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The horrific bird flu that has wiped out 36 million chickens and turkeys, explained

Bird flu currently poses little threat to humans, but it’s hell for the birds.

A white and buff-feathered chicken looks at the camera on a black background. Getty Images/Rizky Panuntun
Kenny Torrella is a staff writer for Vox’s Future Perfect section, with a focus on animal welfare and the future of meat.

The final month of Minnesota Timberwolves basketball was livelier than ever this season, and not just because they nearly upset the Memphis Grizzlies in their first-round playoff series.

During one game in mid-April, a woman glued her hand to the court. A few days later, another woman chained herself to the goal post. The following week, a third woman, dressed as a referee, stormed the court before removing her jacket, exposing a shirt underneath that read “Glen Taylor roasts animals alive.”

The protests, coordinated by the animal rights group Direct Action Everywhere, were aimed at the Timberwolves’ majority owner Glen Taylor. Taylor also owns Rembrandt Enterprises, a large Iowa egg producer that has culled — meaning deliberately killed — 5.3 million of its hens in response to a widespread bird flu outbreak (and then laid off nearly all of its staff).

Alicia Santurio disrupted a Minnesota Timberwolves basketball game in April, protesting the team’s owner, Glen Taylor, who also owns an Iowa egg farm that culled 5.3 million chickens in response to a bird flu outbreak.
Carlos Gonzalez/Star Tribune via Getty Images

The virus, known as the Eurasian H5N1 avian influenza, began tearing through Europe, Asia, and Africa in late 2021 and is still raging, with Europe experiencing its worst bird flu outbreak on record. It was first detected in the US in January and has since spread to at least 32 states, resulting in the death of more than 36 million chickens and turkeys and triggering a spike in egg prices.

While the virus has a near 100 percent mortality rate among infected poultry — and can spread rapidly among birds, especially in packed industrial farming conditions — it’s currently believed to pose little threat to human beings. It only rarely spills over to people, and only to those who come into close contact with infected birds. Even when there are human infections, “the viruses are unable to efficiently transmit between humans,” notes Michelle Wille, a virus ecologist at the University of Sydney.

But when certain strains of avian flu do manage to infect humans, it can be deadly. From 2003 to 2021, a little more than half of the 863 people who contracted an earlier strain of H5N1 died. The H5N1 strain currently spreading appears to be less transmissible and less severe to humans than those that infected people in the past, and only two people have tested positive for the strain — a man in the United Kingdom last December, and a man in Colorado last week.

The Colorado man — a prison inmate who had come in direct contact with presumably infected birds while working at a culling operation as part of a pre-release work program — experienced a few days of fatigue and recovered after being treated with an antiviral drug. Around 10 people who came into contact with him are under close observation.

Beyond the occasional one-off case in close human contacts, the bigger worry is that an unchecked flu that spreads among birds has plenty of opportunities to mutate in a way that allows it to transmit efficiently from person to person, thereby kicking off a new influenza pandemic. A widespread bird flu outbreak in 2005 raised alarm bells and prompted the US Senate to allocate $4 billion to prepare for a possible influenza pandemic — though when a new flu pandemic did break out in 2009, the origin was ultimately found in a swine virus.

So far, the bird flu has mostly been a problem for birds. It’s not the disease that’s killing most of them, however — it’s their owners.

When chicken, turkey, and egg companies detect one infected bird, they kill the whole flock in an effort to slow the spread of the virus. And they’re doing so using a variety of excruciating methods, including spraying birds with a suffocating water-based foam or closing off barn vents to raise temperatures so the birds die by heat stroke, a practice called ventilation shutdown, which can take 1.5 to 3.75 hours to kill them.


“It’s horrendous,” says Craig Watts, a former large-scale chicken farmer and currently a director of field operations for the Socially Responsible Agriculture Project, a nonprofit that advocates against industrial livestock operations. “I’ve been in those houses when the power went out and the generator didn’t kick on. In just a few minutes [the heat] is unbearable. … I can’t imagine that going on for hours and hours.”

According to the Storm Lake Times, a newspaper based near Rembrandt’s operation, the company used ventilation shutdown plus, or VSD+, meaning they also pumped heat into their barns to kill the birds faster, a practice being employed in several states. Rembrandt Enterprises did not respond to a request for comment.

The situation is horrific, but given the industrialized nature of the US poultry industry and its response to past bird flu outbreaks, animal advocates say it’s unsurprising. Nearly all birds raised for meat and eggs in the US are raised on factory farms, where producers raise hundreds of thousands to millions of animals per year. And most of these animals are genetically identical, which could make them more vulnerable to bird flu. Some experts say the intensification of animal farming — raising more animals closer together — could also be increasing the virulence and transmission rate of bird flu strains.

