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Discrepant epidemiological patterns between classical and atypical scrapie in sheep flocks under French TSE control measures
The Veterinary Journal 185 (2010) 338–340
Contents lists available at ScienceDirect
The Veterinary Journal journal homepage: www.elsevier.com/locate/tvjl
Short Communication
Discrepant epidemiological patterns between classical and atypical scrapie in sheep flocks under French TSE control measures Alexandre Fediaevsky a,b,*, Patrick Gasqui b, Didier Calavas a, Christian Ducrot b a b
Agence Française de Sécurité Sanitaire des Aliments, Unité Epidémiologie, 31 Avenue Tony Garnier, F69364 Lyon cedex 07, France INRA, UR 346 Unité d’Epidémiologie Animale, F63122 Saint-Genès-Champanelle, France
a r t i c l e
i n f o
Article history: Accepted 21 June 2009
Keywords: Scrapie Sheep Transmissible spongiform encephalopathy Epidemiology Communicable disease control
a b s t r a c t The occurrence of secondary cases of atypical and classical scrapie was examined in 340 outbreaks of atypical and 296 of classical sheep scrapie detected in France during active surveillance programmes between 2002 and 2007. The prevalence of atypical scrapie in these flocks was 0.05% under selective culling and 0.07% under intensified monitoring i.e. not significantly different from that detected during active surveillance of the general population (P > 0.5), whereas these figures were much higher for classical scrapie (3.67% and 0.25%, respectively, P < 10 5). In addition the number of atypical scrapie cases per outbreak did not indicate clustering. The results suggest that atypical scrapie occurs spontaneously or is not particularly contagious, and that the control measures in force allowed appropriate control of classical scrapie but were not more efficient than active surveillance in detecting cases of atypical scrapie. Ó 2009 Elsevier Ltd. All rights reserved.
The ability of sheep to transmit transmissible spongiform encephalopathies (TSEs) in natural conditions, as demonstrated for classical scrapie (CS), caused fear of a possible spread of bovine spongiform encephalopathy (BSE) in sheep (Baylis et al., 2002) and concerns that scrapie could hide BSE. The current European TSE surveillance programme in small ruminants led to the discovery of atypical scrapie (AS), a ubiquitous though rare disease that occurs mostly in old animals and preferentially those carrying certain prion protein (PRNP) genotypes (Benestad et al., 2008, 2003; Fediaevsky et al., 2008). The epidemiological and pathological features of AS are generally considered to indicate that the disease would not be contagious under natural conditions or only at a very low level (Benestad et al., 2008). However no formal evidence has been put forward and, in many countries, stringent control measures are still applied to flocks following the detection of a single AS case. The prevalence of secondary cases of AS and CS, in sheep from French flocks subjected to scrapie control measures between 2002 and 2007 was investigated in this study. Two types of measures, depending on the history of the case could apply following the detection of an index case of scrapie, whatever its type. These were (1) selective culling when the case had stayed in the same flock for its entire life; this culling of sheep genetically susceptible to CS is applied to case flock mates, to any sheep from the birth cohort of the case which had been reared with the case during its first * Corresponding author. Address: Agence Française de Sécurité Sanitaire des Aliments, Unité Epidémiologie, 31 Avenue Tony Garnier, F69364 Lyon cedex 07, France. Tel.: +33 473624050; fax: +33 473624548. E-mail address: [email protected] (A. Fediaevsky). 1090-0233/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.tvjl.2009.06.019
year of life and to all offspring when the case was a female; (2) intensified monitoring when the case had stayed in different flocks; this monitoring is applied in the flocks where the case had stayed and consisted of testing all adult sheep leaving the flock for the slaughterhouse or rendering plant over a 3 year period. It could be replaced by selective culling if further cases were detected in those flocks. All TSE tests related to the same index case were linked to the number of outbreaks and held in a national database. Some outbreaks were excluded from the analysis because of poor record-keeping. Before 2007, secondary cases detected within CS outbreaks were not always typed by the national reference laboratory, and undetermined cases were assumed to be CS. This was confirmed by data obtained from 2007 onwards, in which AS accounted for <2% of