From what I have read about prions, there really is reason to doubt whether they cause BSE and scrapie. The reason is that prions are not the only misfolded proteins with the ability to cause other proteins to misfold, and yet are the only proteins that seem to be able to transmit disease the way they do. I have read about misfolded proteins that have been proven to transmit disease, but they all seem to only be able to transmit if there is an underlying condition. None of the scientists who think prions cause the disease have an explanation for why prions are the only misfolded protein that can infect healthy organisms.

Mad cow disease and Scrapie can infect individuals that are healthy however! It turns out that healthy organisms and cells have machinery that break down misfolded proteins faster than they can form, and developing a disease caused by misfolded proteins requires something to be wrong with the machinery in order for the disease to develop. Scientists skeptical of prions think the BSE/scrapie agent, whether it be Spiroplasma, a virus, or something else, not only causes prions to form, but also disrupts the machinery that degrades misfolded proteins. The result is the prion can accumulate faster than it can be removed. As of today, prion theory does not have an agreed upon mechanism for the different strains of disease that exist either, while the alternative theories do.

The source below describes what I talked about, how prions are the only proteins that can transmit disease to healthy individuals, which if you ask me, is reason to be skeptical.
https://royalsocietypublishing.org/doi/full/10.1098/rstb.2008.0073