Mad cow disease is a fatal brain disease in cattle caused not by a virus or bacterium, but by a misfolded protein called a prion. Its scientific name is bovine spongiform encephalopathy, or BSE, because it leaves the brain riddled with tiny holes, giving it a sponge-like appearance under a microscope. The disease drew global alarm in the 1990s when scientists confirmed it could jump to humans through contaminated beef, causing a deadly illness called variant Creutzfeldt-Jakob disease (vCJD).
How Prions Damage the Brain
Every healthy cow (and every human) naturally produces a normal prion protein on the surface of brain cells. In mad cow disease, that normal protein gets refolded into an abnormal shape. The misfolded version then acts like a template, converting nearby normal proteins into copies of itself. These abnormal proteins clump together into sticky aggregates that build up in brain tissue.
What makes prion diseases unusual is that the clumps themselves aren’t directly toxic. Instead, the damage happens when misfolded proteins interact with normal prion proteins still anchored to the surface of brain cells, triggering a toxic signal that kills neurons from the outside in. As more and more neurons die, the brain develops the characteristic spongy holes. There is no immune response that can clear the misfolded proteins, and no treatment that can stop the chain reaction once it starts.
How It Spread Through Cattle
The epidemic traces back to a practice in industrial farming: recycling leftover cattle parts into feed. After slaughter, the portions of a cow not sold for human consumption were cooked, dried, and ground into powder, then mixed into feed for other livestock. If any of that material came from a cow carrying BSE, the abnormal prions survived the processing and infected whatever animal ate the feed. A cow typically picks up the infection during its first year of life, but symptoms don’t appear for years.
This created a loop. Infected cows were slaughtered, rendered into feed, and fed back to more cows, silently amplifying the disease across herds. The United Kingdom was hit hardest during the late 1980s and 1990s, with over 180,000 cattle diagnosed at the peak. There are two forms of BSE: classical, caused by contaminated feed, and atypical, which appears to arise spontaneously at very low rates in older cattle. The only case of classical BSE ever found in the United States was in 2003, in a cow imported from Canada.
The Human Form: Variant CJD
When humans eat beef contaminated with BSE prions, particularly tissue from the brain or spinal cord, they can develop variant Creutzfeldt-Jakob disease. This is distinct from classic CJD, a rare brain disease that typically strikes people around age 68 and begins with rapidly progressing dementia. Variant CJD affects a much younger group, with an average age of onset around 28, and follows a different pattern.
The first symptoms are psychiatric: anxiety, depression, apathy, delusions, or social withdrawal. Many patients also develop persistent, painful sensory symptoms, such as burning or tingling sensations that don’t go away. These early signs can look like a mental health condition, which makes the disease difficult to recognize at first. Neurological problems, including poor coordination, involuntary muscle jerks, and vision changes, typically develop four or more months after the initial symptoms. Dementia sets in later in the course. From the first symptoms to death, vCJD takes more than a year, compared to four to five months for classic CJD.
The incubation period, the gap between eating contaminated beef and showing symptoms, is believed to span years or even decades, which made tracking the disease’s spread especially difficult during the UK outbreak. Around 230 cases of vCJD have been identified worldwide since 1996, the vast majority in the United Kingdom. The disease is invariably fatal. No treatment exists that can slow or reverse it.
How vCJD Is Diagnosed
Definitive diagnosis requires examining brain tissue, either through biopsy or after death. But a key noninvasive tool is MRI. In vCJD, brain scans show a distinctive bright signal in a region at the back of the thalamus called the pulvinar. This “pulvinar sign” is highly accurate when seen in the right clinical context, and a specific MRI sequence called FLAIR is the most sensitive way to detect it. Other areas of the brain, including the dorsomedial thalamus and the tissue surrounding the brain’s central canal, also light up on imaging in most cases.
Some tests that work well for classic CJD are unreliable for the variant form. Certain brain-wave patterns typical of classic CJD don’t appear in vCJD, and a spinal fluid protein marker used to flag classic CJD is less sensitive for detecting the variant. This means clinicians rely heavily on the combination of the patient’s age, the psychiatric-first symptom pattern, and MRI findings to build a diagnosis.
Feed Bans and Food Safety Rules
Governments responded to the BSE crisis with sweeping regulations targeting the feeding loop that spread the disease. In the United States, the FDA banned the use of high-risk cattle materials in all animal feed. Specifically, the brains and spinal cords of cattle 30 months of age and older are classified as prohibited material and must be completely removed during slaughter. The entire carcass of any BSE-positive animal is banned from feed, and the same applies to cattle that weren’t inspected and passed for human consumption (unless they’re under 30 months old or the brain and spinal cord have been removed).
Processing facilities must use separate equipment or containers for prohibited materials and mark them with a visible agent so they can’t accidentally mix with safe material. Rendering plants are required to keep records proving their suppliers have adequate procedures to exclude high-risk tissue. Even rendered fat (tallow) faces restrictions: if it contains more than 0.15 percent insoluble impurities, it cannot be fed to cattle or other ruminants.
These layered safeguards have been effective. The UK’s epidemic peaked in the early 1990s and declined sharply after feed bans took hold. New human cases of vCJD have slowed to a near standstill worldwide.
Where Things Stand Today
BSE has not been eradicated, but it has been reduced to a handful of cases per year globally, almost all of them the atypical form that arises spontaneously rather than through contaminated feed. In 2024, nearly a million cattle were tested across the European Union, and only three atypical BSE cases were found: two in France and Ireland, one in Poland. Four additional cases were reported elsewhere in the world, including one in the United States, one in Brazil, and two in the United Kingdom. Only one of those, a 7.5-year-old beef cow in Scotland, was classical BSE.
The rarity of new cases reflects decades of surveillance and feed regulation rather than the disease disappearing on its own. Prions are extraordinarily durable. They resist cooking temperatures, chemical disinfection, and radiation that would destroy any virus or bacterium. The controls that keep BSE out of the food supply depend on consistent enforcement of feed bans and slaughter protocols, which is why testing programs continue at scale even though the numbers are small.