the scrapie cases detected in CS outbreaks. All statistical analyses were performed with R 2.6.1 for Windows (R Development Core Team, 2008). The estimated prevalence of secondary scrapie cases was determined as described by Fediaevsky et al. (2008) and was computed, together with the 95% binomial confidence interval, for each outbreak and for outbreaks pooled according to type of index case and type of control measure. These prevalences were compared by Chi-Square tests with those obtained from the active surveillance programmes for the same period. The distribution of prevalence of AS in AS outbreaks was compared to the distribution of CS in CS outbreaks for each type of control measure using a Kolmogorov–Smirnov test and the prevalences compared by Chisquare tests. The observed distribution of the number of AS cases in AS outbreaks subjected to intensified monitoring was then
339
A. Fediaevsky et al. / The Veterinary Journal 185 (2010) 338–340 Table 1 Prevalence per type of scrapie and origin of sheep tested in France, together with 95% binomial confidence interval (CI), for the period 2002–2007. Origin of sheep tested
AS
AS outbreak – selective culling AS outbreak – intensified monitoring Active surveillance at slaughterhouse Active surveillance at rendering plant
CS
CS outbreak – selective culling CS outbreak – intensified monitoring AS outbreak – selective culling AS outbreak – intensified monitoring Active surveillance at slaughterhouse Active surveillance at rendering plant
Number of cases
Number of tests
Prevalence (%)
CI 95% (%)
5
10,883
0.05
0.02–0.11
11
15,045
0.07
0.04–0.13
183
308,298
0.06
0.05–0.07
232
290,936
0.08
0.07–0.09
1152
30,223
3.67
3.47–3.89
148
58,407
0.25
0.21–0.30
11
18,988
0.06
0.03–0.10
0
20,900
0.00
0.00–0.02
Prevalence (%)
Type of scrapie
4.00 3.80 3.60 3.40
0.30 0.25
CS
AS
0.20 0.15 0.10 0.05 0.00 AS-C AS-M
SH
RP AS-C AS-M CS-C CS-M
SH
RP
Origin of Animals Tested
70
430,692
0.02
0.01–0.02
387
523,924
0.07
0.07–0.08
AS, atypical scrapie; CS, classical scrapie.
Fig. 1. Prevalence per type of scrapie and origin of sheep tested in France, together with 95% binomial CI for the period 2002–2007. AS, atypical scrapie outbreak; CS, classical scrapie outbreak; C, selective culling; M, intensified monitoring; SH, active surveillance at slaughterhouse; RP, active surveillance at rendering plant.
compared to a theoretical distribution assuming that the total number of cases in each outbreak followed a binomial distribution depending on the number of animals tested and on an individual probability of disease, p. The hypothesis where p equalled the average prevalence of AS in active surveillance (0.07%) was compared with a hypothesis where p was 2 higher in AS outbreaks (0.14%) than in active surveillance. The number of secondary cases per outbreak was randomly generated and the distribution of the number of outbreaks with, respectively, 1 and >1 AS secondary cases was computed for 10,000 simulations. In total, 340 outbreaks of AS and 296 outbreaks of CS were included. The prevalence of AS was not significantly different under both control measures and under active surveillance (all P > 0.5) (Table 1 and Fig. 1). The prevalence of CS in CS outbreaks was higher when the culling strategy was applied compared to intensified
monitoring, both prevalences being higher than under active surveillance (all P < 10 5). The distributions of the prevalences of AS and CS in AS and CS outbreaks, respectively, were significantly different (all P < 0.05) (Fig. 2), uniform for AS (all P > 0.9) and variable for CS (all P < 10 5). Among the 334 flocks where intensified monitoring was applied, one AS secondary cases was detected in 11 flocks and no AS secondary case was found in 225 flocks. The same distribution was obtained for 7.44% of the simulations and was close to the most frequently simulated distribution (Fig. 3). Assumption of a binomial distribution with uniform probability was consistent with the figures observed in outbreaks but this assumption can be questioned since individual risk of disease is modified by genetic susceptibility (Moreno et al., 2007), which in turn can be modulated by certain risk factors at the farm level (Hopp et al., 2006).
Intensified Monitoring P(Prevalence
P(Prevalence
Selective Culling 1.0
0.8
0.6
0.4
0.2
1.0
0.8
0.6
0.4
0.2
0.0
0.0 0
20
40
60
80
Observed Prevalence (%)
100
0
20
40
60
80
100
Observed Prevalence (%)
Fig. 2. Distributions of the prevalence of secondary cases in AS outbreaks (bold blue line) and in CS outbreaks (thin red line). For instance, the probability that the prevalence of secondary cases of CS in a CS outbreak subjected to selective culling would be <20% was 0.95.
