Mad cow disease, formally called bovine spongiform encephalopathy (BSE), was first detected in the United Kingdom in 1986. The most widely accepted explanation is that it spread through cattle feed containing ground-up animal parts, but how the very first case arose remains a question scientists are still piecing together. The answer involves a strange infectious agent unlike any virus or bacterium, a change in industrial processing methods, and possibly a disease that jumped from sheep to cows.
What Actually Causes BSE
BSE isn’t caused by a virus, bacterium, or parasite. It’s caused by a misfolded protein called a prion. Every cow (and every human) naturally produces a normal version of this protein in the brain and nervous system. In BSE, the protein folds into an abnormal shape. Once that happens, the misfolded protein acts like a template: it attaches to healthy proteins nearby and forces them to refold into the same abnormal shape, creating a chain reaction.
As the misfolded proteins accumulate, they destroy brain tissue. The brain develops a sponge-like texture full of tiny holes, which is where the word “spongiform” comes from. Affected cattle lose coordination, become aggressive or anxious, lose weight, and eventually die. There is no treatment and no recovery.
The Meat-and-Bone Meal Theory
For decades, the cattle industry used a protein supplement called meat-and-bone meal, made by grinding and heat-treating slaughterhouse waste (bones, organs, and other leftover tissue from cattle and sheep). This recycled animal protein was a cheap way to boost nutrition in cattle feed. In the late 1970s, rendering plants in the UK changed their production process. One long-standing hypothesis is that this change allowed an infectious prion to survive the treatment and remain active in the finished feed.
The original suspicion was that the prion came from sheep. Sheep have their own prion disease called scrapie, which has existed for centuries. The idea was that scrapie-infected sheep tissue entered the rendering system, survived the new, less rigorous processing, and infected cattle when they ate the contaminated feed. Once those cattle were slaughtered and their remains were rendered back into more feed, the cycle repeated and amplified, turning a single case into an epidemic.
However, a British government inquiry concluded that scrapie-contaminated feed was likely not the original source of BSE, and that the change in rendering practices wasn’t necessarily what allowed the agent to survive. Instead, the inquiry suggested BSE may have first appeared spontaneously in a cow through a random genetic mutation, and then the feeding system amplified it into a widespread outbreak. In other words, the feed didn’t create the disease, but it turned what might have been a one-off event into an epidemic affecting millions of cattle.
New Evidence on the Sheep Connection
Despite the British inquiry’s conclusions, the sheep theory hasn’t been fully dismissed. A 2025 study published in the Proceedings of the National Academy of Sciences provided experimental evidence that a type of sheep prion disease called atypical scrapie can, when transmitted to cattle, transform into the classical BSE prion. Atypical scrapie is considered a spontaneous disease in sheep, occurring sporadically in herds worldwide, even in regions that have never had classical scrapie.
The researchers found that this transformation happens through a process called “conformational shift,” where the cattle’s own prion protein forces the incoming sheep prion into a new shape, one that matches the BSE prion exactly. This means the disease didn’t need to simply “survive” the species jump from sheep to cow. It could have fundamentally changed form during that jump, emerging as something new. This finding reopens the possibility that sheep prions seeded the original BSE cases, even if the resulting disease looked nothing like scrapie.
Classical vs. Spontaneous BSE
Scientists now recognize two broad categories of BSE. Classical BSE (C-type) is the feed-borne form responsible for the UK epidemic. It spreads when cattle eat feed contaminated with the abnormal prion protein. This is the form that killed over 180,000 cattle in the UK and was detected in 25 countries.
Atypical BSE (L-type and H-type) occurs spontaneously at very low levels in all cattle populations, particularly in animals eight years or older. It doesn’t appear to be linked to contaminated feed. Think of it as a rare, random misfolding event that can happen in any aging cow’s brain. The existence of atypical BSE supports the idea that the very first case of classical BSE could have started the same way: a single spontaneous case in one cow, whose remains entered the feed supply and kicked off the cycle.
How It Jumped to Humans
Ten years after BSE was first identified in cattle, doctors in the UK identified an unusual form of Creutzfeldt-Jakob disease in young patients. In 1996, this was formally described as variant CJD (vCJD). The patients were far younger than typical CJD patients, and the disease progressed differently. By 1997, mouse transmission studies confirmed that vCJD was caused by the same prion agent as BSE. Biochemical analysis showed that the misfolded proteins found in BSE-infected cow brains and vCJD-infected human brains shared the same structural signature.
The link was contaminated beef products. Tissue from the brain, spinal cord, and other nervous system parts of infected cattle had entered the human food supply during the height of the epidemic in the late 1980s and early 1990s, before the risks were fully understood. Between 1996 and 2023, 233 people worldwide died of vCJD, with the vast majority of deaths occurring in the United Kingdom.
How the Outbreak Was Stopped
The single most important intervention was banning the use of animal-derived protein in cattle feed. The UK implemented this ban in 1988, and the U.S. Food and Drug Administration followed with its own feed ban in 1997. Without the recycling of infected tissue back into feed, the chain of transmission broke. Case numbers in UK cattle peaked in 1992 and declined sharply afterward.
Today, additional safeguards are in place. Slaughterhouses remove “specified risk materials,” the brain, spinal cord, and other tissues most likely to harbor prions, from all cattle before processing. In the United States, a surveillance program tests roughly 25,000 animals per year, targeting cattle that show signs of central nervous system problems or that are unable to walk, underweight, injured, or found dead. This level of testing can detect a single BSE case per million animals in the national herd with 95 percent confidence, consistent with international guidelines from the World Organisation for Animal Health.
The epidemic ultimately revealed how industrial farming practices could amplify a rare biological event into a crisis. A disease that may have started in a single animal, whether through a random mutation or a species jump from sheep, spread to millions of cattle and hundreds of people because the system fed animals back to animals. Removing that link in the chain effectively ended the outbreak.