U.S. could use hyperthermia, asphyxia to fight bird flu

Federal agriculture authorities may elect to kill some bird flocks through combined hyperthermia and asphyxia in their efforts to save other flocks during highly infectious disease outbreaks.

Citing delays in depopulating flocks in response to this year’s highly pathogenic avian influenza virus outbreak, Department of Agriculture officials indicated they will consider whether to shut barn doors and vents to depopulate flocks more quickly than they can through other methods.

Dr. T.J. Myers, associate deputy administrator for Veterinary Services in the USDA Animal and Plant Health Inspection Service, said selective use of that depopulation method—known as ventilation shutdown­—could prevent disease spread because of its speed. It could replace the days long work of immersing batches of birds in carbon dioxide or water-based foam by depopulating an entire barn population in less than 24 hours.

Dr. Karen Burns Grogan, who spoke to JAVMA on behalf of the American Association of Avian Pathologists and is executive vice president of the American College of Poultry Veterinarians, said the method would be used only during an animal health emergency with potential for disastrous consequences. And it could save the lives of millions of birds.

“If our traditional depopulation methods are not applicable or useful or available, then we want to have alternative methods available to effectively depopulate diseased populations,” she said. “We’re not talking about healthy birds. These are birds that have tested positive, and we are trying to eliminate the spread of a very deadly virus and to prevent the pain and suffering of potentially millions of other birds.”

AAAP information indicates ventilation shutdown combines rises in heat and carbon dioxide levels, which causes birds to become lethargic, lose consciousness, and die.

During this past spring’s outbreak of a highly pathogenic H5N2 avian influenza virus, APHIS worked with farmers and contractors to depopulate sometimes millions of chickens per egg farm and hundreds of thousands of birds per turkey farm. The virus resulted in the death of about 50 million birds, including more than 40 million chickens.

Delays in depopulating infected birds aided the rapid spread of the virus, and APHIS since has set a goal of depopulation within 24 hours of presumptive positive identification of a highly pathogenic avian influenza virus infection, according to a USDA policy statement published in September.

“There is strong evidence that a delay in depopulation results in an exponential increase in the total amount of HPAI virus shed into the environment by infected poultry, highlighting the imperative for rapid depopulation to control and contain an outbreak,” according to the statement.

Carbon dioxide and foam remain the primary tools for depopulation, but ventilation shutdown is among the adjunct methods usable if the primary tools are insufficient. It should be used only after concluding that no other methods could give timely assurance that the virus would not spread, according to APHIS.

Dr. Burns Grogan said that, in addition to sealing the barns during ventilation shutdown, those conducting the depopulation may also turn on heaters within the houses. The time needed between shutdown and death will vary depending on the poultry house population and design, but death of the entire flock likely would occur within several hours.

She also expects that ventilation shutdown would be used only under veterinarian supervision and with USDA guidance.

Chad Gregory, CEO of the United Egg Producers, provided a statement that rapid depopulation is the most humane and responsible response to an avian influenza outbreak, and the organization welcomes guidance from the USDA and others who are researching options for delivering humane deaths.

“Egg farmers understand that hard decisions must be made during this time of rapid disease spread, but there is widespread agreement that acting to depopulate within the first 24 hours is the most effective strategy to mitigate the spread of AI,” he said.

The AVMA published in May 2015 a statement that, because the highly pathogenic avian influenza virus was spreading faster than could be controlled through conventional means, exceeding capacities of regulators to depopulate using preferred methods, “the use of less ideal methods that result in a quick death for birds and support disease containment may be necessary.” But that statement implored use of such a method only on a case-by-case basis and after “extreme care” to ensure it was a justified last resort.

The AVMA expresses support only for foam-based methods of depopulating poultry birds, but an AVMA panel is studying depopulation methods in an effort to produce guidance similar to what the Association provides on euthanasia.

The Humane Society of the United States opposes the use of ventilation shutdown, likening the method to “mass baking of live chickens.” Dr. Michael Blackwell, chief veterinary officer for the HSUS, said in a statement that the method is a “miserable and protracted” means of killing birds.

Dr. Blackwell said in an interview that he thinks the influenza virus can be contained when found within a facility, providing time to start using other depopulation methods while most birds remain healthy.

“There should be, under conditions of containment, sufficient time to use reasonably humane methods for this terrible task,” he said.

But Dr. Blackwell also said ventilation shutdown could be warranted under extreme circumstances, and he has asked that the USDA define its conditions for use.

Dr. Myers noted that the H5N2 virus could persist in wild dabbling ducks. And APHIS, with the U.S. Fish and Wildlife Service and U.S. Geological Survey, has increased surveillance for highly pathogenic avian influenzas in an effort to provide warning when the viruses are present.

APHIS officials hope to collect 30,000 to 40,000 samples from wild birds by summer 2016.

The Canadian Wildlife Health Cooperative also plans to test live and dead wild birds across the country for the presence of avian influenza. The testing is on behalf of the Canadian Food Inspection Agency.

Dr. Myers also thinks farms are better prepared in case of another incursion from the H5N2 or another highly pathogenic avian influenza virus.

Disease controls set up before the 2015 outbreak had been intended to stop pathogens at farm borders, but migrating birds spread the virus to the ground just outside barns, Dr. Myers said. He thinks the outbreak showed a need for a change in the biosecurity concept, focused on keeping environmental pathogens out of barns rather than from crossing borders.

But addressing the spread of viruses among multiple barns on a single farm—such as those connected by a single egg collection belt—is a substantial long-term challenge that will require structural changes, Dr. Myers said.