§ The Countess of Mar
asked Her Majesty's Government:
Whether the current strain typing tests for transmissible spongiform encephalopathies are sufficiently sensitive and discriminatory to determine that apparently similar diseases appearing in the same or different species are unequivocally of the same origin.
The methods currently available form the cornerstone of research in this field because they have been used historically to study scrapie, and more recently BSE. Transgenic models have considerable potential to produce more rapid results and more precisely define strain type. They may also prove to be more sensitive. The current characterisation for spongiform encephalopathy agents involves inoculating infected tissue into a panel of mice of different genotypes. When this is done with the BSE agent a consistent pattern of incubation periods and lesion profiles results which is seen not only with isolates of the BSE agent from cattle but also after experimental passage through other species (goats, sheep, mice) and from field isolates of the spongiform encephalopathy diseases in cats, greater Kudu and nyala. Under the same test conditions a number of different strains of scrapie can be identified. It is also an established feature of scrapie that the nature of the agent in systems of this type changes after experimental passage through a different species. This process is partly attributed to adaption to a new host, but it has also been recognised that the original isolate may have consisted of a mixture 75WA of strains, of which only one, the most pathogenic to the mouse inoculated, dominates once in the mouse. The work done on BSE has revolutionised this work by determining that strain characteristics are retained in another species. The apparent presence of only one recognised 'wild type' strain in cattle is a vital factor.
This has enabled such a protocol to be of potential value in determining whether any CJD cases originate from BSE. Strain typing of CJD is however in its infancy, and characterisation of CJD strains will need to take place in parallel with correlation with BSE isolates. The system is also now being used to characterise isolates of CJD, but as yet no results are available as the experiment is still in progress.