A descriptive study of the prevalence of atypical and classical scrapie in sheep in 20 European countries

Abstract

Background: The development of active surveillance programmes for transmissible spongiform encephalopathies of small ruminants across Europe has led to the recent identification of a previously undetected form of ovine prion disease, 'atypical' scrapie. Knowledge of the epidemiology of this disease is still limited, as is whether it represents a risk for animal and/or public health. The detection of atypical scrapie has been related to the use of only some of the EU agreed rapid tests. Information about the rapid tests used is not, as yet, available from public reports on the surveillance of transmissible spongiform encephalopathies in small ruminants. We collected detailed results of active surveillance from European countries to estimate and to compare the prevalence of atypical scrapie and classical scrapie in sheep for each country stratified by each surveillance stream; healthy slaughtered and found dead adult sheep.

Results: From the 20 participating countries, it appeared that atypical scrapie was detected in Europe wherever the conditions necessary for its diagnosis were present. In most countries, atypical scrapie and classical scrapie occurred at low prevalence level. The classical scrapie prevalence estimates were more variable than those for atypical scrapie, which appeared remarkably homogeneous across countries, surveillance streams and calendar years of surveillance. Differences were observed in the age and genotype of atypical scrapie and classical scrapie cases that are consistent with previous published findings.

Conclusion: This work suggests that atypical scrapie is not rare compared to classical scrapie. The homogeneity of its prevalence, whatever the country, stream of surveillance or year of detection, contrasts with the epidemiological pattern of classical scrapie. This suggests that the aetiology of atypical scrapie differs from that of classical scrapie.

Figure 1

Quantity of samples examined by tests A, F and G. In red, five countries processed more than 50,000 tests. In orange, one country processed between 20,000 to 50,000 tests. In yellow, four countries processed 5,000 to 20,000 tests. In pale yellow, five countries, performed less than 5,000 tests. In blue, four countries did not perform any of these tests. In grey, 11 countries did not answer the questionnaire (including Malta).

Figure 2

Cases detected in sheep in active surveillance. In red, four countries detected AS only. In strip red and yellow, ten countries detected AS and CS. In yellow, three countries detected CS only. In yellow with black spots, one country detected TSE unclassified case. In green, two countries didn't detect any case. In grey, 11 countries did not answer the questionnaire (including Malta). The period covered ranges from 2002 to 2006. AS cases may have been detected by other means than through active surveillance.

Figure 3

Reported AS cases by test. Percentage (number in brackets) of AS cases detected in sheep through active surveillance in 20 EU countries between 2002 and 2006 per test or combination of tests.

Figure 4

Sheep CSPE (‰) in healthy slaughter surveillance. Crude prevalence estimates are represented with their 95% confidence intervals. The dashed line represents the mean CSPE in healthy slaughter for all the countries and all the years. Graph is restricted to country-years with more than 500 tests because confidence intervals were too large to fit on the graph with an appropriate scale. For 2006, some countries could not provide data for the full year. In Switzerland, surveillance was conducted from July 2004 to June 2005 and was referred to as a single year (2004).

Figure 5

Sheep CSPE (‰) in fallen stock surveillance. Crude prevalence estimates are represented with their 95% confidence intervals. The dashed line represents the mean CSPE in fallen stock for all the countries and all the years. Graph is restricted to country-years with more than 500 tests because confidence intervals were too large to fit on the graph with an appropriate scale and not showing Slovenia due to higher prevalence. For 2006, some countries could not provide data for the full year. In Switzerland, surveillance was conducted from July 2004 to June 2005 and was referred to as a single year (2004).

Figure 6

Sheep CSPE (‰) in fallen stock surveillance in Slovenia. Crude prevalence estimates are presented with their 95% confidence intervals.

Figure 7

Sheep TSEs prevalence (‰) in active surveillance in Cyprus. Crude prevalence estimates are presented with their 95% confidence intervals.

Figure 8

Sheep ASPE (‰) in healthy slaughter surveillance. Crude prevalence estimates are represented with their 95% confidence intervals. The dashed line represents the mean ASPE in healthy slaughter for all the countries and all the years included in ASPE calculation. Graph is restricted to country-years with more than 500 tests A or G because confidence intervals were too large to fit on the graph with an appropriate scale. For 2006, some countries could not provide data for the full year. In Switzerland, surveillance was conducted from July 2004 to June 2005 and was referred to as a single year (2004).

Figure 9

Sheep ASPE (‰), in fallen stock surveillance. Crude prevalence estimates are represented with their 95% confidence intervals. The dashed line represents the mean ASPE in fallen stock for all the countries and all the years included in ASPE calculation. Graph is restricted to country-years with more than 500 tests A or G because confidence intervals were too large to fit on the graph with an appropriate scale.) For 2006, some countries could not provide data for the full year. In Switzerland, surveillance was conducted from July 2004 to June 2005 and was referred to as a single year (2004).

Figure 10

Age distribution of AS and CS cases according to stream of surveillance (in %).

Figure 11

Genotype distribution of AS and CS cases. Figure for 414 AS cases and 844 CS cases detected through active surveillance in fallen stock or healthy slaughter.