Pigs Do Fly! Implications for Influenza

The influenza genome is segmented. Eight pieces of single-stranded RNA encode for 11 proteins: PB2, PB1, PB1-F2, PA, HA, NP, NA, M1, M2, NS1, and NS2. The segmentation allows influenza of different subtypes infecting the same host to trade segments like card players on a Friday night. Most of the resulting viruses will express phenotypes for the worse, but a small subset may be transformed into strains more infectious in their usual hosts or to a new host species.

This reassortment accounts in part for the origins of this year’s pandemic. Livestock pigs have long hosted their own version of seasonal H1N1, evolutionarily related to our own. From 1930-1998 the pig version evolved only slightly. But starting in 1998, the virus was subjected to a series of reassortment events. In North America, an aggressive swine H1N1 emerged with internal genes of a human H3N2 virus and an avian influenza. That virus subsequently spread across pig populations, with limited transfer to humans, usually to farm workers, who routinely offer the influenza virus human test subjects every step in its evolution.

In early 2009 a previously undescribed influenza, what we now know as swine flu H1N1 (2009), emerged in humans in central Mexico and spread around the world. Three of the new virus’s segments appeared to be from the classical swine influenza (HA, NP, NS), three from the North American H3N2-avian swine recombinant we just described (PB2, PB1, PA), and two from a Eurasian swine recombinant (NA, M) that originated in birds. In short, every one of the new H1N1’s genetic segments is most closely related to those of influenzas circulating among swine.

Something of a mystery remains, however. How did genomic segments from influenza circulating on opposite sides of the world get together? The most parsimonious explanation is that the final reassortment event occurred among swine where the first human outbreaks emerged in Mexico. A second possibility, with the first human infections estimated months before the first recognized cases in Veracruz, the strain emerged in Asia where all three of the influenzas that would form sfH1N1 (2009) have been found.

While the specifics of the strain’s origins remain unclear, the general mode of swine influenza’s movement is now clearer. Unlike as in avian influenzas, no migratory waterfowl can be offered as an obvious intermediate host by which the viruses are transported long distance. By sfH1N1 (2009)’s emergence, only hog populations were implicated. As hog transport is facilitated by human handlers alone, there can be no mistake that human agency has played a defining role in the spread of swine flu’s source strains.

With the globalization of the livestock filiere, the distances over which food animal populations are transported have expanded to continental and even intercontinental scales. Data is scarce, but Baltussen et al. (2009) report that in 2007 alone 22 million live piglets and slaughter pigs were traded across Europe. Their figure 2 shows the main routes of hog transport among European countries.

The surge in livestock miles goes hand in hand with the global spread of a corporate model of vertically integrated husbandry associated with farm consolidation and increases in head count per farm. By way of structural adjustment programs and neoliberal free trade agreements, agribusinesses are moving company operations to the Global South and Eastern Europe to take advantage of cheap labor, cheap land, weak regulation, and domestic production hobbled in favor of heavily subsidized agro-exporting.

But as David Burch explains, companies are also engaging in sophisticated corporate strategy. Agribusinesses are spreading their production line across much of the world. The CP Group, for one, now the world’s fourth largest poultry producer, has poultry facilities in Turkey, China, Malaysia, Indonesia, and the United States. It has feed operations across India, China, Indonesia, and Vietnam. It owns a number of fast food chain restaurants throughout Southeast Asia.

A supply chain arrayed across multiple countries allows companies the means by which to compensate for any interruptions in business, including of their own making. The CP Group operates joint-venture poultry facilities across China, producing 600 million of China’s 2.2 billion chickens annually sold. When an outbreak of deadly bird flu occurred in a farm operated by the CP Group in Heilongjiang Province, Japan banned poultry from China. CP factories in Thailand filled the market gap by increasing exports to Japan.

Despite working out much of this over the past year, it never occurred to me the lengths to which agribusinesses are willing and able to move their wares. A recent Winnipeg Free Press article describes a Manitoba company’s newly implemented efforts at flying thousands of pigs out of Winnipeg to Germany before a truck trip to Russia. As the video below shows, it’s Winnipeg to Krasandor on the other side of the world in four days. Even if pigs are rarely flown so far, still an open question, the agro-economic pressures placed on increasing the geographic extent of livestock transport are obvious.

The implications for influenza are fundamental. First, the scale of transport increases the likelihood previously isolated influenza serotypes can trade genomic segments, as occurred for this year’s transcontinentally recombinant H1N1. Second, increasing the virus’s geographic scope should select for deadlier strains. A renewable supply of susceptibles–ever available on the next horizon–is thought to act as a primary fuel for the evolution of virulence.

When pigs fly may be a bad time.


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