Bovine spongiform encephalopathy: a new disease of cattle?

Bovine spongiform encephalopathy (BSE) was first recognized in Great Britain in 1985. Most believe that the disease is of recent origin initiated by feeding rendered animal protein from scrapie-infected sheep to cattle, then perpetuated by feeding rendered infected cattle to other cattle. This paper explores an alternative hypothesis that BSE existed in cattle populations in an unrecognized form for a much longer time until amplified by changes in the rendering process that allowed cattle to cattle transmission to occur. This viewpoint is supported by observations that transmissible mink encephalopathy, a disease that first occurred 45 years ago, is likely caused by feeding downer cows to mink, and that the sporadic form of Creutzfeldt-Jakob disease occurs spontaneously with no evidence of natural transmission. This epidemiologic scenario on the origin of BSE has important implications for prevention of the disease in BSE-free countries. Mainly, emphasis needs to put on practices of feeding animal protein to cattle rather than in reducing the prevalence of sheep scrapie. If BSE is already present in the cattle population, the major threat becomes feeding cows to cows.