking for clues to the makeup of the killer virus, scientists discovered that Sw/HK/915/04, the influenza strain of the Hong Kong pig, shared seven of its eight genes with the strain of 2009.
The 2009 flu epidemic stripped back the veil on the dangers of our interconnected world.
The shared genes suggest to scientists that the 2009 virus may have evolved by mixing genomic segments of viral strains in pigs from around the world. Global shipments of pigs are seldom tracked and researchers did not establish a direct link between the infected Hong Kong pig and the 2009 outbreak in Mexico. “We don’t really have information about pig viruses in Mexico,” says Gavin Smith, an evolutionary biologist based at Duke-NUS Graduate Medical School, part of the National University of Singapore, and lead author of a Nature paper on the origins of the 2009 epidemic. “It was probably generated locally but genetic components could have come from Asia.”
Smith echoes what international health officials and scientists say may be the most worrisome aspect of viruses today: the ease and speed with which some travel across the globe. The 2009 flu epidemic, they say, stripped back the veil on the dangers of our interconnected world. “Pathogens evolve and spread in a world where our mobility is very different than it was 50 to 100 years ago,” says Dirk Brockmann, a theoretical physicist at Northwestern University, who studies the spread of infectious diseases, including the 2009 virus, classified as H1N1. “When a pathogen emerges that has human transmissibility, like the H1N1 virus, that is a massive risk.”