Transmission of BSE

There are different scientific hypotheses concerning the origins of BSE.

1. One theory is that BSE existed in undetectable levels in the British cattle population prior to 1988 and its increase in incidence was the result of numerous coincident changes in the industry or environment.
2. Another theory suggests BSE initially appeared as a spontaneous event in cattle in the 1980's and then spread unimpeded by the current industry and environment.
3. Epidemiological data also suggest that BSE in Britain may have resulted from feeding cattle rendered protein produced from the carcasses of scrapie-infected sheep. The practice of using products such as meat and bone meal in cattle rations as a source of protein has been common for several decades.Changes in rendering operations in the 1970's and early 1980's may have allowed transmission of the infective agents to cattle; resulting in the large number of cases that developed.
   • in particular the redering process was changed to exclude a solvent extraction step
   • this resulted in meat and bone meal (MBM) with a higher fat content in addition to the high protein content of MBM

BSE has not been identified in the US, possibly because of the low ratio of sheep to cattle. For perspective, there are 40 million sheep and 12 million cattle in Great Britain (a 3 to 1 ratio) in a total land area similar to the state of Oregon. In the US there are 10 million sheep and 100 million cattle (a 1 to 10 ratio). Thus the chance of Scrapie being transmitted from sheep to cattle is less in the US than the UK.

The infective agent is present in nerve tissue and neither blood nor meat have been shown to be infective. This is the basis for the excluding all brain and spinal cord from meat and bone meal for animal feeds.

Details about Changes in Processing and Feeding Practices which may have contributed to BSE spread.

BSE is considered a "common source" epidemic, meaning that animals contract the disease from a common element in their environment. Semen, chemicals, autosomal inheritance, biologics and pharmaceutical have been ruled out as the common source. In trial circumstances maternal transmission can occur, and it appears that the species barrier between animals can be crossed in created extremes such as where infected material is injected into the brain of another animal. Development of infection appears to be related to size of inoculum.

BSE has been experimentally transmitted to other animals including rodents, cats and primates.

click on the figure for additional information on transmission and the potential for BSE to infect sheep