BSE: Science & research - Experimental BSE in sheep
BSE has never been found in the UK sheep flock. However, sheep ate the same feed (meat and bone meal) that is thought to have been responsible for the BSE epidemic in cattle. In addition, the clinical signs of scrapie and experimental BSE in sheep are very similar and there is a possibility that BSE in sheep may have occurred but been masked by scrapie.
Research has been commissioned to investigate various aspects of the possibility of BSE in sheep, including:
- Investigation of the possibility that BSE entered the national flock through contaminated animal feed
- Transmission routes for BSE in sheep
- The stability of the BSE agent in sheep
- Mixed infection of scrapie and BSE in the same animal
Investigation of the possibility that BSE entered the national flock through contaminated animal feed
Experimental studies have shown that sheep are susceptible to BSE by the oral route and the resulting disease has the same strain characteristics as cattle BSE. Two long-term projects are underway to determine the pathogenesis of the disease. Preliminary results indicate that a BSE infection in sheep would give a pattern of disease in both the Central Nervous System (CNS) and also tissues outside of the CNS, such as in the spleen or lymph nodes, similar to scrapie (projects SE1428, SE1929).
A retrospective search for BSE was conducted of all scrapie positive cases identified by passive surveillance in GB between 1st January 1998 and 31st October 2001, with prospective testing of both passive and active surveillance samples from November 2001 onwards. This search used diagnostic tools that were able to discriminate between experimental BSE in sheep and classical scrapie. Approximately 2,500 UK scrapie samples have been tested and no cases of BSE have been found (1).
Reference
1. Stack M, Jeffrey M, Gubbins S, Grimmer S, González L, Martin S, Chaplin M, Webb P, Simmons M, Spencer Y, Bellerby P, Hope J, Wilesmith J & Matthews (2006) Journal of General Virology 87(Pt 7): 2099-2107. Full text is available here.
The fact that no case of BSE in sheep has ever been detected indicates that the prevalence of BSE in UK sheep must be very low or zero. In 2006 the SEAC Sheep subgroup 
were asked to consider whether the risk from BSE in sheep could be quantified and for their views on the latest science underpinning the national scrapie plan. The Sheep Subgroup concluded that the most likely prevalence of BSE in the UK sheep flock was zero, and in the worst case no more than ten flocks would be infected.
Transmission routes of BSE: Could BSE be maintained in the national flock?
It is important to determine whether BSE can be spread and maintained naturally in sheep. Two complementary studies are underway to address this issue, in particular the possibility of maternal transmission (projects SE1946, SE1424).
Research has already shown that BSE can be transmitted from one generation to the next in a highly infected experimental sheep flock. The research is at too early a stage to determine whether horizontal transmission has also occurred (projects SE1946, SE1424).
Studies are underway to determine the minimum oral dose of exposure which will result in BSE infection in various breeds of sheep. Also the effect of age at challenge on the susceptibility of a sheep to BSE is being investigated (projects SE1844, SE1846).
The stability of the BSE agent in sheep
One of the major questions is, if BSE had been maintained in the national flock what would the disease look like today and could it be identified by current diagnostic tests? Ongoing research shows that when BSE is transmitted from sheep to sheep experimentally it retains its BSE-specific characteristics. This indicates that if BSE is present in the current sheep population it would be recognisable as BSE and therefore detected by current tests (projects SE1435, SE1945).
Further information on the methods to detect BSE in sheep is available in TSE diagnostics.
Mixed infection of scrapie and BSE in the same animal
There is the possibility that a sheep could be infected with both scrapie and BSE at the same time. It is unknown what signs of disease would occur and whether a BSE infection could be positively identified from the diagnostic test results. Research is underway to investigate whether BSE would be detected in a mixed infection. Due to the low number of classical scrapie cases the probability of a mixed infection is low (projects SE1429 & SE1436).
Action taken
Data from the Defra-funded sheep TSE research has been used in numerous risk assessments and used to ensure that current TSE controls are proportionate and based on the latest scientific knowledge. For example, contingency planning for BSE in sheep and the review of the NSP Ram Genotyping Scheme (RGS).
Links to further information
Page last modified: 18 March, 2008