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Photo by Joshua Roberts / Alamy stock photo

Chickens at a Delaware farm in 2013. The American Veterinary Medical Association is seeking comment on guidelines for veterinarians who are called upon to cull large numbers of birds or other livestock.

Debate over the ethics of applying ventilation shutdown, a controversial method of killing livestock through heat stroke and suffocation, is again rising to the fore amid the publication in the United States of a new set of professional guidelines for killing animals en masse.

A highly anticipated update to the American Veterinary Medical Association's Guidelines for the Depopulation of Animals has been published in draft form and made available to the association's more than 100,000 veterinarian members. They have until Jan. 30 to comment on the draft ahead of the completion of a final version later this year.

An analysis of the 531-page document by the VIN News Service indicates that the AVMA's Panel on Depopulation still deems ventilation shutdown, while undesirable, to be an acceptable course of action in certain circumstances for swine and poultry. The range of those circumstances, however, has been narrowed compared with the existing guidelines with regards to killing swine.

Moreover, more humane methods of depopulation — such as spraying animals with nitrogen-infused foam — have been introduced to the guidelines for the first time.

Although the AVMA is not a regulatory body, its guidelines are incorporated into depopulation policy enforced by the U.S. Department of Agriculture. According to the AVMA, the new draft guidelines represent more than two years' worth of deliberation by more than 70 individuals, including veterinarians and animal scientists, among other professionals.

Demand for methods to kill a large number of animals can arise for various reasons. They may be harboring fast-spreading disease, including zoonotic disease, which is transmissible to humans. They may be hurt during a natural disaster, and the humane response is to end their suffering. Or they could be an economic burden to their owner due to an oversupply of livestock.

Various methods of depopulation are at veterinarians' disposal, some more appropriate for certain types of animal than others, and each comes with its own advantages and disadvantages. For example, killing pigs via gunshot is considered among more humane methods because it is quick and relatively painless. But it isn't practical for a factory farm looking to dispatch thousands of pigs at once.

Ventilation shutdown (VSD) involves sealing up a building in which animals are housed, causing them to die from a lack of oxygen and rising heat. Their death can be hastened by pumping heat and/or carbon dioxide into the sealed building, a method known as ventilation shutdown plus (VSD+). Even then, it can take an hour or longer for animals to die.

Other methods of depopulation include stunning animals with penetrating bolts, electrocution, gassing with carbon monoxide or smothering in foam of various types. Each comes with challenges, such as procuring the necessary equipment and raw materials. Oftentimes, livestock producers prefer VSD because it is cheaper and easier to perform.

The AVMA's existing depopulation guidelines, published in 2019, offer latitude to use VSD+ on poultry and swine in a number of situations. For example, for swine, they state that VSD+ is "permitted in constrained circumstances," adding that potential justifications might include "constraints on zoonotic disease response time, human safety, depopulation efficiency, deployable resources, equipment, animal access, disruption of infrastructure, and disease transmission risk."

The existing guidelines categorize depopulation methods as either "preferred," "permitted in constrained circumstances" or "not recommended." In the draft updated guidelines, these categories are labeled "Tier 1," "Tier 2" and "Tier 3," respectively. Though the names have changed, the scenarios listed as acceptable in each category are more or less the same as before.

In a notable change, the use of VSD+ in swine has been downgraded from "permitted in constrained circumstances" in the existing guidelines to Tier 3 in the draft update. Methods categorized in Tier 3 are those that have "limited to no evidence" to support their use, according to the draft guidelines. Examples of situations in which they may yet be used include "structural collapse or compromise of buildings housing animals, complete inability to safely access animals for a prolonged period of time or any circumstance that poses a severe threat to human life."

The use in swine of VSD alone — without added heat and/or carbon dioxide — is clearly discouraged under any circumstances. The new draft states: "There is no data to support that VSD by itself will result in complete and effective depopulation for swine. Its use is not recommended in swine."

For poultry, the key advice is little changed. The 2019 guidelines and the draft update both classify VSD+ in the second category, and VSD alone is classified in the third category.

"Ventilation shutdown alone as a depopulation method is a last resort and must only be considered when all other options have been thoughtfully considered and ruled out," the draft states in relation to poultry. It references the ongoing outbreak of highly pathogenic avian influenza (HPAI) as a circumstance in which the use of VSD alone might be necessary to stop the spread of the virus as quickly as possible.

Mixed response from the profession

Some practitioners, including animal welfare advocates, have a lukewarm view of the draft updated guidelines, welcoming some changes — such as the downgrading of VSD+ for pigs to the lowest classification — while expressing disappointment that some elements are unchanged.

Other practitioners maintain that the new guidelines strike the right balance between upholding animal welfare and recognizing the need to resort to less-desirable means of depopulation when time is limited.

The Veterinary Association for Farm Animal Welfare (VAFAW), a nonprofit that identifies itself as the only veterinary group in the U.S. devoted solely to the welfare of production animals, hosts on its website a letter to the AVMA criticizing several elements of the draft document. The VAFAW has invited any concerned veterinarians and veterinary students to sign the letter. The full list of signatories will be attached to the letter as an appendix.

"We recognize and support several improvements in this updated edition, including the addition of higher-welfare, nitrogen-based depopulation methods for poultry and pigs in Tier 1," the letter states. "These practical, scalable methods, including nitrogen whole-house gassing and high-expansion nitrogen-filled foam, result in rapid loss of consciousness without pain and with little or no distress."

A primary author of the letter is Dr. Gwendolen Reyes-Illg, who works with both VAFAW and the Animal Welfare Institute, a nonprofit based in Washington, D.C. Among the letter's key requests is that VSD+ be downgraded to the bottom tier for poultry and described explicitly as "not recommended."

