Transmissible Spongiform Encephalopathies

The Transmissible Spongiform Encephalopathies Programme provides a primary source of high quality scientifically based advice on all aspects of TSEs, with special reference to BSE and scrapie.

The continuing decline in the number of cases of BSE in the UK, and similar trends in other EU member states, has re-focused attention on the cost-benefits of surveillance, the prohibition of using animal by-products in ruminant feed and the TSE specified risk material (SRM) controls. This triad of animal (feed ban) and human (SRM) protection measures, and their monitoring system (surveillance) needs to be proportionate to the risks and based on sound scientific evidence. Any revisions will challenge VLA and Defra scientists for several years to come.

The VLA TSE Programme co-ordinates a network of active and passive BSE surveillance activities, including monitoring the testing and culling of animals in the framework of BSE eradication measures. In the past year, the number of BSE cases in cattle has dwindled to approximately 13 per million animals over 24 months and the average age of these cases has risen to 144 months.
This continues a trend in the UK, now seen throughout Europe, with an incidence of BSE decreasing from over 1000 cattle per million with an average age of eight to nine years (see graph below). This evidence is currently being reviewed by the European Food Safety Authority in their considerations into raising the age before cattle (for human consumption) need to be tested for TSE.
Surveillance of TSEs in small ruminants remains a priority while our understanding of the complexity and variability of TSE agents in sheep and goats continues to be challenged. VLA acts as the OIE, FAO and European Community Reference Laboratory (CRL) for TSEs. In this latter capacity we convene and stimulate investigation of unusual isolates of TSEs found in the UK and other member states through a Strain Typing Expert Group (STEG) forum.

The STEG is composed of leading European experts in the field and has met regularly in the past year to sift through new data. This has included French, UK and Cypriot TSE cases which, by rapid testing, had biochemical or immuno-histological properties outside the normal range for scrapie. The STEG’s current concern is the stability and range of characteristics of BSE in small ruminants and its possible masking by co-infection with scrapie.

Fears of BSE in other species, and its recognition, have been a stimulus for several lines of research and surveillance over the past years. Since 2003, the Food Standards Agency has funded a collaborative project between VLA and the Moredun Research Institute aimed at testing the transmissibility of cattle BSE to red deer and we now know that deer can be infected with BSE. The red deer BSE phenotype is distinct from the endogenous cervid TSE, chronic wasting disease (CWD). This gives us the confidence that, should the current EU surveillance for cervid TSE yield cases, the CRL and STEG would be able to discriminate CWD and BSE in red deer.

Natural transmission of scrapie is considered to occur preferentially at, or around, birth. Therefore lambs in an infected flock will inevitably be exposed to the disease and are likely to be infected at a very young age. The question is: how much, and when does this exposure exactly occur?

VLA has identified that certain husbandry methods, such as feeding lambs stored colostrum or cross-fostering in and between flocks, are risk factors for the spread of scrapie. We have shown the transmission of scrapie from scrapie-affected dams to lambs has occurred by feeding from birth milk and colostrums during the later stages of scrapie-infection. Evidence of infection was detected as early as 44 to 46 days in the distal ileum of two lambs by immuno-histochemistry for abnormal prion protein (PrPD) and widespread infection inferred by RAMALT testing by 190 to 210 days of age. Both donor ewes and lambs were of the susceptible VRQ/VRQ genotype and the donor ewes were sourced from a VLA research flock with approximately 10% prevalence of natural infection.