Mad cow disease, formally called bovine spongiform encephalopathy (BSE), traces back to a practice that began in the United Kingdom in the 1980s: feeding cattle processed remains of other animals. The exact first spark is still debated, but the disease was amplified into an epidemic by mixing infected animal tissue into cattle feed, turning herbivores into unwitting cannibals.
How the Disease First Appeared
Two main theories explain how BSE originated. The first points to scrapie, a similar brain disease that has existed in sheep for centuries. When sheep carcasses were processed into a protein-rich feed supplement called meat and bone meal, the infectious agent may have survived the rendering process and jumped species into cattle. The second theory, which a British government inquiry ultimately favored, suggests that BSE arose spontaneously through a random genetic mutation in a single cow. That cow’s remains were then rendered into feed and given to other cattle, amplifying what might have been a one-off event into a widespread outbreak.
Regardless of which theory is correct, both converge on the same accelerant: meat and bone meal. Throughout the 1980s, this rendered animal protein was a standard nutritional supplement in the UK cattle industry. Once even a small amount of infectious material entered the supply, the recycling of cattle remains back into cattle feed created a self-perpetuating loop. Each generation of infected animals produced more contaminated feed, and the disease spread through herds across Britain and eventually to other countries.
Prions: The Unusual Cause
BSE is not caused by a bacterium or virus. It is caused by prions, which are misfolded proteins. Every mammal’s brain naturally contains a normal version of a protein called PrP. In a healthy brain, this protein is mostly structured in coiled shapes called alpha-helices. A prion is the same protein twisted into a different, flattened shape dominated by structures called beta-sheets. While the normal protein is about 47% coiled and only 3% flat, the misfolded version flips to 43-54% flat structure.
What makes prions so dangerous is their ability to recruit. When a misfolded prion contacts a normal brain protein, it forces that protein to refold into the wrong shape. That newly misfolded protein then converts others, creating a chain reaction. Over time, clumps of these abnormal proteins accumulate in the brain, destroying neurons and leaving the tissue riddled with tiny holes, giving it a sponge-like appearance under a microscope. This is where the name “spongiform encephalopathy” comes from.
Prions are extraordinarily tough. They resist heat, chemical disinfectants, and standard hospital sterilization procedures that would destroy bacteria, fungi, and viruses. Normal cooking temperatures do not neutralize them. This resilience is a key reason why rendered animal feed remained infectious even after processing.
How It Spreads to Humans
For years, the British government assured the public that BSE posed no risk to people. That changed in March 1996, when researchers reported that 11 British teenagers and young adults had died of a new illness called variant Creutzfeldt-Jakob disease (vCJD). This human form of the disease was linked to eating beef contaminated with BSE prions, particularly tissues from the brain and spinal cord of infected cattle.
Not all parts of a cow carry the same risk. Prions concentrate in nervous system tissue. The highest-risk materials in cattle of any age are the tonsils and the last section of the small intestine. In cattle 30 months and older, the brain, spinal cord, eyes, skull, and the nerve clusters attached to the brain and spinal cord are also classified as high-risk. These tissues are now banned from the human food supply in the US and many other countries. Muscle meat, the cuts you would typically buy as steaks or roasts, carries far lower risk.
What BSE Looks Like in Cattle
Infected cattle don’t show symptoms for a long time. The incubation period ranges from 2 to 8 years, meaning a cow can carry the disease for years before anyone notices something is wrong. When symptoms do appear, they include changes in temperament such as nervousness or aggression, abnormal posture, difficulty standing, loss of coordination, and decreased milk production. Affected animals may lose body condition even without losing their appetite. The disease is always fatal once symptoms begin.
Diagnosis is primarily done after death. Brain tissue is examined using techniques that detect the abnormal prion protein, either through staining methods that make prion deposits visible under a microscope or through rapid screening tests that can process many samples at once. There is no reliable live test for BSE in cattle, which is why surveillance programs rely on testing the brains of slaughtered and dead animals.
Feed Bans and Modern Safeguards
The single most effective measure against BSE has been banning the practice that caused it. In 1997, the US FDA prohibited feeding mammalian protein to cattle and other ruminants. Later regulations went further, banning the use of high-risk cattle materials in feed for any animal species. Specifically, the brains and spinal cords from cattle 30 months and older, the entire carcass of any BSE-positive animal, and certain rendered fats with high levels of tissue residue are all prohibited from all animal feed.
Countries like Australia took even broader steps, banning imports of meat and bone meal and live cattle from any country that had reported BSE cases. These feed bans, combined with the removal of specified risk materials from slaughter lines, have dramatically reduced the disease. Between 2016 and 2024, only three cases of classical BSE were reported worldwide, all in the United Kingdom. A small number of “atypical” BSE cases still appear sporadically in older cattle around the world. These atypical cases are thought to arise from rare, spontaneous prion misfolding rather than from contaminated feed, much like the mutation that may have started the epidemic in the first place.
Why the Risk Is Low but Not Zero
The BSE epidemic peaked in the early 1990s with nearly 37,000 confirmed cases in UK cattle in 1992 alone. Today, the disease has been reduced to a handful of cases globally per year. The combination of feed bans, removal of high-risk tissues at slaughter, and ongoing surveillance in dozens of countries has made the food supply far safer than it was three decades ago.
The residual risk comes from the nature of prions themselves. They cannot be cooked away, they can incubate silently for years, and atypical cases still arise spontaneously at very low rates. This is why the regulatory infrastructure, the feed bans, the testing programs, the removal of specified risk materials, remains in place even though the epidemic is effectively over.