There are it would seem to me three possibilities here. First that the infected cow was feed infected feed, which would seem to me unlikely. Second, as the Japanese evidence suggests, prions can occur naturally or are the result of some environmental condition. Prions are the reverse transcription of a protein that is crucial for the retention of memory. The third possibility is that the disease has jumped from one species (Elk) to another (cattle).
Why do I think it is unlikely that the animal was feed infected feed? Because that has been the focus of all attempts to prevent the spread of the disease and is easily accomplished compared to efforts such as the Japanese to test all animals (they have a very small herd). As the beef industry is an important export sector the USDA has been particularly vigilant in this area. Also contrary to all the hype, the disease does not spread easily. The protein in question is naturally occurring in the brains of mammals and is crucial in memory retention processes. The prion that causes the disease is simply a revers image of that protein (the atoms are arrange in the opposite manner). It seems more likely that this reversal occurs naturally periodically in very small numbers and is not usually a problem unless certain cultural practices cases it to spread. As there as been no wide spread out break of mad cow disease but rather two isolated cases it seems more likely that increased awareness and improved testing procedures are revealing a natural process that we were previously unaware of.
The Holstein was unable to walk on its own when it arrived at the slaughterhouse, a "downer" in industry slang.
Despite its incapacity, a USDA vet cleared the animal for slaughter after being told the cow had suffered injuries during a recent calving; a postmortem confirmed that assessment.
Piffka, if you are not already familiar with them you might look at these two web sites. The first is by Stanley Prusiner, the discoverer of Prions, I have included a link that leads directly to his article.
The Prion Diseases by Stanley Prusiner
The second is a more general article on Prions with several useful links.
Formerly a rare disease that affected less than one per million in most countries, one worst case scenario predicts that the incidence of CJD in the UK will escalate from, on average, fifty annually to claim ten thousand Britons by the year 2000, and a further ten million by the year 2010. Another predicts that half the British people, some 30 million, will be left Brain-dead by CJD.