IT'S. THE. FEED. If this is a prion disease, than it comes from the animal feed. This is from 30 years of experience in the UK. Mad Cow Disease was and is caused almost entirely by allowing animals to eat brain and spinal tissue infected with the disease. Not a conspiracy, a well known, BASIC fact that means that in Britain (and elsewhere) it is now strictly illegal to allow culled dairy cows back into their own feed (yes, animal remains were added to herbivour feed for years). Mad Cow Disease has been pretty much eliminated from Britain after years of strict rules on feed; the number of human deaths has been much lower (so far) than feared - hopefully due to years of enforcing rules on what bits of cow can enter human consumption.
What are you feeding your farmed deer? Is it standard cattle feed or similar? Does it have cheap slaughterhouse reject animal protein in it? Then, when you've thought about that... if deer have, basically, Mad Cow Disease... how prevalent is it in the cattle you are eating?