Mad cow disease spread through cattle feed made from other cattle
Britain's mad cow epidemic was driven by a misfolded protein that survives normal cooking and was recycled back into cattle through meat-and-bone meal. More than 184,000 British cattle were diagnosed between 1986 and 2015, about four million were killed to stamp it out, and by 2017 only four cases were reported worldwide.
Bovine spongiform encephalopathy is an incurable, invariably fatal brain disease of cattle. Its agent is a prion, a version of a normal brain protein whose alpha-helix structure has flipped into a beta sheet. Each misfolded molecule coaxes healthy ones into the same shape in an accelerating chain, and the resulting clumps kill nerve cells, leaving the brain riddled with holes. Ordinary cooking or heat treatment does not destroy it.
Signs appear only after a long incubation, which the World Health Organization suggested in 2002 was roughly four to five years. Affected animals may stagger, tremble, overreact to stimuli, turn nervous or aggressive, lose weight and give less milk; decline to collapse and death then takes weeks to months. There is still no reliable test in living animals, so confirmation depends on examining brain tissue after death.
Cattle are thought to have caught it from feed containing remains of cows that developed the disease spontaneously or products from sheep with scrapie, and feeding such meal to young dairy calves amplified the UK outbreak, which peaked in 1993. A British government inquiry, however, rejected scrapie as the origin and pointed to an unidentified event in the 1970s. Several million infected cattle are believed to have entered the food chain.
In people, the disease is believed to cause variant Creutzfeldt–Jakob disease, with the greatest risk from eating brain, spinal cord or gut tissue; 233 cases had been recorded worldwide by 2024. Controls now keep British animals over 30 months out of food and feed, require testing of older cattle in continental Europe, and bar specified risk materials, such as brain and spinal cord, from feed in North America. Feed bans have sharply cut cases wherever the disease occurred.
Source: Bovine spongiform encephalopathy
