Surveillance and simulation of bovine spongiform encephalopathy and scrapie in small ruminants in Switzerland

Abstract

Background: After bovine spongiform encephalopathy (BSE) emerged in European cattle livestock in 1986 a fundamental question was whether the agent established also in the small ruminants' population. In Switzerland transmissible spongiform encephalopathies (TSEs) in small ruminants have been monitored since 1990. While in the most recent TSE cases a BSE infection could be excluded, for historical cases techniques to discriminate scrapie from BSE had not been available at the time of diagnosis and thus their status remained unclear. We herein applied state-of-the-art techniques to retrospectively classify these animals and to re-analyze the affected flocks for secondary cases. These results were the basis for models, simulating the course of TSEs over a period of 70 years. The aim was to come to a statistically based overall assessment of the TSE situation in the domestic small ruminant population in Switzerland.

Results: In sum 16 TSE cases were identified in small ruminants in Switzerland since 1981, of which eight were atypical and six were classical scrapie. In two animals retrospective analysis did not allow any further classification due to the lack of appropriate tissue samples. We found no evidence for an infection with the BSE agent in the cases under investigation. In none of the affected flocks, secondary cases were identified. A Bayesian prevalence calculation resulted in most likely estimates of one case of BSE, five cases of classical scrapie and 21 cases of atypical scrapie per 100'000 small ruminants. According to our models none of the TSEs is considered to cause a broader epidemic in Switzerland. In a closed population, they are rather expected to fade out in the next decades or, in case of a sporadic origin, may remain at a very low level.

Conclusions: In summary, these data indicate that despite a significant epidemic of BSE in cattle, there is no evidence that BSE established in the small ruminant population in Switzerland. Classical and atypical scrapie both occur at a very low level and are not expected to escalate into an epidemic. In this situation the extent of TSE surveillance in small ruminants requires reevaluation based on cost-benefit analysis.

Figure 1

Discriminatory PrP^d immunohistochemistry. Classical Scrapie (a+b) and ovine BSE (c+d) are discriminated by using two antibodies linking at distinct epitopes of the prion protein. The core-binding MAb R145 shows intraneuronal immunolabeling in classical scrapie (a) and ovine BSE (c) while the n-terminal-binding MAb 12B2 only detects intraneuronal labeling in classical scrapie (b) whereas PrP^d in ovine BSE remains undetected in the neurons. Two examples, one of a goat (ID 15355, e and f) and one of a sheep (ID S1RS, g and h) exemplify the findings in the cases under investigation. For the results of the complete sample set refer to table 4.

Figure 2

Discriminatory Western Immunoblot for case 30842. Brain tissue homogenate of sheep 30842 along with BSE-positive, scrapie-positive and negative controls was digested with proteinase K and MAbs P4 and 6H4 were used for PrP^res detection and discrimination of BSE and scrapie by Western immunoblot. The pattern of the di-, mono- and non- glycosylated moieties of case 30842 shows typical features of classical scrapie with regard to molecular mass of non-glycoslated PrP^res (lower band) and the reactivity with the n-terminal MAb P4.

Figure 3

Forecasting TSE prevalences in Switzerland. Means (dots) and 90% probability intervals (light lines) of the number of infected animals and flocks forecasted over 70 years by the simulation models are shown for small ruminant BSE, classical scrapie (CSC) and atypical scrapie (ASC).