The avian flu outbreak that has more than doubled egg prices across the country has also led to the death of more than 48 million birds in a dozen states, according to the US Department of Agriculture.

Iowa, the hardest hit, has euthanized more than 31 million birds, including approximately 40% of the state’s 60 million laying hens, according to Randy Olson, executive director of the Iowa Poultry Association. Turkey farmers in the state, while affected to a lesser degree, also have suffered. Minnesota, the leading turkey producer, has lost nearly 9 million turkeys.

The massive challenge of disposing of these sick birds illustrates the scale of chicken farming in the US. When avian flu infects a single bird on a chicken farm, the whole population has to be destroyed in order to stop the spread. In Iowa, for example, where an egg farm holds anywhere from 70,000 to 5 million birds, infection means slaughtering an unimaginable number of animals.

“It’s reasonable when we see these outbreaks to wonder if they are a manifestation of the unsustainability of the system,” says Suzanne McMillan, senior director of the farm and animal welfare campaign at the American Society for the Prevention of Cruelty to Animals (ASPCA). “Bird flu is a window into how today’s poultry flocks live day to day, in intensive confinement and unsanitary conditions. It’s an unnatural, unsustainable situation.”

When an infected bird is detected on a farm, it is immediately quarantined and the USDA works in conjunction with the farmer to determine the best method for disposing of the exposed flock. “It is important to realize that each [avian influenza] outbreak incident is unique and involves site specific conditions that need to be considered in making the best disposal decision for the situation at the site,” reads a 2006 paper by the Environmental Protection Agency.

The reality is often much more emotional than that language might imply. “These farmers spend their entire careers caring for their animals and to see the disease affect their flocks … is an emotionally devastating event,” Olson says.

For egg-producing chickens, the majority of which are housed in cages, euthanasia by carbon-dioxide gas is the only method approved by the USDA.

Meanwhile for turkeys and broiler chickens, which are floor-reared (ie not kept in cages), the use of water-based foam similar to that used by firefighters is the most effective way to euthanize a large flock in a short period of time, says Beth Carlson, North Dakota’s deputy state veterinarian.

Approved for use by the USDA in 2006, the water-based foam suffocates the birds, which are typically herded into an enclosed area of the barn so that the foam can be deployed more quickly.

The foam’s active ingredient, propylene glycol, breaks down “relatively quickly (within several days to a week) in surface water and in soil”, according to the US Agency for Toxic Substances and Disease. The foaming agent, lauryl alcohol, is acutely toxic to marine animals, but has been deemed harmless to human health.

The administration of the foam – either by nozzle or, more commonly for commercial farms, a foam generator – is a rolling process, meaning workers apply foam to birds at one end of the house, and then work their way, contained area by contained area, to the opposite end.

The foam needs to be six to 10 inches higher than the birds in order to suffocate them. Pumping in enough foam to cover their heads can take up to an hour.

“Individual birds are only exposed for a matter of seconds,” says Dr Eric Benson, a professor in the University of Delaware’s animal and food sciences department, who co-authored a paper on mass euthanasia methods for poultry. He says animals typically die within a minute.

In April, two turkey farms in North Dakota were infected and more than 100,000 birds euthanized as a result. Both farms used water-based foam.

“Euthanasia is never a pleasant thing to have to do,” Carlson says. But foam “is fast and minimally stressful for the birds, which means it’s less stressful for the people involved”.

Not all agree that using foam is humane. “We could do better,” says Michael Blackwell, chief veterinary officer at The Humane Society of the United States, who likens death by foam to “cuffing a person’s mouth and nose, during which time you are very much aware that your breathing has been precluded”.

Unlike Benson, he estimated that average death takes between three and seven minutes: “For any animal, that would be distressing.”

For floor-reared flocks, euthanasia by water-based foam is the cheapest and most efficient method available, as open-air barns do not need to be covered and the application requires fewer people to administer. And yet, says Blackwell, it is possible for farmers to use more humane alternatives by tarping floor-reared animals and administering inert gases, such as nitrogen and argon, which cause the birds to drift off, as if going to sleep.

