Hog Ties

Crates

Before the 1997 outbreak of H5N1 and the realization that the 1918 virus was also purely avian in origin, pigs were thought to be the prime “mixing vessels” by which human and avian influenza viruses could potentially mix and match their genes to create strains capable of infecting humans. All influenza viruses use cellular sialic acid receptors to dock onto cells and infect them. The sialic acid receptors lining the intestines of ducks have what are called alpha-2,3 linkages, whereas human lungs have sialic acid receptors with alpha-2,6 linkages. Influenza strains tend only to bind effectively with one type of receptor or the other. This difference in receptor compatibility helps form a species barrier, keeping the bird strains in the birds and the human strains in the humans.¹²⁸⁶ How then, could a hybrid bird-human virus ever arise?

Researchers discovered that the respiratory tracts of pigs have both types of molecular linkages. Since pigs display both bird-type and human-type virus receptors, pigs could potentially be simultaneously infected with both bird flu and human flu. With both viruses slobbering throughout the pigs’ respiratory systems, the strains might re-assort to create a hybrid virus that could recognize human-type receptors but retain enough avian novelty to escape pre-existing human immunity. This could theoretically trigger mild pandemics as were seen in 1957 and 1968,¹²⁸⁷ thought to be mild because humans retained at least some immune memory to the human fraction of the hybrid virus.

There is building evidence that this scenario may be more than just speculation; pigs may indeed play some role in avian influenza’s adaptation to people. During the 1918 pandemic, millions of pigs fell ill. The question at the time was did they give it to us or did we give it to them?¹²⁸⁸ Our current understanding of the chain of events was that the 1918 pandemic H1N1 virus started, as we presume it always does, in waterfowl spread to domesticated poultry. It then spread to humans and dead-ended in pigs.¹²⁸⁹ Since then, the H1N1 virus has circulated in pig populations, becoming one of the most common causes of respiratory disease on North American pig farms.¹²⁹⁰ Then something strange started to happen in 1998.

Throughout the century, influenza viruses had established a stable H1N1 lineage in U.S. pigs. But in August 1998, a barking cough resounded throughout a North Carolina pig farm in which all the thousands of breeding sows fell ill. An aggressive H3N2 virus was discovered, the type of influenza that had been circulating in humans since 1968. Not only was this highly unusual—only a single strain of human virus had ever previously been isolated from an American pig population—but upon sequencing of the viral genome, researchers found that it was not just a double reassortment (a hybrid of human and pig virus, for example) but a never-before-described¹²⁹¹ triple reassortment, a hybrid of three viruses—a human virus, a pig virus, and a bird virus. “Within the swine population, we now have a mammalian-adapted virus that is extremely promiscuous,” explains one molecular virologist, referring to the virus’s proclivity to continue to snatch up genes from human flu viruses. “We could end up with a dangerous virus.”¹²⁹²

Within months, the virus showed up in Texas, Minnesota, and Iowa.¹²⁹³ Within a year, it had spread across the United States.¹²⁹⁴ Pigs did not begin to fly. The rapid dissemination across the country was blamed on long-distance live animal transport.¹²⁹⁵ In the United States, pigs travel coast to coast. They can be bred in North Carolina, fattened in the corn belt of Iowa, and slaughtered in California.¹²⁹⁶ While this may reduce short-term costs for the pork industry, the highly contagious nature of diseases like influenza (perhaps made further infectious by the stresses of transport)¹²⁹⁷ needs to be considered when calculating the true cost of long-distance live animal transport.

What led to the emergence of this strain in the first place? What changed in the years leading up to 1998 that facilitated the surfacing of such a unique strain? It is likely no coincidence that the virus emerged in boss hog North Carolina, the home of the nation’s largest pig farm.¹²⁹⁸ North Carolina has the densest pig population in North America and boasts more than twice as many corporate swine mega-factories as any other state.¹²⁹⁹

The year of emergence, 1998, was the year North Carolina’s pig population hit ten million, up from two million just six years before.¹³⁰⁰ At the same time, the number of hog farms was decreasing, from 15,000 in 1986 to 3,600 in 2000.¹³⁰¹ How do five times more animals fit on almost five times fewer farms? By cramming about 25 times more pigs into each operation. In the 1980s, more than 85% of all North Carolina pig farms had fewer than 100 animals. By the end of the 1990s, operations confining more than 1,000 animals controlled about 99% of the state’s inventory.¹³⁰² Given that the primary route of swine flu transmission is thought to be the same as human flu—via droplets or aerosols of infected nasal secretions¹³⁰³ —it’s no wonder experts blame overcrowding for the emergence of new flu virus mutants.