Dena Jones of the Animal Welfare Institute says the 2014-2015 bird flu outbreak in the US, which led to the culling of more than 50 million animals — the largest cull in US history — didn’t prompt any real change in the industry. Instead, mega operations that raise millions of birds per year are continuing to be built across the country, from Oregon to Wisconsin and West Virginia to North Carolina as US chicken and egg consumption rises.

“We’re doubling down on this same system by raising more animals with less genetic diversity and higher density in larger operations, and all of that contributes to making it difficult to humanely kill an animal during an emergency,” Jones said.

There are culling methods that kill the birds much quicker than ventilation shutdown, such as spraying them with nitrogen-filled foam or gassing them in small enclosures, a method some producers are using to address this outbreak. There’s also a race to create an effective bird flu vaccine that could be used to slow the spread of future outbreaks, a race the USDA is partially funding.

Considering the speed at which bird flu spreads among commercial poultry flocks, and how painful it is for infected birds, the industry has no choice but to mass cull. But the USDA’s approval of ventilation shutdown in 2015 and the rise of its use in recent years, combined with the slow pace of vaccine approval and adoption, mean that for the time being, the birds themselves will continue to receive little consideration in the fight against bird flu. The ongoing expansion and intensification of US animal agriculture, along with a rise in animal disease outbreaks, might also mean that we need to learn how to live with the bird flu and the looming threat it poses.

Bird flu spread, explained

Migratory waterfowl, like ducks, geese, and terns, are the natural hosts of highly pathogenic avian influenza strains, but can largely — though not always — carry and spread the virus without showing symptoms.

Wild birds rarely come into direct contact with farmed chickens and turkeys, most of which are raised in large indoor barns — especially in more developed economies — but instead spread the virus when their fecal droppings, saliva, or nasal secretions contaminate animal feed or land on surfaces like farmworkers’ clothing or farm equipment. Researchers say the global poultry trade also contributes to the worldwide spread of the bird flu through the import and export of infected poultry.

Once any birds test positive for the virus, the whole flock is culled, as the flu can quickly spread to the tens of thousands of other hens, chickens, or turkeys in a single farm. And the flu itself is agonizing for infected poultry. Chickens have trouble breathing and suffer from extreme diarrhea, and sometimes develop swelling around their head, neck, or eyes. Turkeys’ wings can become paralyzed and they might experience tremors.

A turkey farm in Illinois in 2019. Over the last few months, millions of chickens and turkeys have been culled — primarily at large-scale farms — to slow the spread of the bird flu.
Daniel Acker/Bloomberg via Getty Images

Bird flu outbreaks have been recorded in commercial poultry flocks since at least the 19th century, but the frequency accelerated — and became a bigger issue in the poultry industry — starting in 1997, when an outbreak of H5N1 in Hong Kong chicken farms led to 18 infections in people, six of whom died. Officials responded by culling all 1.3 million chickens in Hong Kong in the winter of 1997-98. Since then, outbreaks have occurred around the world every few years.

And not much beyond mass culling can be done to slow the spread once it starts. Adel Talaat, a professor of microbiology at the University of Wisconsin-Madison, says we should improve disease surveillance and farm biosecurity to help prevent new outbreaks and slow the spread, but a vaccine that could reliably reduce transmission would go a long way.

At the moment, there aren’t any highly effective vaccines on the market, but Talaat is working to develop one using a database of thousands of avian influenza antigens to create a “composite” vaccine that he hopes will protect against current and future virus strains. “Our job is to try to stop this cycle of transmission,” Talaat says. “Because if you stop the cycle of transmission you will be able to basically stop the mutation and stop the replication of the virus.”

A scientist with gloved hands hold a flask toward the camera.
Adel Talaat, a professor of microbiology at University of Wisconsin-Madison, is developing a bird flu vaccine he hopes can be used to slow the spread of future bird flu outbreaks.
Jeff Miller/UW-Madison

He estimates it could take up to five years until he completes his work and hopefully receives USDA approval, and says a mass vaccination program in the early phase of a bird flu outbreak could be effective at slowing the spread of the virus.

“In a big country [like the US], once we start seeing any one case, we know it’s going to go throughout the states — state by state — so we really should start an aggressive campaign for vaccination right away,” Talaat says.

Aside from the ineffectiveness of currently available bird flu vaccines, they’re also made in such a way that it’s impossible to distinguish vaccinated, non-infected birds from infected birds. And because no country wants to import meat from potentially infected birds, the vaccines have been a non-starter. Talaat hopes his vaccine will solve this long-standing problem.

A spokesperson with the National Turkey Federation told Vox over email that the trade group “supports vaccine development and believes it can be done relatively quickly. However, World Organisation for Animal Health (OIE) rules impose severe trade penalties for vaccine use, and we are encouraging USDA to work aggressively for a change in those rules.”

“Decisions on vaccinations require many data and we’re investigating an avian influenza vaccine that could distinguish from the wild-type virus,” Rick Coker, a USDA spokesperson, said over email. “We do not have a time frame on any potential vaccine or how it would be used.”

