BSE is back in the news here in the UK thanks to a new detection in Scotland. I haven't looked into transmissible spongiform encephalopathies much for a decade or so, but as I understand it the general model for TSEs is that an initial spontaneous misfolding of the protein PrP catalyses the misfolding of other PrP proteins until (over a fairly long incubation period) spongiform structural changes, amyloid plaque formation and other pathological changes cause CNS disease typically manifesting as behavioural changes. Ingesting a dose of misfolded protein can infect new hosts of the same species, and thus farming practices which include feeding animal byproducts to the same species can inadvertently amplify transmission, which resulted in the BSE epidemic in the UK back in the 1990s. Rarely, this transmission process can cross the species barrier.
I can find references to spontaneous scrapie in sheep and spontaneous CJD in humans, but very little on classical BSE in cattle. I do remember that there was a theory that BSE orginated when cattle were fed meat-and-bone-meal (MBM) from scrapie-afflicted sheep. I have found one peer-reviewed article suggesting that at least one type of atypical BSE (H-BSE) can occur spontaneously. I have also found one popular-science article that sounds like it describes spontaneous BSE, but it's unclear whether this is C-BSE or the H-BSE finding above (it's behind a paywall) and it's over ten years old, so I don't know what further developments there have been in this area.
Can classical BSE (C-BSE) arise spontaneously in cattle populations, and if so at what rate?