Today I learned

Not scabies (caused by tiny, parasitic bugs), but scrapie. Scrapie is the sheep form of bovine spongiform encephalopathy, and it's caused by an abnormal protein in the brain--nothing to do with parasites. The misshapen protein can be found in the brain and spinal cord, and it turns out that grinding animals up wholesale to turn them into meat and bone meal can spread those abnormal proteins to the animals eating their ground up cousins.

Similar illnesses are found in other animals like elk (chronic wasting disease) and humans (Creutzfeldt-Jakob disease, kuru). If I remember right, spongiform encephalopathies are usually rare conditions that come from gene mutations. That's why BSE (sensationalized as "mad cow disease") made headlines 30-40 years ago--these sick sheep and cows were showing up in unexpected numbers because of consuming tainted feed, and there was a lot of uncertainty around whether or not humans could develop CJD from eating tainted beef and mutton. It's really, really unlikely to happen--the last I'd read about it, the people who were confirmed to have contracted CJD all had a super uncommon genetic quirk that made their normal proteins less able to maintain their healthy shape and increased their risk for disease.