Volume 2: Science
2. The Spongiform Encephalopathies - knowledge existing in 1986
Summary

2.180 This chapter provides a brief account of the main features of the TSEs which would have been known by December 1986, when scientists were faced with the possible emergence of a new TSE in cattle. The key points can be summarised as follows:

1. TSEs are non-inflammatory, infectious diseases causing progressive degeneration of the nerve cells of the brain associated with a characteristic spongy appearance of affected parts of the brain, sometimes including deposits of insoluble protein (amyloid).
2. Infection with a TSE is caused by an unknown infective agent, resistant to treatments which inactivate viruses. Unlike most infections, the incubation period is measured in years and months, rather than weeks and days. This suggests that the agent is an unusual 'slow virus' which is non-immunogenic. Another hypothesis which is gaining ground and has strong support is that the agent is a self-replicating protein, a host protein that has undergone post-translational modification (ie, has undergone modification following synthesis).
3. Work on scrapie and scrapie transmission to experimental mice shows that the degree of susceptibility or resistance to infection is determined by genetic factors in the affected animal. TSE agents isolated from different sources also show variation in their effects. In fact, the incubation period and pattern of disease in the animal depends on the interaction between the particular TSE isolate and the genotype of the host.
4. There is evidence from sheep that scrapie can be transmitted vertically from ewe to lamb, and that lateral transmission occurs from pasture contamination. Maternal transmission does not appear to occur in other species.
5. While most cases of CJD are sporadic, some are familial, in which case the disease is inherited as an autosomal dominant trait. Iatrogenic cases of TSE can occur as a result of hormones, vaccines, transplants and surgical instruments being contaminated with the TSE agent.
6. Although natural transmission of scrapie between sheep and goats, and of CWD between mule deer and Rocky Mountain elk, has been recorded, no other instance of natural transmission between species is known. As at December 1986, published data supported the proposition that scrapie did not cause CJD. Apart from uncertain anecdotes, no case of TSE was known in cattle. However, cross-species transmission of scrapie and CJD has been achieved in experimental mice and certain other species both orally and by parenteral inoculation.
7. In scrapie-affected sheep, bioassays have shown that infectivity is found mainly in the central nervous system (CNS) and in the lymphoreticular system (LRS). Infectivity in the LRS can be demonstrated well before clinical signs of the disease. As to the CNS, it appeared that infectivity progressed along the spinal cord to the brain, again before clinical signs appeared.