In the spring of 2013, a deadly coronavirus began to spread across the United States. Within a year it had reached 32 states, sweeping through dense populations that lacked immunity to the new pathogen. Though researchers scrambled to curb the disease, by the following spring, the epidemic claimed some 8 million lives—all of them pigs.
The pathogen responsible, Porcine Epidemic Diarrhea Virus (PEDv), poses no danger to humans. But among its hosts, pigs, the virus ravages their bodies with severe gastrointestinal disease. The 2013 outbreak killed an estimated 10 percent of the nation’s pigs in a matter of months. Struggling to make ends meet with limited supplies, pork producers pushed their prices to record highs as farmers nursed the dying and sick—most of which were newborn piglets—by the thousands.
“It was extremely devastating,” says Don Davidson, a veterinarian with the Ohio-based food company Cooper Farms. “The losses were huge. Months later … you could just see there weren’t as many pigs in market.”
By summer of 2014, the diarrheal disease had mostly petered out, partly because of a combination of increased diagnostic efforts and the growing immunity of the nation’s pig population. But perhaps the biggest factor in ending the epidemic was behavioral: a near-universal ramp-up in farms’ attention to cleaning, disinfection and isolation, says Michaela Trudeau, an animal coronavirus researcher at the University of Minnesota. These enhanced biosecurity measures “are one set of things we turn to over and over again to keep our pigs safe,” she says.
As the world battles another dangerous coronavirus, the human pathogen SARS-CoV-2, similar lessons could prove valuable once again. People aren’t pigs, and SARS-CoV-2—a respiratory virus—does not cause the same illness as PEDv. But this new coronavirus is vulnerable to many of the tactics that brought its predecessors to heel. In both cases, “it comes down to cooperation,” Trudeau says. “The more people [working to] contain it, the better off we’ll be.”
A cosmopolitan disease
The human population first discovered PEDv in the early 1970s, when veterinarians in Britain noticed pigs falling ill with bouts of watery diarrhea that couldn’t be traced back to any known pathogens. Because many cases were mild and the disease mostly spared piglets, farmers largely shrugged off the outbreaks.
Then, as the decades passed, the virus snuck across international borders and mutated. By 2010, strains emerging in China were sparking massive outbreaks that felled newborn pigs in droves.
In April of 2013, PEDv made landfall in the United States, flaring up first in Ohio and then Indiana and Iowa. Alarmed farmers and researchers ran tests for the typical spate of diarrheal microbes, yielding negative after negative. By the time PEDv was identified as the likely culprit, the pathogen had started to spread east and west. Although American veterinarians knew of the virus’ existence abroad, few had given serious thought to a PEDv cross-continental jump. American pigs had no immunity to fight off the new pathogen, and no vaccines or treatments were available.
The virus “had never made it to the U.S., and we thought it would stay that way,” says Montserrat Torremorell, an animal health expert at the University of Minnesota. “We weren’t ready.”
Likely stemming from the same ancestor that had yielded China’s deadly strains, the American variant of the virus proved a formidable foe. “It’s extremely infectious” in low doses, says Davidson, who explains he once heard that “the eraser on top of a No. 2 pencil could hold enough PED virus to infect every pig in the United States.”
Once swallowed by an unsuspecting piglet, the pathogen would travel to the gut and infect and destroy the finger-like nutrient-absorbing projections called villi on the walls of the small intestine. Newborn piglets were plagued with intense bouts of diarrhea, driving extreme dehydration that proved lethal within a few days of infection: On many farms, the death rates in the youngest swine “were just about 100 percent,” Davidson says.
Though older pigs remained mostly resilient to the disease’s most severe effects, they weren’t immune to infection. Even in the absence of symptoms, they shed and spread the virus through their feces, seeding new outbreaks as unsuspecting farmers shipped their swine stocks nationwide.
“This industry is structured in such a way that pigs move, and not just by a few miles,” Torremorell says. Domesticated pigs are, not unlike humans, a fairly cosmopolitan group. In a single lifetime, a pig may make several trips of a thousand miles or more, such as when it’s ready for sale or slaughter.
Humans, too, played a major role in transmission. While PEDv can’t infect people, the pathogen used them as its oblivious chauffeurs, hitching rides to new swine hosts as farmers, feed suppliers and veterinarians traveled from place to place. Hardy enough to persist for several days outside the pig body, the virus clung to clothes and glommed onto the soles of shoes. It planted itself onto equipment and coated the insides of trailers and trucks.
Worst of all may have been the virus’ ability to fester for weeks in feed, giving the pathogen a straight shot into the guts of its hosts. “Feed ingredients have unique access to our pigs,” says Megan Niederwerder, a veterinary virologist at Kansas State University. “That definitely was not at the forefront of our minds.”