340
A. Fediaevsky et al. / The Veterinary Journal 185 (2010) 338–340
Individual Probability = 0.07%
6 4 0
2
Number of Outbreaks with More than One Secondary Case
4 3 2 1 0
Number of Outbreaks with More than One Secondary Case
5
8
Individual Probability = 0.14%
0
5
10
15
20
5
10
Number of Outbreaks with One Secondary Case
15
20
25
30
35
Number of Outbreaks with One Secondary Case
Fig. 3. Density of probability of the distribution for 10,000 simulations of the number of outbreaks with 1 or more secondary cases, assuming a binomial distribution and an individual probability of AS of 0.07% (left panel) and 0.14% (right panel) given the number of animals tested per outbreak. The black cross indicates the observation (11 outbreaks with 1 secondary case and 0 outbreaks with P1 secondary cases).
However, with a hypothesis of an individual risk of 0.14%, the observed distribution was obtained for only 0.24% of the simulations and was far from the most frequently observed distribution (Fig. 3). In contrast with AS, the prevalence of secondary cases of CS was much higher than under active surveillance, as has already been reported (Tongue et al., 2005) and, as expected, due to the contagiousness of CS. The variability in CS prevalence could be due to differences in genetic structure of the affected flocks, the time elapsed since contamination of the flock with CS and the effect of certain farming practices on infectious disease spread (Corbière et al., 2007). As different types of measures could apply to animals from the same outbreak, the observed differences in prevalence according to type of measure should not be over-interpreted and could be mostly due to difference in genetic resistance. These results confirm that AS is not or is only minimally contagious. Irrespective of the type of control measure applied in AS outbreaks, they did not lead to the detection of a higher proportion of AS cases than active surveillance and the appropriateness of such measures is therefore questionable. In contrast the same control measures led to significant elimination of infected animals in CS outbreaks. Conflict of interest statement None of the authors of this paper has a financial or personal relationship with other people or organisations that could inappropriately influence or bias the content of the paper.
Acknowledgments The authors would like to thank Dr. Thierry Baron (AFSSA Lyon) and his team for typing historical secondary scrapie cases and Patrice Chasset (Ministry of Agriculture) for excellent technical assistance in data sourcing. This study was partially funded by the French Ministry of Agriculture. References Baylis, M., Houston, F., Kao, R.R., McLean, A.R., Hunter, N., Gravenor, M.B., 2002. BSE – a wolf in sheep’s clothing? Trends in Microbiology 10, 563–570. Benestad, S.L., Arsac, J.N., Goldmann, W., Nöremark, M., 2008. Atypical/Nor98 scrapie: properties of the agent, genetics, and epidemiology. Veterinary Research 39, 19. Benestad, S.L., Sarradin, P., Thu, B., Schönheit, J., Tranulis, M.A., Bratberg, B., 2003. Cases of scrapie with unusual features in Norway and designation of a new type Nor98. Veterinary Record 153, 202–208. Corbière, F., Barillet, F., Andréoletti, O., Fidelle, F., Laphitz-Bordet, N., Schelcher, F., Joly, P., 2007. Advanced survival models for risk-factor analysis in scrapie. Journal of General Virology 88, 696–705. Fediaevsky, A., Tongue, S.C., Nöremark, M., Calavas, D., Ru, G., Hopp, P., 2008. A descriptive study of the prevalence of atypical and classical scrapie in sheep in 20 European countries. BMC Veterinary Research 4, 19. Hopp, P., Omer, M.K., Heier, B.T., 2006. A case-control study of scrapie Nor98 in Norwegian sheep flocks. Journal of General Virology 87, 3729–3736. Moreno, C.R., Moazami-Goudarzi, K., Laurent, P., Cazeau, G., Andréoletti, O., Chadi, S., Elsen, J.M., Calavas, D., 2007. Which PrP haplotypes in a French sheep population are the most susceptible to atypical scrapie? Archives of Virology 152, 1229–1232. R Development Core Team, 2008. R: A Language and Environment for Statistical Computing. R Foundation for Statistical Computing. Tongue, S.C., Webb, P., Simmons, M.M., Gubbins, S., 2005. Prevalence of scrapie infection in cull animals from 14 scrapie-affected flocks in Great Britain. Veterinary Record 157, 480–482.
