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A Chicken Disease More Deadly Than Avian Flu Could Destroy the Poultry Industry

Marek's disease can decimate poultry stocks in a matter of days. The good news: a vaccine exists. The bad news: it makes the virus even deadlier.
Photo via Flickr user sashafatcat

By now, you should already know about bird flu. M.I.A. made a song about it way back in 2007 (well, sorta), and more recently it's become the reason why US poultry farms have exterminated over 45 million chickens. It's nasty, virulent stuff.

But avian influenza, as it's also known, might not be the harbinger of the chicken apocalypse. In fact, US Secretary of Agriculture Tom Vilsack announced last week that the USDA is currently in the process of testing a bird flu vaccine with a reported 100-percent success rate.


Time to celebrate? No, not quite yet.

PBS Newshour reported this week that an even deadlier disease could decimate poultry stocks in a matter of days. Marek's disease, caused by a form of herpesvirus, can result in afflicted chickens developing widespread tumors and paralysis before killing them.

A vaccine for Marek's exists. Worryingly, however, it won't stop vaccinated chickens from spreading the virus to their unprotected brethren.

Even more alarming is the fact that the vaccine actually makes the disease kick into overdrive, allowing it to spread longer and faster, according to a study recently published in PLOS Biology.

As PBS Newshour explains, the Marek's vaccine is considered "leaky" because it "keeps a microbe from doing serious harm to its host, but doesn't stop the disease from replicating and spreading to another individual." Better vaccines—such as those for polio and measles in humans—provide lifelong protection against both infection and transmission.

While Marek's only affects fowl, it still manages to wreak havoc on chicken farmers, costing them $2 billion in losses each year.

Andrew Read, an evolutionary biologist who co-led the study, told PBS that "hot" (a.k.a. some of the deadliest) strains of Marek's can kill unvaccinated birds in ten days. "There is no human virus that is that hot," he said. "Ebola, for example, doesn't kill everything in ten days."

The vaccine helped stave off mass chicken deaths when it was introduced in the 1970s, but symptoms of Marek's have only become worse since then.


"Previously, a hot strain was so nasty, it wiped itself out. Now, you keep its host alive with a vaccine, then it can transmit and spread in the world," Read told PBS. "So it's got an evolutionary future, which it didn't have before."

That was reflected in the study, too. In the course of testing the effects of virus strains with varying degrees of hotness on both vaccinated and unvaccinated chickens, one mildly virulent strain got even hotter. Control chickens died faster when they were exposed to vaccinated chickens than unvaccinated chickens.

So, what does that mean for our chicken future? First and foremost, every chicken on the planet must be vaccinated against Marek's—and most of the 20 billion of them are. But the virus could continue to evolve and severely devastate the poultry industry.

Its unclear if the new avian influenza vaccine being trialled by the USDA is leaky, but previously existing vaccines certainly are—and that could lead to a scenario similar to that with Marek's.

As always, it's a bad day to be a chicken.