From: http://www.theguardian.com/vital-signs/2015/jul/14/bird-flu-devastation-highlights-unsustainability-of-commercial-chicken-farming

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.”