Contents lists available at ScienceDirect
The Veterinary Journal journal homepage: www.elsevier.com/locate/tvjl
Short Communication
Discrepant epidemiological patterns between classical and atypical scrapie in sheep flocks under French TSE control measures Alexandre Fediaevsky a,b,*, Patrick Gasqui b, Didier Calavas a, Christian Ducrot b a b
Agence Française de Sécurité Sanitaire des Aliments, Unité Epidémiologie, 31 Avenue Tony Garnier, F69364 Lyon cedex 07, France INRA, UR 346 Unité d’Epidémiologie Animale, F63122 Saint-Genès-Champanelle, France
a r t i c l e
i n f o
Article history: Accepted 21 June 2009
Keywords: Scrapie Sheep Transmissible spongiform encephalopathy Epidemiology Communicable disease control
a b s t r a c t The occurrence of secondary cases of atypical and classical scrapie was examined in 340 outbreaks of atypical and 296 of classical sheep scrapie detected in France during active surveillance programmes between 2002 and 2007. The prevalence of atypical scrapie in these flocks was 0.05% under selective culling and 0.07% under intensified monitoring i.e. not significantly different from that detected during active surveillance of the general population (P > 0.5), whereas these figures were much higher for classical scrapie (3.67% and 0.25%, respectively, P < 10 5). In addition the number of atypical scrapie cases per outbreak did not indicate clustering. The results suggest that atypical scrapie occurs spontaneously or is not particularly contagious, and that the control measures in force allowed appropriate control of classical scrapie but were not more efficient than active surveillance in detecting cases of atypical scrapie. Ó 2009 Elsevier Ltd. All rights reserved.
The ability of sheep to transmit transmissible spongiform encephalopathies (TSEs) in natural conditions, as demonstrated for classical scrapie (CS), caused fear of a possible spread of bovine spongiform encephalopathy (BSE) in sheep (Baylis et al., 2002) and concerns that scrapie could hide BSE. The current European TSE surveillance programme in small ruminants led to the discovery of atypical scrapie (AS), a ubiquitous though rare disease that occurs mostly in old animals and preferentially those carrying certain prion protein (PRNP) genotypes (Benestad et al., 2008, 2003; Fediaevsky et al., 2008). The epidemiological and pathological features of AS are generally considered to indicate that the disease would not be contagious under natural conditions or only at a very low level (Benestad et al., 2008). However no formal evidence has been put forward and, in many countries, stringent control measures are still applied to flocks following the detection of a single AS case. The prevalence of secondary cases of AS and CS, in sheep from French flocks subjected to scrapie control measures between 2002 and 2007 was investigated in this study. Two types of measures, depending on the history of the case could apply following the detection of an index case of scrapie, whatever its type. These were (1) selective culling when the case had stayed in the same flock for its entire life; this culling of sheep genetically susceptible to CS is applied to case flock mates, to any sheep from the birth cohort of the case which had been reared with the case during its first * Corresponding author. Address: Agence Française de Sécurité Sanitaire des Aliments, Unité Epidémiologie, 31 Avenue Tony Garnier, F69364 Lyon cedex 07, France. Tel.: +33 473624050; fax: +33 473624548. E-mail address: [email protected] (A. Fediaevsky). 1090-0233/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.tvjl.2009.06.019
year of life and to all offspring when the case was a female; (2) intensified monitoring when the case had stayed in different flocks; this monitoring is applied in the flocks where the case had stayed and consisted of testing all adult sheep leaving the flock for the slaughterhouse or rendering plant over a 3 year period. It could be replaced by selective culling if further cases were detected in those flocks. All TSE tests related to the same index case were linked to the number of outbreaks and held in a national database. Some outbreaks were excluded from the analysis because of poor record-keeping. Before 2007, secondary cases detected within CS outbreaks were not always typed by the national reference laboratory, and undetermined cases were assumed to be CS. This was confirmed by data obtained from 2007 onwards, in which AS accounted for <2% of the scrapie cases detected in CS outbreaks. All statistical analyses were performed with R 2.6.1 for Windows (R Development Core Team, 2008). The estimated prevalence of secondary scrapie cases was determined as described by Fediaevsky et al. (2008) and was computed, together with the 95% binomial confidence interval, for each outbreak and for outbreaks pooled according to type of index case and type of control measure. These prevalences were compared by Chi-Square tests with those obtained from the active surveillance programmes for the same period. The distribution of prevalence of AS in AS outbreaks was compared to the distribution of CS in CS outbreaks for each type of control measure using a Kolmogorov–Smirnov test and the prevalences compared by Chisquare tests. The observed distribution of the number of AS cases in AS outbreaks subjected to intensified monitoring was then
339
A. Fediaevsky et al. / The Veterinary Journal 185 (2010) 338–340 Table 1 Prevalence per type of scrapie and origin of sheep tested in France, together with 95% binomial confidence interval (CI), for the period 2002–2007. Origin of sheep tested
AS
AS outbreak – selective culling AS outbreak – intensified monitoring Active surveillance at slaughterhouse Active surveillance at rendering plant
CS
CS outbreak – selective culling CS outbreak – intensified monitoring AS outbreak – selective culling AS outbreak – intensified monitoring Active surveillance at slaughterhouse Active surveillance at rendering plant
Number of cases
Number of tests
Prevalence (%)
CI 95% (%)
5
10,883
0.05
0.02–0.11
11
15,045
0.07
0.04–0.13
183
308,298
0.06
0.05–0.07
232
290,936
0.08
0.07–0.09
1152
30,223
3.67
3.47–3.89
148
58,407
0.25
0.21–0.30
11
18,988
0.06
0.03–0.10
0
20,900
0.00
0.00–0.02
Prevalence (%)
Type of scrapie
4.00 3.80 3.60 3.40
0.30 0.25
CS
AS
0.20 0.15 0.10 0.05 0.00 AS-C AS-M
SH
RP AS-C AS-M CS-C CS-M
SH
RP
Origin of Animals Tested
70
430,692
0.02
0.01–0.02
387
523,924
0.07
0.07–0.08
AS, atypical scrapie; CS, classical scrapie.
Fig. 1. Prevalence per type of scrapie and origin of sheep tested in France, together with 95% binomial CI for the period 2002–2007. AS, atypical scrapie outbreak; CS, classical scrapie outbreak; C, selective culling; M, intensified monitoring; SH, active surveillance at slaughterhouse; RP, active surveillance at rendering plant.
compared to a theoretical distribution assuming that the total number of cases in each outbreak followed a binomial distribution depending on the number of animals tested and on an individual probability of disease, p. The hypothesis where p equalled the average prevalence of AS in active surveillance (0.07%) was compared with a hypothesis where p was 2 higher in AS outbreaks (0.14%) than in active surveillance. The number of secondary cases per outbreak was randomly generated and the distribution of the number of outbreaks with, respectively, 1 and >1 AS secondary cases was computed for 10,000 simulations. In total, 340 outbreaks of AS and 296 outbreaks of CS were included. The prevalence of AS was not significantly different under both control measures and under active surveillance (all P > 0.5) (Table 1 and Fig. 1). The prevalence of CS in CS outbreaks was higher when the culling strategy was applied compared to intensified
monitoring, both prevalences being higher than under active surveillance (all P < 10 5). The distributions of the prevalences of AS and CS in AS and CS outbreaks, respectively, were significantly different (all P < 0.05) (Fig. 2), uniform for AS (all P > 0.9) and variable for CS (all P < 10 5). Among the 334 flocks where intensified monitoring was applied, one AS secondary cases was detected in 11 flocks and no AS secondary case was found in 225 flocks. The same distribution was obtained for 7.44% of the simulations and was close to the most frequently simulated distribution (Fig. 3). Assumption of a binomial distribution with uniform probability was consistent with the figures observed in outbreaks but this assumption can be questioned since individual risk of disease is modified by genetic susceptibility (Moreno et al., 2007), which in turn can be modulated by certain risk factors at the farm level (Hopp et al., 2006).
Intensified Monitoring P(Prevalence
P(Prevalence
Selective Culling 1.0
0.8
0.6
0.4
0.2
1.0
0.8
0.6
0.4
0.2
0.0
0.0 0
20
40
60
80
Observed Prevalence (%)
100
0
20
40
60
80
100
Observed Prevalence (%)
Fig. 2. Distributions of the prevalence of secondary cases in AS outbreaks (bold blue line) and in CS outbreaks (thin red line). For instance, the probability that the prevalence of secondary cases of CS in a CS outbreak subjected to selective culling would be <20% was 0.95.
