Mad Cow: Why Deer and Elk?

Current thinking is that Mad Cow Disease (BSE) is spread by the practice of using ground up slaughterhouse remains containing prion-infected neural tissue as feed for other cattle. Seems plausible. Our friendly beef industry claims that the US herds are not infected.

However, some herds of deer and elk in the US West have chronic wasting disease, a disease similar to BSE. To my knowledge, deer and elk are not naturally cannibalistic. What mechanism can account for the appearance and spread of this disease in deer and elk? Is someone feeding these populations with the same type of meal formerly used in UK?

The DNR says: