People get mad cow disease by eating beef products contaminated with prions, the misfolded proteins that cause bovine spongiform encephalopathy (BSE) in cattle. The human form of the disease is called variant Creutzfeldt-Jakob disease (vCJD), and it develops after prions from infected cattle cross the species barrier through the digestive system, eventually reaching the brain. In rare cases, the disease has also spread between humans through blood transfusions.
Prions and How They Cause Damage
Unlike bacteria or viruses, prions are not living organisms. They are normal brain proteins that have folded into an abnormal shape. Once a misfolded prion enters the body, it triggers neighboring proteins to misfold as well, creating a chain reaction. These abnormal proteins clump together in brain tissue, gradually destroying neurons and leaving the brain riddled with tiny sponge-like holes. This is what gives BSE its name: the brain literally takes on a spongy appearance under a microscope.
What makes prions especially dangerous is their near-indestructibility. They resist boiling, irradiation, dry heat, alcohol, and most chemical disinfectants. Standard cooking temperatures do nothing to neutralize them. Laboratory decontamination requires autoclaving at 134°C (273°F) for 18 minutes or outright incineration. No amount of grilling, roasting, or frying a contaminated piece of meat will make it safe.
Which Parts of the Animal Carry the Risk
Prions do not distribute evenly throughout a cow’s body. They concentrate in nervous system tissue and certain organs, which is why regulators classify specific parts as “specified risk materials.” In cattle 30 months and older, the highest-risk tissues include the brain, spinal cord, skull, eyes, and vertebral column. In cattle of any age, the tonsils and a section of the small intestine called the distal ileum also carry risk.
Muscle meat (the steaks and roasts most people eat) carries far lower risk than these nervous system tissues. The concern historically was that slaughterhouse practices could allow brain or spinal cord material to contaminate other cuts of meat, particularly in mechanically separated beef, where machines strip remaining tissue from bones. Modern regulations now prohibit the use of these high-risk materials in human food.
Human-to-Human Transmission
Eating contaminated beef is the primary route, but vCJD has also been transmitted through blood transfusions. Five patients are suspected to have acquired vCJD or a vCJD infection after receiving blood products from a donor who later developed the disease. All of these transfusion-associated cases occurred before 1999, when the UK adopted universal leukodepletion, a filtering process that removes white blood cells from donated blood. Prions in vCJD appear to have a particular affinity for white blood cells, which explains why filtering them out reduced the risk.
Surgical instruments pose a theoretical concern as well, since prions cling to metal surfaces and survive standard sterilization. However, no confirmed cases of vCJD have been traced to surgical transmission.
The Long Incubation Period
One of the most unsettling aspects of vCJD is how long it takes to appear. The incubation period is measured in years, not weeks or months. People who developed symptoms in the mid-1990s likely ate contaminated beef a full decade earlier, in the mid-1980s. The typical incubation period is roughly 10 years, though it could potentially be longer in some individuals.
vCJD tends to strike younger people than classic CJD, a related but distinct prion disease that typically affects people over 55. Classic CJD arises spontaneously or through genetic mutations and is not linked to contaminated beef. Variant CJD patients often first experience psychiatric symptoms like depression and anxiety before neurological decline sets in. Once symptoms begin, the disease is always fatal.
How Feed Bans Broke the Cycle
BSE spread through cattle populations because of a specific industrial practice: feeding cattle ground-up remains of other cattle. When a cow with BSE was rendered into animal feed, its prions survived the processing and infected the next generation of animals. This created an escalating cycle, particularly in the UK during the 1980s and early 1990s.
The first major intervention was a 1997 FDA regulation banning mammalian protein from cattle feed. A later enhancement expanded the ban further, prohibiting high-risk cattle materials from feed for all animal species, not just ruminants. The rules require that brains and spinal cords from cattle 30 months and older be excluded from any animal feed, that carcasses of BSE-positive cattle be entirely removed from the feed supply, and that rendering facilities use separate equipment for prohibited materials. Even rendered fat (tallow) faces restrictions if it contains more than 0.15% insoluble impurities, which could indicate contamination with nervous tissue.
Ongoing Surveillance
The USDA’s BSE surveillance program tests roughly 25,000 cattle per year, targeting populations where the disease is most likely to appear, such as animals that died on farms or showed neurological symptoms before slaughter. In fiscal year 2024, about 22,800 samples were tested. The program is designed to detect a single BSE case per million animals in the national herd with 95% confidence.
These combined measures, feed bans, removal of high-risk tissues from the food supply, and ongoing testing, have made vCJD extraordinarily rare today. The vast majority of cases occurred in the UK during the 1990s and early 2000s, linked to the peak of the BSE epidemic in British cattle. For anyone eating commercially sold beef in countries with modern BSE controls, the practical risk is extremely low.