Prions and Mad Cow Disease (BSE)

Picture a sheet of paper that was folded the wrong way. Now imagine it crumples every good sheet it touches. A prion works just like that bad sheet of paper.

The viruses that give you a cold and the bacteria that cause food poisoning all carry genetic information, because they need it to multiply inside your body. A prion has no genetic material at all. It is just one lump of protein, and that alone can pass on a disease.

Normal prion protein sits quietly on the surface of many cells in people and animals, including nerve cells. The trouble starts when its 3D shape gets bent out of place. This can happen on its own, because of a gene mutation, or because a misfolded prion came in from outside.

A misfolded prion nudges the normal proteins next to it. Then even the normal proteins get bent into the bad shape, one after another, and the misfolded form spreads out of control.

Cows are plant eaters that live on grass. So how did they catch such a frightening protein disease? The cause was a feed that people made.

Farmers wanted bigger cows that grew faster and gave more milk and meat, so they fed them animal-based feed. It was called meat-and-bone meal, made by finely grinding the leftover parts and bones of slaughtered cattle and sheep. Plant-eating animals were fed this.

Nobody knows for sure where the first misfolded prion came from. One theory says it came from sheep with a brain disease called scrapie. Another says it arose on its own in a cow. What is clear is that nerve tissue from infected animals got mixed into feed and spread from cow to cow. Misfolded prions hold up against ordinary cooking heat and disinfection, so they survived even the heating step used to make the feed.

In the end, misfolded prions built up in the brains of cows that ate the contaminated feed. Their brain cells were destroyed. They stumbled, became aggressive, and bovine spongiform encephalopathy, or mad cow disease, broke out in whole herds.

Misfolded prions shocked the world because they can pass to people too. People who ate high-risk parts of cattle with mad cow disease developed variant Creutzfeldt-Jakob disease (vCJD), which damages the brain.

Ordinary cooking, like grilling or boiling, barely reduces how infectious these agents are. So control focuses not on recipes but on removing the risky parts from the start. Prions gather less in lean meat and more in nerve tissue such as the brain, eyes, and spinal cord, and in the last part of the small intestine. These parts are named specified risk materials (SRM), and they are cut off and thrown away at slaughter.

Many countries have also banned feeding ruminants the meat-and-bone meal of other ruminants. For foods made with ingredients from cattle, some rules block imports or treat them separately, depending on where the cattle came from and which parts were used. That is why ingredients like gelatin from cattle hides and bones can fall under these rules.