For years, chronic wasting disease (CWD), caused by prions – abnormal, transmissible pathogenic agents – has been spreading stealthily across North America, with concerns voiced primarily by hunters after spotting deer behaving strangely.

The prions cause changes in the hosts’ brains and nervous systems, leaving animals drooling, lethargic, emaciated, stumbling and with a telltale “blank stare” that led some to call it “zombie deer disease”.

Its discovery in Yellowstone, whose ecosystem supports the greatest and most diverse array of large wild mammals in the continental US, represents an important public wake-up call, says Dr Thomas Roffe, a vet and former chief of animal health for the Fish & Wildlife Service, a US federal agency.

Roffe had been predicting CWD would reach Yellowstone for decades, warning that both the federal government and the state of Wyoming needed to take aggressive measures to help slow its spread.

“The BSE [mad cow] outbreak in Britain provided an example of how, overnight, things can get crazy when a spillover event happens from, say, livestock to people,” Anderson says.

Dr Raina Plowright, a disease ecologist at Cornell University, says CWD should be viewed against a backdrop of dangerous emerging zoonotic pathogens that are moving back and forth across species barriers between humans, livestock and wildlife globally.

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