We're in the midst of the largest-ever bird flu outbreak in the United States. More than 23 million turkeys and chickens have been affected since December.
Iowa, Minnesota, and Wisconsin have all declared states of emergency, and five months in, the outbreak keeps spreading with no sign of slowing down, forcing farms to halt production, slaughter millions of birds, and lay off workers.
But most people haven't even heard about the outbreak yet — because, so far, it hasn't spread to humans. And that's the scary question that keeps disease experts up at night: could bird flu somehow spread to people? The longer these avian flu viruses circulate in poultry, the higher the risk that they mutate into something people can catch.
"That's why we monitor H5 [avian flu] viruses very closely," said Joseph Bresee, chief of the epidemiology branch of the influenza division at the Centers for Disease Control and Prevention. "There's always a chance they'll adapt in a way that'll make it easier to infect humans."
Since December, three types of avian flu — H5N8, H5N1, and H5N2 — have been detected in 18 states and two Canadian provinces. None of the circulating pathogens here have been known to make people sick, although there's another version of H5N1 that has infected nearly 650 people in Asia and the Middle East since 2003, with deadly results.
"The H5 viruses in Asia are different from the H5 viruses we're seeing here," Breese explained. "But avian flu viruses, as long as they're spreading in animals, are going to continue to mutate and adapt."
How this massive flu outbreak in birds could leap into humans
First, let's be clear: the risk that these pathogens spread to people is low, though not zero. As Tom Philpott points out at Mother Jones, "Public health officials have been warning for decades that massive livestock confinements make an ideal breeding ground for new virus strains." In particular, scenarios like the one that's playing out in the Midwest right now pose a threat.
There are two potential scenarios in which an outbreak like this could make the leap across species and spread to humans.
First, there's a process called "reassortment." Flu viruses have eight segments of their DNA that are independent from one another. Different viruses can swap whole gene segments with each other, essentially popping apart and reattaching to form new viruses.
So for bird flu to spread to humans, an avian influenza virus and a human influenza virus would have to meet within the same cell and swap genetic material, in order to create a new virus that can infect humans. The most likely place for this to occur is in a pig, since pigs have receptors for both bird flu and human flu. Once this new virus is created, a person would then need to be exposed to the infected pig and catch the virus that way.
Second is a slower process: a single avian influenza virus can mutate through generations into something that can affect humans. All viruses mutate as they jump from host to host, and over time those mutations can alter the severity of the virus or how it's transmitted, Bresee explained.
Time is a big factor in whether either of these two conditions might arise: the longer the flu sticks around in birds, the more chances it has to mutate or reassemble into something that can infect humans.
Even if bird flu does jump to humans, that doesn't ensure a pandemic
Now here's a crucial caveat: even if a bird flu virus does make the leap from birds to humans, it doesn't necessarily mean we'll see a deadly pandemic.
David Quammen, author of the book Spillover, points out that having a virus that makes the jump from one species to humans isn't enough to ensure a worldwide health threat. The virus also needs to be able to sicken or kill humans, and it needs to be easily spread from person to person, through a cough or sneeze.
"There's a lot of unpredictability with influenzas," Quammen adds, "because they mutate." But again, the longer an outbreak persists, the more opportunities the virus has to mutate or reassemble in ways that can harm people.
Right now, bird flu is a bigger risk to farmers than to the general public
The CDC is very concerned about this outbreak from a human health perspective, Bresee said. "This is clearly a very large outbreak in poultry. For humans, that's important because both the size and geographic distribution of the outbreak, as that expands, represents a bigger and bigger threat to humans. More and more humans will be exposed to viruses, so from our standpoint, this is a very significant outbreak."
For now, however, the people who work on farms are at a greater risk than the general public. That's because even if bird flu did mutate in a way that made it susceptible to humans, those infected birds are unlikely to be shipped to market. (You can read more about the CDC's recommendations for the public here.) The US Department of Agriculture has programs to help contain outbreaks — and euthanize flocks — as soon as potentially harmful pathogens are detected.
"We have production systems that are highly efficient, and they all have a biosecurity program," said Rodrigo Gallardo, assistant professor in the poultry medicine program at the University of California Davis. "We also have the best surveillance system in the world in terms of identifying outbreaks."
This wasn't the case in many parts of Asia, where bird flu did spread to the general public. In the places were the outbreak began, poultry was often raised in unsanitary conditions, or alongside other mammals like pigs, increasing the risk of recombination. Many Asian countries didn't have robust surveillance systems to identify outbreaks at the earliest stage.
Even so, Gallardo warned about being too complacent. "This has been an extended outbreak. It started with different viruses in January than what we're seeing now," he said. "What we need to hope is that through the summer — because of the dryness and the weather changes — the virus will die."
For now, officials are taking a range of precautions. In addition to closely monitoring the circulating viruses and tracking their mutations, they are closely watching people who work with poultry for signs of infection. The CDC has also advised them to wear the same protective outfits as Ebola workers, including respirators and goggles.
"The important message," the CDC's Bresee said, "is to make sure we protect humans, because the chance is not zero, that's for sure."