The origin and transmission of BSE to hundreds of thousands of cattle has been widely attributed to using cattle rendered protein produced from the carcasses of scrapie-infected sheep or cattle with a previously unidentified form of transmissible spongiform encephalopathy as livestock feed. This widespread practice of feeding substances derived from animals to other animals (even of the same species) was sharply curtailed following the BSE epidemic in the U.S. and Europe, but never fully eliminated. For example, livestock feed regulations in the U.S. continue to allow the feeding of certain mammalian slaughter by-products to calves as well as the inter-species feeding of rendered animal proteins, such as feeding poultry waste and litter to ruminants. USDA also announced that since the discovery of the infected cow, they have identified 10 different feed suppliers that delivered animal feed to the farm. Determining if the cow became sick from feed is an area where investigators are focusing close attention.