Pig CAFOs influence timing of human flu seasons, study shows

The enormous numbers of animals concentrated in industrial pig farms are changing the pattern of flu seasons, by providing flu viruses a place to jump between humans and animals and multiply faster than they otherwise would, according to new research from North Carolina — a state that is second only to Iowa in pig production.

Using publicly available data, researchers at Duke University mapped the locations of swine in concentrated animal feeding operations (CAFOs) in North Carolina, which sells almost 10 million hogs per year and accounts for more than 14 percent of the U.S. pig market. They crossmatched that information with data on the occurrence of flu in the state during four flu seasons, from 2008 to 2012. They found that in two of those flu seasons, 2009-2010 and 2010-2011, cases peaked significantly earlier than in the other two.

What likely happened, they say, is that the virus circulating in those flu seasons was carried onto farms by workers and spread to the pigs — and as it passed from pig to pig, the virus had a chance to reproduce in a manner that would not have happened in the absence of CAFOs. That much larger amount of virus spread back out into surrounding community, spiking the number of flu cases earlier in the flu season.

The researchers did not have the data to say whether the virus circulating through pigs made the flu season worse overall, nor whether it caused more serious illness in individual people. But the effect on the timing of the season ought to be enough to prompt a public health response, they said.

“We don’t want to be implicating the swine industry as a public health threat; it’s a phenomenon of modern food production that there are heavy concentrations of animals like this,” said Paul Lantos, a physician and expert in geographic information systems who is the first author of the paper, which appears in Clinical Infectious Diseases.

But, he said, health authorities should think about creating surveillance systems around areas where diseases can pass between animals and humans, to detect their crossover. And they should consider emphasizing flu vaccination in communities around CAFOs, to protect farm workers and anyone to whom workers might transmit flu.

“Farm workers are predominantly young healthy males, and they are not the people usually targeted by intensive flu vaccination campaigns; those focus on children, the elderly and the infirm,” Lantos said.

There’s a nuance to the Duke team’s findings, and it has to do with the complex ecology of flu. The years when the North Carolina flu season was affected by local CAFOs were years when the seasons’ main virus was a “swine flu” — that is, a strain of flu that had already partially adapted to pigs.

Flu is a virus that easily swaps genetic segments, and strains that infect humans can incorporate portions of flus that previously passed through birds or pigs. When those segments combine, they can create a new flu that human immune systems have never experienced before, which can lead to more frequent and sometimes more severe illness. The 2009-10 virus was so novel that it sparked a worldwide pandemic, the first in more than 40 years.

But because that virus was already adapted to pigs, it was able to get back onto pig farms and boil up into greater amounts of flu by passing among the animals. That did not occur in the first and fourth seasons the team studied. In 2008-2009, the seasonal flu was not a pig-adapted strain; and by the fourth year, 2011-12, the state data shows there were fewer cases in humans, and presumably fewer to pass along to pigs.

Flu viruses change every year — which is why flu vaccines are adjusted and repeated annually — and their makeup from one year to the next is unpredictable. But it’s been known for a while that pig farms are hot spots for flu transmission. Flu passing from farm workers to pigs, and from pigs to workers, has been documented in Canada, Romania, China, and in several studies in Iowa.

Because of that, the Centers for Disease Control and Prevention recommends that people who work on pig farms, and even people who raise pigs to show at fairs, be vaccinated against flu. But given this research, Lantos said, maybe vaccination should be emphasized for people who also live near pig farms. Farm workers may unknowingly be serving as bridges that allow flu to cross unmonitored back into their families, to their communities and then to the outside world.

Maryn McKenna is a National Geographic contributor, and the author of Superbug and Beating Back the Devil. She last wrote for FERN about how the Netherlands cut antibiotic use on farms. Her new book on antibiotics in agriculture will be published by National Geographic Books/Penguin Random House in 2017She writes regularly about antibiotics and agriculture for Ag Insider.