The H1N1 swine flu virus in North America currently concerning global public health officials is not the first triple hybrid human/bird/pig flu virus to be discovered. The first was discovered in a North Carolina factory farm in 1998. Since the 1918 pandemic, an H1N1 flu virus has circulated in pig populations, becoming one of the most common causes of respiratory disease on North American pig farms.
In August 1998, however, 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-described triple reassortment, a hybrid of three viruses—a human virus, a pig virus, and a bird virus.
Dr. Robert Webster, one of the world’s leading experts of flu virus evolution, blames the emergence 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.” “Within the swine population, we now have a mammalian-adapted virus that is extremely promiscuous,” explained another molecular virologist at the time, referring to the virus’s proclivity to continue to snatch up genes from human flu viruses. “We could end up with a dangerous virus.” This may indeed be what we are now facing.
Within months of the 1998 emergence, the virus showed up in Texas, Minnesota, and Iowa. Within a year, it had spread across the United States. This rapid dissemination across the country has been blamed on long-distance live animal transport. 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. 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) needs to be considered when calculating the true cost of long-distance live animal transport.
What led to the emergence of the North Carolina 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 North Carolina, the home of the nation’s largest pig farm. North Carolina has the densest pig population in North America and reportedly boasts more than twice as many corporate swine mega-factories as any other state.
The year of emergence, 1998, was the year North Carolina’s pig population hit ten million, up from two million just six years before. At the same time, the number of hog farms was decreasing, from 15,000 in 1986 to 3,600 in 2000. How do five times more animals fit on almost five times fewer farms? By crowding 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. 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 secretions —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. As a headline in the trade journal National Hog Farmer announced, “Overcrowding Pigs Pays—If It’s Managed Properly.” 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.”
The swine flu virus discovered this week in California and Mexico appears to be a quadruple reassortment virus incorporating genes from human and avian flu viruses as well as North American and European strains of swine flu. In Europe in 1993, a bird flu virus had adapted to pigs, acquiring a few human flu virus genes, and infected two young Dutch children, even displaying evidence of limited human-to-human transmission.
“Influenza [in pigs] is closely correlated with pig density,” said a European Commission-funded researcher studying the situation in Europe. As such, Europe’s rapidly intensifying pig industry has been described in Science as “a recipe for disaster.” Some researchers have speculated that the next pandemic could arise out of “Europe’s crowded pig barns.” The European Commission’s agricultural directorate warns that the “concentration of production is giving rise to an increasing risk of disease epidemics.” 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. No such limit exists in the United States.
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. This reassorting, Webster’s team concludes, makes the 60 million strong U.S. pig population an “increasingly important reservoir of viruses with human pandemic potential.” “We used to think that the only important source of genetic change in swine influenza was in Southeast Asia,” said Christopher Olsen, a molecular virologist at the University of Wisconsin, Madison. Now, “we need to look in our own backyard for where the next pandemic may appear.”