Transmissible Spongiform Encephalopathies
Danny Matthews Transmissible Spongiform Encephalopathies Programme Manager
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.
Dr Debby Reynolds, Chief Veterinary Officer, opens the Prion Diseases Conference
This year the TSE programme has been epitomised by making steady progress rather than scientific breakthroughs but has continued to encounter some key scientific and political challenges.
The origin of a case of atypical scrapie, diagnosed in a flock of sheep imported from New Zealand during 1998 and 2000 and subsequently bred and managed to maintain its scrapie-free status, proved to be very elusive. The sheep had been maintained as a closed flock in an area of the country where the density of sheep is low and all potential routes of infection were monitored closely. Following several months of extensive retrospective testing of archived samples held at VLA and from farms in New Zealand, the origin was still not confirmed.
Towards the end of the year, the first case of British ‘atypical BSE’ was detected at VLA during a retrospective study. It was somewhat puzzling that 30 such cases had been identified in other European countries but none in Britain. This may simply have reflected the fact that current test methodologies were not available during the bulk of the British epidemic. Atypical cases are usually referred to as L-type or H-type BSEs compared to normal BSE, which is called C-type. This first British case was classified as Htype based on the distribution of bands identified using Western Blotting techniques. The origin of L- and H-type BSEs remains unclear, although some researchers do consider that they may represent spontaneous disease, which consequently could have given rise to the BSE epidemic in the first place. Interestingly, the majority of cases have been found in older cattle, eight years of age or more.
Histopathology laboratory, VLA Lasswade
A major highlight of the year was the convening of an International Conference on Prionic Diseases of Livestock held at the Radisson Edwardian Hotel, Heathrow Airport, London. The conference celebrated 20 years of research into BSE at VLA and commemorated the 10th anniversary of the first recognition of variant CJD. A large audience of scientists, veterinary surgeons and policy makers from national reference laboratories, scientific institutes and enforcement authorities around the world valued the opportunity to be brought up to date on the science of TSEs from a programme specifically aimed at their needs. Topics included epidemiology, pathogenesis, diagnostic tests and breeding for resistance.
TSEs are not easy diseases to study, partly due to the long incubation periods experienced after infection. This has been exemplified in one of our studies that has been running for fifteen years, albeit in two stages, investigating the effect of dose on incubation and the likelihood of infection. Confirmation of BSE in one animal, 110 months after being exposed to 10mg of BSE-infected brain, represents the longest incubation period so far, following experimental challenge. This observation highlights the difficulties all countries will have in determining whether or not their feed controls have finally broken the cycle of re-infection.
A parallel study looked at the relationship between incubation and detection of infected animals using tests on the brain and spinal chord. This was intended to guide policy makers in refining legislation to determine the age at which certain tissues, called specified risk materials (SRM), should be removed from the human food chain. It would also help to determine the most appropriate age at which cattle should be tested. The study demonstrated that where animals received low dose infections, tests were unlikely to be able to detect infected animals until very shortly before they were about to present with clinical signs of BSE. The first detectable signs of disease were confirmed at 44 months post-exposure, which was much higher than the 30 months at which testing currently starts for healthy animals.
Understanding and predicting the tail-end
of the BSE epidemic remains a real
challenge, especially as cases arise in small
numbers in countries following the
tightening of their feed controls. Only as
the risk of food-borne exposures reduce,
will it be possible to focus on the
potential for sporadic and spontaneous
occurrence of cases. Similar challenges are
faced with sheep scrapie, where the
number of cases of classical scrapie
detected in the past year has fallen in all
surveillance streams. However, trends are
more difficult to understand due to the
multiple changes to policy over recent
years, which may have influenced the
prevalence of scrapie to varying degrees.
Number of Cases of BSE in Cattle and Scrapie in Sheep 2003-2006