340
A. Fediaevsky et al. / The Veterinary Journal 185 (2010) 338–340
Individual Probability = 0.07%
6 4 0
2
Number of Outbreaks with More than One Secondary Case
4 3 2 1 0
Number of Outbreaks with More than One Secondary Case
5
8
Individual Probability = 0.14%
0
5
10
15
20
5
10
Number of Outbreaks with One Secondary Case
15
20
25
30
35
Number of Outbreaks with One Secondary Case
Fig. 3. Density of probability of the distribution for 10,000 simulations of the number of outbreaks with 1 or more secondary cases, assuming a binomial distribution and an individual probability of AS of 0.07% (left panel) and 0.14% (right panel) given the number of animals tested per outbreak. The black cross indicates the observation (11 outbreaks with 1 secondary case and 0 outbreaks with P1 secondary cases).
However, with a hypothesis of an individual risk of 0.14%, the observed distribution was obtained for only 0.24% of the simulations and was far from the most frequently observed distribution (Fig. 3). In contrast with AS, the prevalence of secondary cases of CS was much higher than under active surveillance, as has already been reported (Tongue et al., 2005) and, as expected, due to the contagiousness of CS. The variability in CS prevalence could be due to differences in genetic structure of the affected flocks, the time elapsed since contamination of the flock with CS and the effect of certain farming practices on infectious disease spread (Corbière et al., 2007). As different types of measures could apply to animals from the same outbreak, the observed differences in prevalence according to type of measure should not be over-interpreted and could be mostly due to difference in genetic resistance. These results confirm that AS is not or is only minimally contagious. Irrespective of the type of control measure applied in AS outbreaks, they did not lead to the detection of a higher proportion of AS cases than active surveillance and the appropriateness of such measures is therefore questionable. In contrast the same control measures led to significant elimination of infected animals in CS outbreaks. Conflict of interest statement None of the authors of this paper has a financial or personal relationship with other people or organisations that could inappropriately influence or bias the content of the paper.
Acknowledgments The authors would like to thank Dr. Thierry Baron (AFSSA Lyon) and his team for typing historical secondary scrapie cases and Patrice Chasset (Ministry of Agriculture) for excellent technical assistance in data sourcing. This study was partially funded by the French Ministry of Agriculture. References Baylis, M., Houston, F., Kao, R.R., McLean, A.R., Hunter, N., Gravenor, M.B., 2002. BSE – a wolf in sheep’s clothing? Trends in Microbiology 10, 563–570. Benestad, S.L., Arsac, J.N., Goldmann, W., Nöremark, M., 2008. Atypical/Nor98 scrapie: properties of the agent, genetics, and epidemiology. Veterinary Research 39, 19. Benestad, S.L., Sarradin, P., Thu, B., Schönheit, J., Tranulis, M.A., Bratberg, B., 2003. Cases of scrapie with unusual features in Norway and designation of a new type Nor98. Veterinary Record 153, 202–208. Corbière, F., Barillet, F., Andréoletti, O., Fidelle, F., Laphitz-Bordet, N., Schelcher, F., Joly, P., 2007. Advanced survival models for risk-factor analysis in scrapie. Journal of General Virology 88, 696–705. Fediaevsky, A., Tongue, S.C., Nöremark, M., Calavas, D., Ru, G., Hopp, P., 2008. A descriptive study of the prevalence of atypical and classical scrapie in sheep in 20 European countries. BMC Veterinary Research 4, 19. Hopp, P., Omer, M.K., Heier, B.T., 2006. A case-control study of scrapie Nor98 in Norwegian sheep flocks. Journal of General Virology 87, 3729–3736. Moreno, C.R., Moazami-Goudarzi, K., Laurent, P., Cazeau, G., Andréoletti, O., Chadi, S., Elsen, J.M., Calavas, D., 2007. Which PrP haplotypes in a French sheep population are the most susceptible to atypical scrapie? Archives of Virology 152, 1229–1232. R Development Core Team, 2008. R: A Language and Environment for Statistical Computing. R Foundation for Statistical Computing. Tongue, S.C., Webb, P., Simmons, M.M., Gubbins, S., 2005. Prevalence of scrapie infection in cull animals from 14 scrapie-affected flocks in Great Britain. Veterinary Record 157, 480–482.