The story behind the brain-destroying mad cow disease vividly illustrates why it’s not a good idea to eat your own species. For cattle, cannibalism had nothing to do with survival or grisly rituals and everything to do with economics.

The first so-called mad cows (the sickness is formally called bovine spongiform encephalopathy) were identified in 1984 in the U.K. They were probably infected a few years earlier by eating feed derived from the parts of sheep, cows and pigs that people avoided—diaphragms, udders, hooves, spinal cords, brains, and the like. The process of separating the ground-up components of slaughtered animals to make feed—and other products such as soap and wax—is called rendering and has existed for hundreds of years. In the mid-20th century in the U.K., rendering demanded the use of solvents and hours of boiling. The procedures presumably destroyed any pathogens that might have come from diseased creatures—pathogens that include the prion, a dangerous, malformed version of a protein found in all mammals.

In the 1970s the price of oil rose sharply, shooting up 10-fold by 1980. High crude prices, coupled with stagnant economic times, led renderers to seek ways to cut energy costs. So they did away with the solvents and the extended heating, opting instead to separate the parts in a centrifuge. The elimination of the extra cooking steps apparently enabled prions to persist.

Perhaps the first prions came from a cow that spontaneously developed the disease. Or perhaps scrapie, a prion disease of sheep that had been endemic in the U.K. for centuries but did not seem to pose a threat to human health, jumped species to infect bovines. In any case, subsequent rendering of infected cows—and then giving the resulting feed to other cows to eat as a cheap source of protein—amplified the outbreak. The situation echoed the devastation of the Fore people of Papua New Guinea: when the group practiced cannibalistic funerary rites in the early 20th century, it spread a fatal prion disease called kuru.

For the cows of the U.K. and elsewhere—the export of contaminated feed spread the disease globally—the epidemic subsided after regulations banned cannibalistic feed. Animal-health officials last year registered 125 cases worldwide, down from the peak of 37,000 in 1992. The rules came too late to save some 200 people who contracted the human form of the ailment—a small number, thankfully, considering that tens of millions have probably dined on mad cow beef.