Mad Cow Case Appears In California

Wall Street Journal:

The U.S. Department of Agriculture said Tuesday it had confirmed the first new case of mad-cow disease since 2006 after the illness was suspected in a California dairy cow.
The cow was tested as part of the USDA's ongoing surveillance system for the disease. The USDA tests about 40,000 cows per year. This is the fourth case of mad-cow disease reported in the U.S.
Agency officials stressed that no meat from the animal entered the human food supply, though some dairy cows are slaughtered for food. USDA officials said they don't expect any foreign countries to ban U.S. beef because of the new mad-cow case.
USDA Chief Veterinarian John Clifford said scientific studies by the World Health Organization show that humans aren't at risk of contracting the disease through milk consumption. "Milk is safe," he said.

My question about Mad Cow disease is whether it is only transmitted by eating infected brain or nervous system matter?  My understanding is that Creutzfeldt-Jakob disease can be genetic:

 The defective protein can be transmitted by contaminated harvested human brain products, Immunoglobulins (IVIG), corneal grafts, dural grafts or electrode implants (acquired or iatrogenic form: iCJD); it can be inherited (hereditary or familial form: fCJD); or it may appear for the first time in the patient (sporadic form: sCJD). In the hereditary form, a mutation occurs in the gene for PrP, PRNP. Ten to 15 percent of CJD cases are inherited. (CDC)

It would seem to me that there could be two reasons why Mad Cow almost always appears in dairy herds.  One is that they need the cheapest protein sources so they might get contaminated feed.  The other would be that the gene pool is pretty shallow, and it might be an inherited trait.  I would think that is a real possibility in dairy herds.  It still might not rule out the transmissibility to humans, but it is an interesting consideration. 

Well, after looking a little bit, I guess it may be genetic:

Findings published in PLoS Pathogens (September 12, 2008) suggest that mad cow disease also is caused by a genetic mutation within a gene called Prion Protein Gene. The research shows, for the first time, that a 10-year-old cow from Alabama with an atypical form of bovine spongiform encephalopathy had the same type of prion protein gene mutation as found in human patients with the genetic form of Creutzfeldt–Jakob disease, also called genetic CJD for short. Besides having a genetic origin, other human forms of prion diseases can be sporadic, as in sporadic CJD, as well as foodborne. That is, they are contracted when people eat products contaminated with mad cow disease. This form of Creutzfeldt-Jakob disease is called variant CJD.

Interesting.