Modelling approaches for the analysis of BSE surveillance data throughout the European Union, developed on behalf of the European Commission, have shown promise in establishing a standardised methodology for the calculation of BSE prevalence, BSE status and preferred surveillance method. Although subject to further peer review, the model has now been recommended to the OIE as a potential tool that will support compliance with the BSE Chapter of the Zoosanitary Code.

Active surveillance of sheep scrapie ‘threw up’ both scientific and technological challenges in 2003. Significant numbers of sheep samples that tested positive using the rapid BioRad Platelia ELISA were impossible to confirm using immunohistochemistry and Western Blot methods. Initial investigations have led to further proposals to explore the nature of the prion protein detected in these sheep, which appears to be unusually sensitive to enzymatic action. It will be necessary to prove that the brains are infectious and that the results are not due to artefact.

These ‘unclassified’ samples were also unusual in terms of the range of genotypes of sheep affected ARR and AHQ alleles being common.

Core (top) and N-terminal (bottom) Western Blot patterns

Collaboration with other scientists, from France in particular, has highlighted the fact that similar problems were being experienced in other countries. Exchanges of materials and samples have led to the identification of antibodies that may enable confirmation of scrapie infection in such sheep. Although proof of infectivity remains to be completed, the discrete bilaterally symmetrical immunostaining achieved may indicate a form of scrapie that has not previously been detected in Europe.

Another dimension to surveillance for scrapie involved the requirement to recognise BSE in sheep, should it be present in the national flock. VLA co-ordinated a ring trial of molecular test methods which confirmed that all three Western Blot methods, one ELISA method and immunohistochemistry, were in agreement on a panel of samples that included sheep scrapie and experimental BSE in sheep.

Programming the automated immuno-stainer

A ‘live’ ring trial was also conducted on samples from one British flock where initial examinations indicated that the samples were different from ‘normal’ scrapie samples. The trial concluded that although unusual, the isolate did not satisfy the criteria necessary to be classified as BSE in sheep.

Epidemiological investigations were essential to the interpretation of the surveillance results and to understand the potential of the National Scrapie Plan (NSP) in protecting animal and human health. All sheep sampled in the active scrapie survey of 2002/03 were genotyped, and served as a useful baseline for the evaluation of progress with the NSP. Alternative options for the future of the NSP were modelled in terms of their potential for increasing the frequency of the ARR allele within the national sheep flock, and for reducing the prevalence of scrapie infection.

Primary preparation of BSE samples at VLA Newcastle

Amongst the experimental challenges, the transmission of BSE to calves fed as little as 1mg of infected tissue was confirmed. This indicates that no end point has been reached in this oral titration and highlights why all countries have had difficulties in preventing food borne exposure. In oral challenges of sheep with BSE, bioassay data emphasised the contrast in peripheral pathogenesis between cattle and sheep. While cattle can be ‘made safe’ by the removal of key tissues (termed specified risk materials) this is clearly not the case in sheep where peripheral lymph nodes and some viscera are infectious from a very early stage of incubation.

Preliminary results which will have significant policy implications have been obtained from a study aimed at investigating whether historical rendering practices, in particular the extraction of tallow with organic solvents, could have prevented the onset of the BSE epidemic.
Using the mouse adapted BSE strain 301V as a model (due to its high starting titre), it seems that organic solvents did not strip out infectivity from the solid phase (greaves) into the liquid (tallow) phase. Infectivity was, however, detected in the solvent extracted tallow, indicating that bioassay of tallow in other studies may have been compromised by the method of sample preparation. In this study sample preparation had been planned to ensure dispersal of inoculum within the inoculated mouse.