There are also efforts underway to create a gene-edited chicken breed immune to bird flu. But for now, the primary way to prevent the flu from killing poultry is by killing poultry.

Toward less cruel culling methods

During the 2014-2015 bird flu outbreak, the most common culling method in the US entailed spraying turkeys with suffocating water-based foam; with this method, it takes seven to 15 minutes for the birds to die, and it causes significant pain. The second-most common method was gassing hens with carbon dioxide in small enclosures, which can render birds unconscious within 30 seconds.

But according to the USDA, deploying these methods was sometimes too slow to meet the need of depopulating infected flocks within 24 hours. So, at the end of 2015, fearing another wave of outbreaks, the USDA approved ventilation shutdown — closing off air vents so the temperature rises, which can take hours for the birds to die by heat stroke. The USDA now says ventilation shutdown alone, without added heat or CO2, should only be used as a last-resort measure.

Over email, Coker with the USDA told Vox that ventilation shutdown plus should only be used under “constrained circumstances,” like when depopulation by water-based foam or CO2 gassing is not possible. Various factors, like epidemiological information and housing and environmental conditions are weighed by USDA personnel, farm operators, and state officials when deciding whether or not to use VSD+. “Should VSD+ be authorized on-site, responders will carry it out quickly and as humanely as possible,” he said.

Despite the policy to only use it under constrained circumstances, VSD+ has already been employed in at least six states and on millions of birds during this current outbreak.

Will Lowrey, an attorney with the animal rights group Animal Outlook who has submitted public records requests on VSD+, found that in addition to being used on the 5.3 million Rembrandt hens, it has also been used on commercial poultry farms in Kentucky, Delaware, Minnesota (Jennie-O/Hormel), Missouri (Tyson Foods), and Wisconsin.

Producers do have some incentive to use VSD+ over other culling options. To receive reimbursement for costs incurred during depopulation and disposal, they have to use a culling method permitted by the American Veterinary Medical Association (AVMA), a nongovernmental trade group, and VSD+ generally requires less labor and supplies than most other methods. But it’s an inhumane practice.

In the AVMA’s culling guidelines for VSD+, the organization cites research conducted at North Carolina State University in 2016 meant to replicate and study ventilation shutdown. Researchers placed one chicken at a time in a small enclosure and pumped in heat, carbon dioxide, or both. Animal Outlook obtained footage from that experiment via a Freedom of Information Act request and shared it with Marina Bolotnikova for the Intercept. You can view the experiment below (warning: it’s graphic).

In the video, a bird appears to be gasping for air, unable to stand, and according to a veterinarian interviewed by the Intercept, showing signs of attempting to vocalize (the video has no audio). It took around 91 minutes for the birds to die of just ventilation shutdown, 53 minutes when heat was added, 11.5 minutes when carbon dioxide was added, and nine minutes when both heat and carbon dioxide were added. Other research has found that times are much longer for hens in stacked cage systems, as opposed to turkeys and chickens raised for meat who live on barn floors.

A coalition of more than 1,500 veterinarians, appropriately called Veterinarians Against Ventilation Shutdown, say the process is inhumane and are calling on the American Veterinary Medical Association to classify it as “not recommended” for culling. An investigator with Direct Action Everywhere — the group that’s been disrupting Minnesota Timberwolves games — says they entered a Rembrandt facility after depopulation and allegedly found some birds who had survived ventilation shutdown plus.

“On the floor and in the cages we found … upwards of 100 chickens [still alive],” the investigator, who spoke with me on the condition of anonymity, said. “If you [extrapolate that for] the parts of the facility we didn’t go into, maybe several hundred chickens were still stuck in cages or running around loose.”

Jones says more humane methods need to be prioritized, like nitrogen-filled foam and small-enclosure gassing, which knock animals unconscious before they die.

Despite the challenges that come with these methods — increased costs and labor, among others — Watts, the ex-chicken farmer, says change would be a matter of the industry prioritizing animal welfare.

“I hear the industry argument about everything costing too much,” he said. “If they’re serious about animal welfare, you and I [wouldn’t be] having this discussion on what could be done better — they would already be doing it.” He wants to see the industrialized model that dominates US agriculture today — the model he once raised birds in — replaced by farms with smaller flock sizes, and where birds are given outdoor access and more space.

Factory-farming animals is an inherently risky business. And when a system that crams tens of thousands of birds together is faced with a highly-transmissible, lethal virus, that system is largely defenseless. At best, industry can work to minimize harm, but only if it’s willing to pay increased costs. But the conditions on today’s meat and egg farms — and the approval and adoption of ventilation shutdown — demonstrate a drive toward efficiency, not welfare.

“In the short term, it would be my preference to see something more painless and quick” used to cull the birds, says Watts. “In the long term, what we’re looking at is a very flawed system — it’s time to just basically push it off a ledge and reboot and start over.”

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