APHIS has a fact sheet on Johnes (Chronic Wasting Disease) here:

http://www.aphis.usda.gov/publications/a...s_disease08.pdf

They definitively link transmittal through fecal matter (primary source) and saliva/milk (secondary). Several years back, we lost Gertrude the Jersey cow to Johnes-like symptoms. At peak, she was burning through 120#+ of alfalfa + supplements per day and was still rapidly starving to death. Our vet had docs out of Davis and Iowa looking at her case and, although her blood work repeatedly confirmed she had not been exposed to CWD, we ended up putting her down for her sake (she lost approximately 225# in 4 months). We had her shipped to Orange County for necropsy but no reason was found for her condition (no tumors, parasites, negative tissue samples). As we were raising Jerseys for our private milk consumption and we enjoy raw products, we had our herd tested annually for Johnes, TB, etc.