UPDATE ON BOVINE SPONGIFORM ENCEPHALOPATHY
- Heim D & Kihm U.
- Swiss Federal Veterinary Office, Schwarzenburgstr. 161, 3097 Liebefeld, Switzerland
The most important measure to protect consumers was the ban on the use of specified risk material (SRM) for human consumption. At the end of 1989, the UK banned the use of SRM for human consumption - namely brain, spinal cord, tonsils, thymus, spleen and intestines from cattle older than six months. This ban was based on scrapie experiments on infectivity of the different tissues. At the time this ban was put in place there were no available data on BSE; subsequent experiments with BSE have shown that the pathogenesis, and hence the tissue distribution of infectivity in cattle with BSE and sheep with scrapie is different.
In mouse tests on tissues from field cases of BSE in cattle, infectivity has not been recorded outside the central nervous system (brain spinal cord and eyes). In experimental orally induced BSE, infectivity has been found in the distal ileum starting six months after exposure. Furthermore, central nervous tissues and dorsal root and trigeminal ganglia were found to be infective shortly before the onset of clinical signs. In one study, sternal bone marrow collected during the clinical phase of disease was infective; however, one of the possible explanations is that this could have been due to cross contamination of these tissues.
Course of the epidemic in the UK
One of the problems with controlling BSE is that the effect of the measures taken cannot be evaluated until about 5 years have elapsed, which is the average incubation period for BSE. So the effect of the ban on feeding MBM to ruminants did not become clear until 1993. After a peak of 36,000 cases had been reached in 1992, the annual incidence fell. Following more than 178,000 cases in the UK, it would seem that the measures taken, in particular the ban on feeding MBM to all farm animals, implemented in 1996, have been effective. There has been only a few cases of BSE in an animal born after these radical measures which were implemented in August 1996.
b) Bovine spongiform encephalopathy cases outside the United Kingdom
In 1989, the first cases outside the UK occurred in cattle imported from the UK. Not until the end of 1989 were the first indigenous cases reported in Ireland and the European Continent (France, Portugal and Switzerland). In the mid-90s, other countries reported cases of BSE (the Netherlands, Luxembourg, Belgium and Liechtenstein).
In 2000 and 2001 cases of BSE were first diagnosed in more countries in Europe. In September 2001 the first case outside Europe has been diagnosed (Figure 1).
Figure 1: First occurrence of indigenous BSE cases
