La propagación de una gripe aviar que es mortal para las aves de corral plantea la espeluznante pregunta de cómo las granjas logran matar y deshacerse rápidamente de millones de pollos y pavos.

Es una tarea a la que se enfrentan cada vez más las granjas de todo el país, ya que la cantidad de aves de corral muertas en los últimos dos meses ha aumentado a más de 24 millones, con brotes reportados casi todos los días. Algunas granjas han tenido que sacrificar más de 5 millones de pollos en un solo sitio con el objetivo de destruir a las aves en 24 horas para limitar la propagación de la enfermedad y evitar que los animales sufran.

"Cuanto más rápido podamos llegar al sitio y despoblar las aves que quedan en el sitio, mejor", dijo Beth Thompson, veterinaria del estado de Minnesota.

El brote es el mayor desde 2015, cuando los productores tuvieron que sacrificar más de 50 millones de aves. En lo que va del año, ha habido casos en 24 estados, siendo Iowa el más afectado con alrededor de 13 millones de pollos y pavos muertos. Otros estados con brotes considerables incluyen Minnesota, Wisconsin, Dakota del Sur e Indiana.

Las granjas ante la necesidad de sacrificar tantas aves recurren a las recomendaciones de la Asociación Médica Veterinaria Estadounidense. A pesar de que ha desarrollado métodos para matar a las aves de corral rápidamente, la asociación reconoce que sus técnicas "pueden no garantizar que las muertes que enfrentan los animales sean indoloras y libres de angustia". Los veterinarios y los funcionarios del Departamento de Agricultura de EE. UU. también suelen supervisar el proceso.

Uno de los métodos preferidos es rociar espuma contra incendios a base de agua sobre las aves mientras deambulan por el suelo dentro de un establo. Esa espuma mata a los animales al cortarles el suministro de aire.

Cuando la espuma no funciona porque las aves están en jaulas sobre el suelo o hace demasiado frío, el USDA recomienda sellar los establos y colocar dióxido de carbono en el interior, primero dejando inconscientes a las aves y finalmente matándolas.

Si uno de esos métodos no funciona porque el equipo o los trabajadores no están disponibles, o cuando el tamaño de una bandada es demasiado grande, la asociación dijo que el último recurso es una técnica llamada apagado de ventilación. In that scenario, farmers stop airflow into barns, which raises temperatures to levels at which the animals die. The USDA and the veterinary association recommend that farmers add additional heat or carbon dioxide to barns to speed up the process and limit suffering by the animals.

Mike Stepien, a spokesman for the USDA's Animal and Plant Health Inspection Service, said the techniques are the best options when it's necessary to quickly kill so many birds.

"State animal health officials and producers carefully weigh the different options to determine the best option for humane depopulation and do not make such decisions lightly," Stepien said.

Not everyone agrees.

Animal welfare groups argue that all these methods for quickly killing birds are inhumane, though they are particularly opposed to ventilation shutdown, which they note can take hours and is akin to leaving a dog in a hot car. Animal rights groups delivered a petition last year signed by 3,577 people involved in caring for animals, including nearly 1,600 veterinarians, that urged the veterinary association to stop recommending ventilation shutdown as an option.

"We have to do better. None of these are acceptable in any way," said Sara Shields, director of farm animal welfare science at Humane Society International.

Opponents of the standard techniques said firefighting foam uses harmful chemicals and it essentially drowns birds, causing chickens and turkeys to suffer convulsions and cardiac arrest as they die. They say carbon dioxide is painful to inhale and detectible by the birds, prompting them to try to flee the gas.

Karen Davis, of the nonprofit group United Poultry Concerns, urged the veterinary association to stop recommending all of its three main options.

"They're all ways that I would not choose to die, and I would not choose anybody else to die regardless of what species they belong to," Davis said.

Shields said there are more humane alternatives, such as using nitrogen gas but those options tend to be more expensive and could have logistical challenges.

Sam Krouse, vice president of Indiana-based MPS Egg Farms, said farmers feel miserable about using any of the options.

"We pour our lives and livelihoods into taking care of those birds, and it's just devastating when we lose any of those birds," Krouse said. "Everything that we're doing every day is focused on keeping the disease out and making sure that we're keeping our hens as safe as possible."

Officials emphasize that this virus that's spread primarily through the droppings of infected wild birds doesn't threaten food safety or represent a significant public health threat. Sick birds aren't allowed into the food supply and properly cooking poultry and eggs kills any viruses that might be present. And health officials say no human cases of bird flu have been found in the United States during this current outbreak.

Once poultry are dead, farmers must quickly dispose of the birds. They usually don't want to risk the chance of spreading the virus by transporting the carcasses to landfills, so crews typically pile the birds up into huge rows inside barns and combine them with other materials, such as ground up corn stalks and sawdust to create a compost pile.

After a couple weeks of decomposition, the carcasses are converted into a material that can be spread on cropland to help fertilize crops. In some cases, carcasses are buried in trenches on the farm or incinerated.