Changes in Processing and Feeding Practices

Changes in the processing of the feedstock used to prepare offal and meat by-products for meat and bone meal manufacture occurred over the period from the early 1960's through the early 1980's. These changes, coupled with feeding practices in Britain may have contributed led to the spread of BSE. The changes included alterations in processing time, temperature, batch vs continuous processing, use of solvent extraction vs expeller removal of fat and necessity of steam stripping of solvents for processes using solvents. Changes in rendering practices may have potentiated the agent's survival in meat and bone meal.

Lower temperature or shorter time processing, continuous atmospheric pressure processing, and removal of solvent-extraction followed by a second steam-heat treatment have been shown to allow survival of infectivity in rendered meat and bone meal (Taylor et al., 1995). These changes came about for economic reasons and because new animal feed manufacturing equipment could use higher fat raw materials making two step rendering unnecessary.

In the UK, the use of hydrocarbon solvents to extract fat from meat and bone meal (to be used as feed stock in animal feed pelleting equipment) had stopped by 1982, and processing temperatures were lowered to < 165C

In the UK, scrapie was declared a reportable disease in 1993; until then, scrapie infected sheep carcasses could be rendered. Based on annual production in the UK, animal protein feedstock contained at least 14% sheep tissue; because sheep slaughter is very seasonal, it may have been much higher at certain times of the year. In contrast, sheep tissue content was about 0.6% in the Unites States over the same time period (Wilesmith, J.W. et al. 1991).

In the UK, rendered animal protein tends to be distributed nationwide; in the US, rendered animal protein tends to be regional and confined to 150 miles within the processing plant.

In the UK, rendered protein is fed at 4-5% of the daily ration, even to pre-weaned calves. Feeding rendered protein to pre-weaned calves is uncommon in the United States; MBM is fed to weaned calves at less than 2.5% of the daily ration.