Transmissible Spongiform Encephalopathies
The aim of the Transmissible Spongiform Encephalopathy (TSE) Programme is to contribute to the eradication of TSEs in cattle, the eradication or control of scrapie in sheep and the prevention of TSE-infected animals or animal by-products being released into the food chain or wider environment. VLA also acts as the European Community and OIE Reference Laboratory for TSEs and has responsibilities on behalf of Defra for research and surveillance of cattle and small ruminants.
Programme Manager
Eradication of TSEs in cattle
VLA has played a significant role in the fight against BSE and surveillance data has confirmed that, according to the OIE risk classification of fewer than 200 BSE cases per million cattle per year, the UK is considered to be a country with a moderate BSE risk status for the whole of its cattle population. In September 2005, a FVO report concluded that the UK had also made sufficient progress in its enforcement of BSE controls as well as its readiness to identify, register and test bovine animals.
The result of these two major steps has culminated in the removal of the Over Thirty Months Scheme (OTMS) and the Date Based Export Schemes (DBES) resulting in the lifting of the ban on the export of cattle and beef products from the UK.
During the year, atypical cases of BSE were reported from USA, Sweden, Italy, France and Japan. These Born-after- The-Real ban (BARBs) cases, led to Defra commissioning VLA to investigate the PrP gene sequence in BARBs, BSE and cohort controls. Initial analysis has revealed no difference between BARB and BSE cases in the gene open reading frame that might account for differing aetiologies.
Infectivity in cow’s milk and the effect of mastitis is still a concern and we have adapted existing sensitive and rapid BSE screening methods to test cow’s milk. This biochemical testing of milk from BSE-infected cows failed to detect abnormal prion protein, the surrogate molecular marker for TSE agents.
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Eradication or control of TSEs in sheep
The EU strategy of breeding for resistance to TSEs in sheep has been thrown into doubt by the discovery, during the active TSE surveillance programmes of 2002-2005, of a new type of prion-protein related abnormality in sheep previously regarded as resistant to this type of disease. Analysis of the molecular, transmission and pathological characteristics of GB cases, confirmed this atypical scrapie, which is similar if not identical to Nor98 cases found elsewhere in Europe. Doubts about its case definition were resolved by a joint CRL/EFSA group of experts, convened and led by VLA, producing a comprehensive review of what was known about these cases and guidelines for their detection and classification with respect to classical scrapie and experimental BSE in sheep.
Sequencing of the ‘open reading frame’ of the prion protein genes of atypical and classical cases of scrapie has identified AF141RQ and AHQ-allele carrying genotypes as a high risk factor for atypical scrapie, although the ARR allele, the most resistant to developing the clinical signs of classical scrapie, represents a two-fold greater risk of contracting atypical scrapie than the wild-type (ALRQ) allele.
Until August 2005 atypical cases of scrapie in GB were only identified by active surveillance arousing suspicions from sceptical veterinarians and farmers that they were a laboratory artefact. However, between September 2005 and January 2006, VLA has reported atypical scrapie in four, five year old Welsh Mountain sheep presenting as clinical suspects. All four cases showed ataxia and no wool loss (although one was pruritic) but other clinical signs were quite variable.
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