What Ever Happened to Mad Cow Disease?

One of the most existentially frightening illnesses ever to be discovered was mad cow disease, a fatal prion-spread illness that appeared in animals in the 1980s and killed over 200 people during the height of the outbreak in the late 1990s. You don’t hear much about mad cow today, but the threat still looms. Here’s what happened to this nightmare germ.

In 1986, farmers in the UK began to report strange behavior among some of their cows. The affected bovines had trouble walking and getting up. They also started acting increasingly nervous and sometimes violent (hence the “mad” moniker). Within a few weeks to months of these symptoms emerging, the cows would die. Subsequent autopsies revealed that their brains contained a distinct pattern of devastation, characterized by tiny “holes” that left the brain looking spongy underneath a microscope. Soon enough, the ailment had a formal name: bovine spongiform encephalopathy, or BSE. Many experts noticed early on how BSE bore a close resemblance to a well-known fatal disease in sheep and goats, called scrapie.

Mad and dead cattle continued to pile up in the UK, and countries like the U.S. quickly banned the import of British meat. The UK government also took steps to curtail the epidemic, culling millions of cattle deemed to be at risk, though officials denied for years that BSE posed any threat to people. In 1995, however, 19-year-old UK resident Stephen Churchill became the first known case and later fatality of what eventually became called variant Creutzfeldt-Jakob disease (vCJD)—human mad cow.

The culprit behind BSE, scrapie, and vCJD was unlike any disease-causing agent ever seen before: a zombie-esque protein, or prion. Mammals naturally carry a harmless form of the prion protein, though even to this day we’re not sure what it does. But prion proteins can shapeshift into a misfolded version, one that converts normal prions into more of themselves. Over time, the exponential accumulation of these rogue proteins destroys the brain. It can take years to decades for symptoms like slurred speech, a loss of coordination, and dementia to appear, but once they do, death inevitably follows.

We’ve known about some prion diseases like scrapie for over a century, even if scientists at the time hadn’t figured out that prions caused them. Usually, these diseases emerge spontaneously or through inherited mutations that make our prions prone to misfolding. Diseases like Kuru in humans also taught us that prions can be transmissible under certain circumstances, with Kuru infamously spread through the cannibalism of deceased family members and their brains among the Fore people of Papua New Guinea. But the BSE scare established that an animal’s prions could sometimes spark disease in another species entirely—though not easily, in this case.

The original outbreaks of BSE spread far and wide due to industry practices that were especially common in the UK. Thanks to higher soybean costs in the region, many farmers fed their cows less pricey feed that could contain the brainy leftovers of other livestock, including possibly infected sheep and cows. People then got infected by eating meat infused with contaminated brain and spinal cord tissue from BSE cows. These transmission chains were aided by the fact that prions are naturally resistant to most routine forms of decontamination.

But as luck would have it, soybean and other feeds were plenty cheap elsewhere, including in the U.S, meaning this method never became popular worldwide. By the mid-1990s, the UK formally banned the practice, as did most every other country eventually. The UK documented around 180,000 cases of BSE through the first few years of the epidemic, but no other countries reported such large outbreaks (some scientists have argued that countries like France experienced many more cases than reported, though). And even within the UK, annual BSE cases soon dropped off a cliff.

Still, millions of BSE-infected cattle might have entered the food supply before the epidemic was recognized and contained. And many scientists dreaded the potential ticking bomb of new vCJD cases in humans that would explode within a few years. But we got lucky. To date, only around 230 cases of vCJD from a dozen countries have been reported, mostly from the UK, with the peak of annual new cases in 1999.

Research would later show that our risk of catching vCJD from infected beef is crucially influenced by our genetics, with only a small percentage of people carrying the particular version of the prion protein gene that makes them especially vulnerable. And as scary and universally fatal as they are, prion diseases of any kind in people remain very rare.

That’s not to say that the BSE and vCJD outbreaks were nothingburgers. The emergence of BSE changed meat industry practices forever, almost certainly for the better (other widespread measures included banning cow brain and spinal cord tissue from being used to feed humans at all). It’s also why people have been barred from blood donation if they spent time in the UK and nearby countries during the 1980s and 1990s (several cases of vCJD were likely caused by blood transfusion), though countries like the U.S. and Canada are now starting to lift these bans. And while prions were discovered just before the crisis, the media and public attention fueled plenty of funding into deciphering these mysterious agents of disease.

We’re still discovering new things about prions, including the possible origins of the initial BSE outbreaks (maybe it was sheep all along). And the knowledge we’ve gained from studying them has helped scientists better understand neurodegenerative conditions like Alzheimer’s and Parkinson’s. Some scientists, including one of the Nobel Prize winners who discovered prions, even argue that Alzheimer’s and similar illnesses should be considered their own branch of prion disease.

And while mad cow was defeated, it’s not gone. Just like humans and classic CJD, cows have their own sporadic version of BSE that can rarely emerge out of nowhere. Countries even today still have to report and aggressively contain the occasional case of this form. There is also the unlikely but real possibility that delayed cases of vCJD will eventually appear in people with a different genetic make-up than the original victims, or that other unexpected sources of vCJD could be discovered. In 2018, for instance, doctors in New York reported a potential case of vCJD caused by squirrel meat (outside prion experts, however, have been skeptical about the link). And other rare routes of prion transmission exist as well, such as needle stick accidents in the lab.

Prions remain an important public health threat in other animals, too. In deer, moose, and elk, they can cause chronic wasting disease, first discovered in the 1960s. And though the overall prevalence of chronic wasting disease in the U.S. still appears low, it can cause large outbreaks in farmed deer, and there is evidence that the disease will continue to expand and reach new areas of the country (unlike BSE and CJD, chronic wasting disease prions seem to spread more easily between deer). To date, there’s no solid evidence that chronic wasting disease can infect and sicken humans—for now at least.