Based on the size of the flock and local conditions, the dead birds are composted on-site (either indoors or outdoors), composted off-site, buried, moved to landfills or incinerated. Composting – in which dead birds are laid in rows, mixed with a bulking agent such as wood chips or saw dust and then left for approximately 30 days – is the preferred method of disposal, Benson says, because the virus is killed by the heat produced as the birds decompose. The problem with landfills, according to Benson, is that the virus might contaminate the soil or groundwater.

From backyard birds to commercial flocks

The avian influenza outbreak, which originated from the droppings of waterfowl carrying the virus, is showing signs of tapering off as temperatures continue to rise across the country. The virus is weakened by warm, dry conditions.

“The number of infections has certainly slowed in Iowa,” Olson says. “We are hopeful that this current outbreak is near its end.”

However, experts predict that the virus will return in the fall, when cooler, damp weather provides favorable conditions for its spread, and as ducks and other waterfowl complete reverse migration patterns.

While a report from the USDA, which investigated 80 commercial poultry farms, “cannot at present point to a single statistically significant pathway” for the spread of the outbreak, it determined that “a likely cause of some virus transmission is insufficient application of recommended biosecurity practices”.

The USDA report went on to recommend that all equipment and vehicles, as well as workers’ clothing, be disinfected before moving between farms.

“We need to learn lessons from this outbreak and modify biosecurity practices to minimize future outbreaks,” says Olson, citing a lack of consistency in the industry when it comes to auditing avian health hazards, as well as a need for more comprehensive guidelines for vehicle disinfection, as areas that require stricter standards. “This disease does not discriminate. We’ve seen it in backyard flocks; we’ve seen it in wild birds; we’ve seen it all types of commercial housing.”

Hon S Ip, a virologist at the US Geological Survey’s National Wildlife Health Center, disagrees with this assessment. During the initial outbreak starting in December, the disease was detected mainly in wild flocks in the Pacific Flyaway, a bird migration path from Alaska to Patagonia. But “in the Midwest there have been more commercial flocks infected than backyard flocks,” he says.

An ‘unsustainable system’

This is likely, at least in part, because sunshine and warm backyard temperatures are effective at killing the virus, says Dr Michael Greger, director of public health and animal agriculture for the Humane Society of the United States.

Commercial poultry farms, on the other hand, “are designed like a disease incubator”, thanks to dark, moist and crowded conditions.

While factory poultry are more isolated, “when infected, [factory-farmed birds] are subject to wildfire-like outbreaks”, says Michael Davis, author of The Monster at Our Door: The Global Threat of Avian Flu.

On top of that, the genetic makeup of birds found in factory farms is often less diverse than those raised in backyard flocks. Due to the industry’s reliance on homogenous breeding techniques, commercially raised broilers are all pretty much genetically identical. Broilers and turkeys are artificially selected and bred to produce birds that grow quickly – at a rate 300% faster than those birds raised in 1960, according to the ASPCA – and produce as much breast protein as possible, to the point where the birds have a hard time standing upright.

Not only do commercial flocks share a limited gene pool, but some studies have suggested the industry’s vise-like focus on breast meat, in the case of broilers and turkeys, and eggs, in the case of hens, suppresses the birds’ immune systems, a theory known as resource allocation.

When a bird is bred so that all its energy goes to the production of meat or eggs, “something has to give”, says the ASPCA’s McMillan. “The science indicates that a bird’s immunity goes down.”

As Greger puts it: “There is an inverse relationship between accelerated growth and disease resistance, which means faster-growing birds are more susceptible to illness.”

While the USDA terms this outbreak “a wake-up call on biosecurity”, the idea of hermetically sealing farms, which use ventilation fans to keep birds cool, may be too difficult to enforce. “The industrial poultry system, by its very nature, is vulnerable to these kinds of infections,” he says.

It’s the system that is at fault, according to McMillan. “We are forcing birds to live in unbalanced ways, both physically and genetically,”she says. Commercial poultry flocks “are bred to suffer. We force them to live a life of misery, and from that perspective, they are going to be more prone to contracting and spreading disease. These are not healthy, balanced animals.”