With cases of H3N2v influenza on the rise, a look at where we stand and the key role of our system of food animal production in strengthening viruses
The CDC has announced a sharp spike in cases of swine-origin influenza, sometimes known as "swine flu." At least 224 cases have occurred since mid-July, mostly in children living in Indiana and Ohio. This compares with just 12 cases reported nationally in all of 2011. The threat of pandemic influenza may not be imminent, but it is real.
To spark a pandemic, an influenza virus in another species must evolve the ability to infect humans and then spread quickly. So far, we believe those who contracted swine flu this year to have been infected by pigs at agricultural fairs; transmission between humans has not yet been reported. Several developments have made this more likely to occur, though -- including the serious threats introduced by the industrialization of food animal production, which selects for genes that may allow influenza viruses to reach pandemic proportions.
In a country of more than 300 million people, 224 people with a mild flu may seem unremarkable. Viruses evolve quickly, though, and one that develops the ability to infect and spread among humans could wreak havoc. In 2009, another influenza virus infected at least 1.6 million people and killed at least 19,000 worldwide. In 1967, the "Hong Kong flu" killed at least one million people around the globe. The infamous 1917-18 influenza contagion claimed at least 50 million lives.
Viral evolution works much the same as human evolution, though faster. The replication of viral genes is imperfect -- mistakes happen, and these mistakes (mutations) lead to genetic variation between a virus and its progeny. Unlike humans, viruses have no genetic "proofreading" system to catch many of these mistakes. As a result, mutations occur much more frequently. Occasionally, a mutation gives a virus enhanced ability to infect new host cells and reproduce more quickly than its counterparts. An advantageous mutation quickly becomes common throughout a viral population.
The plethora of potential hosts removes a barrier to increased virulence -- a virus can kill its host quickly and still have a good chance of infecting others.
Another process, viral reassortment (which is unique to certain viruses) allows them to acquire vastly different genes in just one generation. The genomes of these viruses consist of short segments of RNA, each separate from the other. When a virus infects a cell, these genes hijack the cellular machinery of the host to replicate themselves. The replicated genes are then packaged into new viruses and released to infect others. If two or more viruses infect the same cell, the genes of all are replicated. When the new viruses are assembled, they may receive genes from all of these viruses -- a new strain can emerge.
Our current model of food animal production factors heavily into viral evolution and transmission. The system -- which is vastly different than it was just a century ago -- provides some efficiency, but it poses grave threats to public health, including increased risk of pandemic influenza.
Beginning in the 1940s, and intensifying recently, small farms were replaced by large, industrial operations that confine thousands or even millions of animals at a single site. The animals are raised in cramped quarters, in constant contact with their waste, and fed corn and soybeans in place of the forage for which their digestive systems evolved.
At any given time there are about one billion poultry and swine total alive in the U.S., and the vast majority of these animals are raised at industrial operations. Each animal is a potential host for influenza viruses. Additionally, the stresses induced by confinement and constant respiratory exposure to high concentrations of ammonia, hydrogen sulfide, and other gases from concentrated waste leave animals more susceptible to viral infections. These conditions allow viruses to infect again and again, increasing the frequency of mutations and viral reassortment, the raw material for evolution.
The practices at these industrial operations can select for dangerous genes. The plethora of potential hosts removes a barrier to increased virulence -- a virus can kill its host quickly and still have a good chance of infecting others. The co-location of swine and poultry operations in some states provides chicken viruses that mutate to infect pigs with a treasure-trove of hosts just up the road. Because humans and pigs are mammals, a swine virus may be more likely to infect humans, given our physiologic similarities.
The interaction between humans and food animals, and our resulting exposure to viruses these animals carry, is now radically different from that at any previous point in history. Earlier generations of farmers may have spent a few hours each day with dozens of animals at most. The workers at industrial operations work all day with hundreds or thousands of birds or pigs. The probability of contracting influenza viruses that have mutated to infect humans is greatly increased. Many of these workers are low-income migrants unprotected by labor laws and without ready access to medical treatment.
A 2007 study in Iowa, the leading swine-producing state in the U.S., determined that rural residents exposed to pigs were almost 55 times more likely than non-exposed individuals to have had contact with influenza virus. If one of these viruses could be transmitted efficiently between humans, an outbreak could occur. Indeed, study participants' spouses who had no contact with pigs were still 28 times more likely than non-exposed non-spouses to have been exposed to influenza. These results suggest that workers may form an effective "bridge population" that spreads influenza off the farm.
Viruses can move in other ways too. The animals at these facilities generate massive quantities of waste--about 335 million tons per year. This waste attracts flies and other animals that can carry viruses far away. Food animals may also be transported hundreds of miles in trucks and the stress of travel can lead them to shed thousands of viruses along the way.
The reasons for the current upswing in swine influenza cases remain unclear, but the more general risk of influenza posed by industrial food animal production is well established. The underlying problems of industrialization deserve our attention for a host of reasons, not the least of which is that a stainable system could help avert disaster.