Starting in the early 1990s, the U.S. pig industry restructured itself after Tyson’s profitable poultry model of massive industrial-sized units.¹³⁰⁴ As a headline in the trade journal National Hog Farmer announced, “Overcrowding Pigs Pays—If It’s Managed Properly.”¹³⁰⁵ The majority of U.S. pig farms now confine more than 5,000 animals each. A veterinary pathologist from the University of Minnesota stated the obvious in Science: “With a group of 5,000 animals, if a novel virus shows up it will have more opportunity to replicate and potentially spread than in a group of 100 pigs on a small farm.”¹³⁰⁶

Europe is facing a similar situation.¹³⁰⁷ Virginia-based Smithfield is the largest pork producer in the world, raking in more than $10 billion in annual revenue and posting record profits in 2005, in part because of its expansion of factory-sized pig farms in Europe.¹³⁰⁸ This trend is raising a stink among both environmentalists¹³⁰⁹ and public health officials.¹³¹⁰ By 1993, a bird flu virus had adapted to pigs, grabbed a few human flu virus genes, and infected two young Dutch children, even displaying evidence of limited human-to-human transmission.¹³¹¹

Denmark is the North Carolina of Europe. In 1970, the number of pig farms with more than 500 animals was zero. Between 1980 and 1994, 70% of the pig farms went out of business at the same time the pig population climbed to more than ten million.¹³¹² Today, this tiny country is the largest exporter of pork in the world.¹³¹³ The world’s bans on Asian poultry because of bird flu, combined with bans on U.S. beef because of mad cow disease, are, according to the chairman of Denmark’s Bacon and Meat Council, “beginning to favourably affect demand.”¹³¹⁴

“Influenza [in pigs] is closely correlated with pig density,” said a European Commission-funded researcher studying the situation in Europe.¹³¹⁵ As such, Europe’s rapidly intensifying pig industry has been described in Science as “a recipe for disaster.”¹³¹⁶ Some researchers have speculated that the next pandemic could arise out of “Europe’s crowded pig barns.”¹³¹⁷ The European Commission’s agricultural directorate warns that the “concentration of production is giving rise to an increasing risk of disease epidemics.”¹³¹⁸ Concern over epidemic disease is so great that Danish laws have capped the number of pigs per farm and put a ceiling on the total number of pigs allowed to be raised in the country.¹³¹⁹ No such limit exists in the United States.

Complicating the U.S. picture, the new swine viruses appear to be crossing back to commercial poultry, as reported in the CDC journal Emerging Infectious Diseases. The investigators warn: “Repeated introductions of swine influenza viruses to turkeys, which may be co-infected with avian influenza viruses, provide opportunities for the emergence of novel reassortments with genes adapted for replication in pigs or even humans.”¹³²⁰ North Carolina is also a top poultry producer. Webster blames the triple assortment of the 1998 virus on the “recently evolving intensive farming practice in the USA, of raising pigs and poultry in adjacent sheds with the same staff,” a practice he calls “unsound.”¹³²¹

With massive concentrations of farm animals within which to mutate, these new swine flu viruses in north America seem to be on an evolutionary fast track, jumping and reassorting between species at an unprecedented rate.¹³²² In 2006, the pig/bird/human triple reassortment strain was isolated from a farm worker in Canada.3175 This reassorting, Webster’s team concludes, makes the 60 million strong¹³²³ U.S. pig population an “increasingly important reservoir of viruses with human pandemic potential.”¹³²⁴ “We used to think that the only important source of genetic change in swine influenza was in Southeast Asia,” says a molecular virologist at the University of Wisconsin. Now, after H5N1, “we need to look in our own backyard for where the next pandemic may appear.”¹³²⁵

There have been about a hundred documented human deaths from swine flu in the last 30 years,¹³²⁶ including a young pregnant woman in the United States in 1988.¹³²⁷ On day 1, she visited a county fair in Wisconsin. On day 4, she started exhibiting signs of the flu. On day 11, she was hospitalized and intubated on a ventilator for respiratory failure. On day 14, labor was induced and she gave birth to a healthy baby. On day 18, she died. CDC laboratory analysis showed she was killed by a swine flu virus that she presumably contracted at the agriculture fair.¹³²⁸ H5N1 and the virus of 1918 help argue, however, that pigs may not be needed to produce killer viruses with pandemic potential.¹³²⁹

It turns out that the human respiratory tract, like the pig respiratory tract, has bird flu-type receptors after all, eliminating the need for an intermediate porcine host.¹³³⁰ Human beings can be directly infected with bird flu. Sufficiently adapted for efficient human-to-human transmission, a wholly avian virus could potentially infect billions since there may be no prior immunity in humans. In this way, H5N1 has been said to be following in the 1918 virus’s footsteps,¹³³¹ though in actuality the footprints, so to speak, have since been weathered away.¹³³² Instead of reassorting to form hybrids in some sort of transitional species mixing vessel, H5N1 is directly attacking the human species, as the 1918 virus is presumed to have done, via an “adaptation of a smoldering avian progenitor.”¹³³³ The head of the American Public Health Association conjectured, “This organism is following the historical [pandemic] playbook step by step.”¹³³⁴

That avian progenitor may have acquired some mammalian adaptive traits cycling through pigs on aquaculture farms or wallowing though flooded rice fields awash with domestic ducks,¹³³⁵ but there is still a vast gulf between a harmless waterborne intestinal duck virus and a killer airborne respiratory human virus. The influenza virus has the mutation machinery to bridge that gap, but it needs a lot of test tubes.

Although pork is the most popular meat in the world,¹³³⁶ the biggest piggeries still only may contain tens of thousands of animals.¹³³⁷ The biggest egg farms confine more than a million.¹³³⁸ It may take six months for a piglet to reach slaughter weight, but much of the global broiler chicken population is hatched and killed in as few as six weeks, dramatically multiplying the annual number of new viral hosts. More than 45 billion chickens pass through the world every year, compared to only about 1 billion pigs. Spread wing to wing, the number of chickens killed every day would wrap more than twice around the world’s equator.¹³³⁹ Never before has the influenza virus had such an opportunity.