In an interview with VIN News, Reyes-Illg said she also would have liked to see the guidelines discuss ways to reduce animals' vulnerability to circumstances that result in depopulation, such as capping the number of individuals that can be kept in a single barn or strengthening food supply chains so they're more resilient to shocks like the Covid-19 pandemic. "The guidelines also present depopulation as a 'rare occurrence,' even as it has become routine in animal agriculture, in response to HPAI as well as to financial issues, like corporate bankruptcy," Reyes-Illg said.

Dr. George Bates, a veterinarian in Chambersburg, Pennsylvania, has previously expressed concerns about the existing guidelines on message boards of the Veterinary Information Network, an online community for the profession and parent of VIN News. When contacted by VIN News about the new draft guidelines, he, too, questioned why VSD+ had been moved down to the third tier for swine only.

"A glaring hypocrisy is that while VSD+ is not recommended for swine, it is still considered by the panel as acceptable — under 'constrained circumstances' — to bake poultry to death," he said. "Basically, the AVMA panel seems more intent on ratifying what are already standard industry practices rather than setting a higher standard that could truly make depopulation more humane."

Among practitioners who are more supportive of the guidelines is Dr. Michelle Kromm, an avian medicine specialist and consultant for the poultry industry who assisted in their production. Kromm said that although she understands the revulsion to VSD+, the death of entire flocks from HPAI is a "devastating outcome."

It may be harder for people to grasp why depopulation is a viable solution if they've never seen a barn full of turkeys succumbing to HPAI, she said.

Kromm has witnessed fatality rates in excess of 95% from HPAI in infected flocks. "What makes the situation of not depopulating even worse is the incredible amount of virus that the birds shed into the barn environment, creating what I picture as clouds of viral particles putting other animals in the neighborhood at greater risk. Allowing infected flocks to die from the disease is tragic, and while some argue that depopulation is even worse, what I find most troubling is failing to do everything possible to protect other at-risk animals."

Decision making during end-of-life situations is "incredibly complex," Kromm said, adding that it's not always possible to predict every aspect of an emergency situation. "It is critical that as many tools as possible be thoroughly considered and available to handle the diverse array of circumstances encountered."

Dr. Justin Kieffer, an associate professor at Ohio State University and member of the AVMA's Panel on Depopulation, said the guidelines need to recognize that killing large numbers of birds can be a logistically challenging task, particularly when it comes to caged laying hens.

"It is incredibly difficult to apply any type of foam system to these birds compared to floor-raised broilers and turkeys," Kieffer told VIN News. "VSD+ can be applied relatively quickly in these situations — usually HPAI outbreaks — to depopulate these barns to prevent days of suffering of these birds and minimize risk to humans tasked with removing the birds." By contrast, he said, swine can be removed from their housing to deploy other forms of depopulation far more easily.

Water-based foam, blunt force trauma also in focus

Another form of depopulation introduced in the draft guidelines is water-based foam, which kills by blocking the airway. Unlike foam infused with nitrogen or carbon dioxide, no gases are added to aid in the onset of unconsciousness.

For swine, water-based foam in the draft guidelines has been placed in the Tier 1 category, sparking contention among practitioners.

The letter that Reyes-Illg helped write asks that the method be lowered to Tier 3. The letter quotes an animal welfare panel of the European Food Safety Authority that death by water-based foam is "highly painful" and "equivalent to death by drowning or suffocation."

Bates, the veterinarian in Chambersburg, shared the VAFAW's enthusiasm for nitrogen-based foam and its concern about water-based foam. "The latter method is simply a form of dry drowning and cannot by any stretch be considered ‘humane' or acceptable in any air-breathing species," he said.

By contrast, Ohio State University's Kieffer contends that water-based foam acts in a similar way to foam infused with nitrogen or carbon dioxide, in that all three methods fundamentally result in anoxia — a severe deprivation of oxygen. "There are some differences in the timing of cessation of movement between those three methods in our studies, but again, they all work in the same general manner, and they are a few minutes in duration versus one hour for VSD+."

Studies assessing the effectiveness of different foam-based methods have produced varying results. Research into water-based foam conducted by Kieffer and colleagues at OSU found it could cause pigs to lose consciousness in as little as 191 seconds with a 13-second margin of error. As for nitrogen-infused foam, research funded by the National Pork Board from dues collected from U.S. pork producers showed a loss of consciousness in an average time range of 63.33 to 169.25 seconds — with the time affected by factors like the pigs' age and size. Separate research conducted by Swedish scientists found that nitrogen-infused foam rendered pigs unconscious in an average of 57 seconds.

Kieffer refuted the notion that using water-based foam is akin to drowning. "The airways of animals depopulated with water-based foam are not filled with water, as it would be in the case of drowning," he said. "The lesions present in the lungs of these animals are not consistent with those found in drowned animals or people. We have new, unpublished data that will be reported out on that here this year."

Countering those assertions, Reyes-Illg said a paper currently in pre-print indicates that pigs killed with water-based foam are more likely to have pulmonary edema — an accumulation of fluid in the lungs. That finding, she maintains, chimes with previous veterinary forensics research involving dogs. Still, Reyes-Illg contends that death by airway obstruction puts animals in a "severe negative welfare state," whether they're considered to be drowning or not.

On a separate issue, signers of the critical letter also want to see another method for killing pigs — manual blunt force trauma — reclassified from Tier 1 in the draft guidelines to Tier 3. Although the method may result in instantaneous loss of consciousness, a high level of skill is required. The method can cause "prolonged and significant pain and distress when performed imperfectly," the letter states.

Kieffer noted that blunt force trauma is sanctioned in the AVMA's euthanasia guidelines (a separate document published in 2020) for euthanizing neonatal piglets, although the document notes there is a higher risk of error compared with other physical methods and advises practitioners to seek more effective alternatives.