Mega-Farms Are Driving the Threat of Bird Flu

It’s still unclear what caused that initial jump from wild birds, which are the natural reservoirs of the virus, to commercial poultry flocks and then to cows, but some research suggests that changing migration patterns caused by warmer weather are creating conditions conducive to the spreading of viruses. Some wild birds are migrating earlier than usual, hatching juvenile birds in new or different habitats.

“This is leading to a higher number of young that are naive to the virus,” Prist explained. “This makes the young birds more infectious—they have a higher chance of transmitting the virus because they don’t have antibodies protecting them.

“They’re going to different areas and they’re staying longer,” Prist added, “so they have higher contact with other animals, to the other native populations, that they have never had contact [with] before.”

That, researchers believe, could have initiated the spillover from wild birds to poultry, where it has become especially virulent. In wild birds, the virus tends to be a low pathogenic strain that occurs naturally, causing only minor symptoms in some birds.

“But when we introduce the virus to poultry operations where birds live in unsanitary and highly confined conditions, the virus is … able to spread through them like wildfire,” said Ben Rankin, a legal expert with the Center for Biological Diversity, an advocacy group. “There are so many more opportunities for the virus to mutate, to adapt to new kinds of hosts and eventually, the virus spills back into the wild and this creates this cycle, or this loop, of intensification and increasing pathogenicity.”

Rankin pointed to an analysis that looked at 39 different viral outbreaks in birds from 1959 to 2015, where a low pathogenic avian influenza became a highly pathogenic one. Out of those, 37 were associated with commercial poultry operations. “So it’s a very clear relationship between the increasing pathogenicity of this virus and its relationship with industrial animal raising,” Rankin said.

Some researchers worry that large farms with multiple species are providing the optimal conditions for more species-to-species transfer. In North Carolina, the second-largest hog-producing state after Iowa, some farmers have started raising both chicken and hogs under contracts that require huge numbers of animals.

“So you’ve got co-location at a pretty substantial scale of herd size, on a single property,” said Chris Heaney, an associate professor of environmental health, engineering, epidemiology, and international health at the Bloomberg School of Public Health. “Another concern is seeing it jump into swine. That host, in particular, is uniquely well suited for those influenza viruses to reassort and acquire properties that are very beneficial for taking up residence in humans.”