New Study Shows Staggering Increase in Heatstroke-Induced Chicken Culls

A new study published in the journal Animals concludes that U.S. pig and poultry producers are increasingly inducing heatstroke to cull farmed animals en masse. As a rift grows in the nation’s veterinary community and the country experiences the worst outbreak of avian flu in history, the study’s authors urge the American Veterinary Medical Association to recommend “more humane methods.” 

Mass culls — often referred to as “depopulation” by the industry — have dramatically increased on industrial animal farms since the onset of the pandemic. Now with highly virulent strains of avian influenza spreading through poultry farms, more than 50 million U.S. birds have been culled prior to when they would normally be slaughtered.

To make matters worse, the methods used for these culls have changed too. According to the study, poultry producers have increasingly turned to controversial methods like ventilation shutdown plus — a process by which all ventilation to a barn is closed and fans are turned off as the temperature is increased — ultimately causing the animals to die of heatstroke.

While figures for all poultry farms across the U.S. aren’t known, the study did find a significant increase in the use of ventilation shutdown plus in depopulations carried out by poultry factory farms. Between February and August of last year, a staggering 51 percent of commercial depopulations relied on ventilation shutdown, either on its own or in addition to other methods, to cull birds. That’s a dramatic increase from just seven years prior — between 2014 and 2016, the study found, commercial poultry operations used the ventilation shutdown method in only 4 out of 224 of its mass culls. 

Heatstroke Kill Methods Cause Pain, Delirium and Fear in Birds

The rising use of “heatstroke-based” methods “is profoundly detrimental to animal welfare,” the study concludes. Heatstroke can cause animals to suffer pain, nausea, thirst, anxiety, dizziness and delirium among other reactions, the authors write. Effects may go beyond our current knowledge, too, as researchers caution that it is not known how long animals exposed to shutdown plus may remain conscious. 

This new study is significant, says its lead author and a veterinarian and advisor to the nonprofit Animal Welfare Institute, Dr. Gwendolen Reyes-Illg, in part because there have only been two other published peer-reviewed papers examining the use of heatstroke for depopulation, “despite tens of millions of birds, and hundreds of thousands of pigs, being killed by these methods in recent years.” 

“Neither previous paper addressed the animal welfare implications of mass killing by induction of heatstroke,” says Reyes-Illg, who was not involved with the earlier studies. But the welfare implications are potentially severe — in examining animals suffering from heatstroke, Reyes-Illg told the Guardian that she has seen “chunks of mucosa and blood come pouring out of the rectum and vomiting of blood is common as well.” 

“Veterinarians have an ethical responsibility to protect and advance animal welfare,” she argues, “yet there seems to be little awareness within the professional community that heatstroke-based depopulation methods are becoming the dominant means of mass on-farm killing of birds in the U.S., causing untold suffering.” 

USDA and Industry Look to Veterinary Association for Depopulation Guidance

The Animal Welfare Institute calls shutdown plus “the most inhumane” of culling methods available, noting that even though the association guidelines are technically voluntary, they are relied on by the USDA in guiding oversight of the industry’s depopulations amid disease outbreaks. 

The industry relies on these guidelines too, making it known that their methods are approved by the association, the study authors say. “This effectively endows the U.S. veterinary profession with substantial power and responsibility when it comes to animal depopulation.”

The authors argue this issue “represents an urgent ethical problem for the U.S. veterinary community” requiring the profession to “champion needed change.” But the association has yet to take a stance against the culling method it recommends. 

In its last depopulation guidelines published in 2020, the group wrote that the addition of heat, or heat plus carbon dioxide, effectively achieves death in an acceptable amount of time. The shutdown of ventilation on its own is slower, and therefore is not recommended by the association. 

Yet a growing number of veterinarians say the association’s guidelines do not go far enough. Ventilation shutdown — in any form — is fiercely opposed by animal protection groups and, increasingly, veterinarians. 

Dr. Ernie Ward, member of Veterinarians Against Ventilation Shutdown, wrote in a petition to the association that the practice causes animals to suffer “unbearable agony and a slow death.” The group’s petition was denied in November 2022.

Some veterinarians are afraid to criticize the animal agriculture industry that employs so many of their colleagues. Veterinarian Daniela Castillo writes that she hesitated to comment on shutdown plus, for example, having seen “conversations become adversarial and even hostile, resulting in a fracturing of the veterinary profession.” Yet Castillo ultimately decided “this topic is so important that I cannot be silent.” 

A Push for Policy Change

In addition to the association changing its stance on ventilation shutdown plus to “not recommended” and suggesting “potentially more humane options, such as high-expansion, nitrogen gas-filled foam,” Reyes-Illg hopes the association will put in place standards that help to reduce the chance that farmed animals will not need to be culled in the first place.

“I would like to see the association engage a multidisciplinary team of veterinarians and ethicists to develop strategies for decreasing our patients’ risk of dying by depopulation,” she says. 

Dr. Reyes-Illg hopes that the association will also recommend that the USDA require producers to establish “emergency response plans” laying out how their farms would be depopulated within a span of 48 hours “without resorting to VSD+Heat or other inhumane methods.”

Ultimately, meaningful change may require a new mandate in the form of legislation regulating culls, such as the proposed Transparency in Depopulation Act. 

Says Reyes-Illg, potential improvements could include developing and requiring vaccines against avian flu or policies that require a limit in animal stocking densities — “so that a failure of biosecurity in a single barn doesn’t lead to the destruction of millions of birds.”

This piece has been updated to reflect that Dr. Reyes-Illg is the lead author of the new study and was not involved in the two earlier studies on heatstroke-based culling.