This is why it is so difficult to detect prion disease in a living being. A brain or tonsil biopsy might find Creutzfeldt-Jakob disease in a human, for example, but only if doctors examine an infected part. The defective proteins survive the rendering process that turns an animal's carcass into industrial fats and gelatin on the one hand, and meat and bone meal on the other. The meal is an effective and cheap protein that helps animals grow and produce milk. When it became apparent that turning herbivores into carnivores was the likely cause of BSE, Britain forbade feeding ruminant meat and bone meal to cattle in 1988, but continued to export the material, thus spreading the disease to other countries.
Scientists consider the inexpensive meat that comes from old dairy cows to be the most dangerous.
Some of the cheapest meat is stripped by machines and high-pressure jets from the bone, which is likely to be highly infectious in a sick cow. The EU's standing scientific committee estimated that each batch contains meat from about 1,000 animals, any one of which could infect the whole, and expose as many as 400,000 persons to the agent. Even the most dedicated vegetarian cannot avoid cattle products, which enter a vast range of goods from cigarette filters to soap. Tallow made from animal fat is used in everyday objects from carpets to television sets. In general, only between one-third and a half of the animal is eaten.