- Cattle became infected during 1981-1988 by the ingestion of (RDP) which was rendered at 120-129⁰C whereas 134-138⁰C is necessary to destroy scrapie.

- The chances of feed contamination by infected brains were increased by the new large-scale rendering methods: no solvent extraction of fat from bone marrow; continuous processing instead of batch processing. The agent is ingested in RDP and 1 gram of food can infect.

- No isolation of BSE from lymphoid tissues has been reported by mouse inoculation studies as if brains are the main source.

- Although all incorporation of RDP was banned in July 1988 feed continued to contaminate for a variety of reasons: (i) RDP is incorporated into pig rations which could be fed to cattle or repelletted for cattle; (ii) the milling machinery and augers were contaminated by pig food; (iii) Stored stocks of food were in circulation. Calves born after 1990 have been infected at 10% the rate of those born in 1986 and may have been infected from such food or by their mothers (see later).