Sheep route: The most formidable BSE theory till date. BSE could have originated in cattle fed with meat and bone meal (MBM) from sheep or goats infected by scrapie, a similar TSE.

Captive or wild animals: Possible infection from a TSE-infected animal kept in a zoo or farm or wild animal. No record for such transmission so far.

Pesticide route: During the 1980s and early 1990s, cattle in the UK were subjected to compulsory treatment with an organo-phthalimido-phosphorus pesticide, phosmet, to prevent damage from warble flies. Phosmet may have changed prions in susceptible cattle embryos, initiating BSE.

bacterial route: Some studies indicate that cattle, sheep and humans with TSEs have autoantibodies to brain neurofilaments. Some of these cross-react with some bacteria. Possibly these antibodies react with neural proteins and alter their structure and activity.

Human route: Possible infection from human remains in MBM imported from Indian subcontinent