Quantifying the risk from ovine BSE and the impact of control strategies

Fryer, Helen; Baylis, Matthew; Sivam, Kumar; McLean, Angela R.

doi:10.1098/rspb.2007.0353

Cited by 11 publications

(17 citation statements)

References 33 publications

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“…The occurrence of atypical cases in the sheep population under selection justifies continued monitoring for scrapie prevalence. Since BSE can infect sheep, this monitoring is also essential for understanding the theoretical risk of BSE in the sheep population [16,21]. …”

Section: Discussionmentioning

confidence: 99%

Trends in genotype frequency resulting from breeding for resistance to classical scrapie in Belgium (2006~2011)

2013

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In sheep, susceptibility to scrapie is mainly determined by codons 136, 154, and 171 of the PRNP gene. Five haplotypes are usually present (ARR, ARQ, ARH, AHQ, and VRQ). The ARR haplotype confers the greatest resistance to classical scrapie while VRQ renders animals most susceptible. In 2004, the European Union implemented a breeding program that promotes selection of the ARR haplotype while reducing the incidence of VRQ. From 2006 to 2011 in Belgium, frequency for the ARR/ARR genotypes increased from 38.3% to 63.8% (n = 6,437), the ARQ haplotype diminished from 21.1% to 12.9%, and the VRQ haplotype decreased from 2.0% to 1.7%. The status of codon 141, a determinant for atypical scrapie, was also evaluated. Out of 27 different breeds (n = 5,163), nine were abundant. The ARR/ARR frequency increased in eight of these nine major breeds. The selection program has had a major impact on the ARR haplotype frequency in Belgium. However, the occurrence of atypical scrapie represents a critical point for this program that warrants the continuous monitoring of scrapie. Additionally, genotype frequencies among the breeds varied greatly. Texel, a breed that is common in Belgium, can still be selected for due to its average ARR frequency.

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Section: Discussionmentioning

confidence: 99%

Trends in genotype frequency resulting from breeding for resistance to classical scrapie in Belgium (2006~2011)

2013

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“…To date, surveillance using biochemical tests that can discriminate between experimental ovine BSE and scrapie has failed to detect any evidence of naturally BSE-infected sheep (Thuring et al, 2004;Eloit et al, 2005;Sharpe et al, 2005;Stack et al, 2006), although there have been one confirmed case (Eloit et al, 2005) and one suspected case of natural BSE in goats. The risk to the human population of BSE in sheep requires further investigation and key to estimating such a risk is the calculation of the prevalence of BSE in sheep at any given point in time, which is in turn dependent on the susceptibility of sheep to the BSE agent and the efficiency of subsequent sheep-to-sheep and sheepto-human passage (Baylis, 2002;Ferguson et al, 2002;Kao et al, 2002;Fryer et al, 2007). Inaccurate data on BSEsusceptible ovine genotypes could potentially have a significant effect on the assessment of risk of BSE in sheep to the human consumer (Kao et al, 2003).…”

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confidence: 99%

Protective effect of the T112 PrP variant in sheep challenged with bovine spongiform encephalopathy

Saunders

Lantier

Cawthraw

et al. 2009

Journal of General Virology

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Sheep with an ARQ/ARQ PRNP genotype at codon positions 136/154/171 are highly susceptible to experimental infection with bovine spongiform encephalopathy (BSE). However, a number of sheep challenged orally or intracerebrally with BSE were clinically asymptomatic and found to survive or were diagnosed as BSE-negative when culled. Sequencing of the full PRNP gene open reading frame of BSE-susceptible and -resistant sheep indicated that, in the majority of Suffolk sheep, resistance was associated with an M112T PRNP variant (TARQ allele). A high proportion (47 of 49; 96 %) of BSE-challenged wild-type (MARQ/MARQ) Suffolk sheep were BSE-infected, whereas none of the 20 sheep with at least one TARQ allele succumbed to BSE. Thirteen TARQ-carrying sheep challenged with BSE are still alive and some have survival periods equivalent to, or greater than, reported incubation periods of BSE in ARR/ARR and VRQ/VRQ sheep.

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“…A full description of the model variables, parameters and equations is presented in Fryer et al 2007 [23], reproduced in Boden et al 2010 [16].…”

Section: Methodsmentioning

confidence: 99%

An Economic Evaluation of Preclinical Testing Strategies Compared to the Compulsory Scrapie Flock Scheme in the Control of Classical Scrapie

et al. 2012

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Cost-benefit is rarely combined with nonlinear dynamic models when evaluating control options for infectious diseases. The current strategy for scrapie in Great Britain requires that all genetically susceptible livestock in affected flocks be culled (Compulsory Scrapie Flock Scheme or CSFS). However, this results in the removal of many healthy sheep, and a recently developed pre-clinical test for scrapie now offers a strategy based on disease detection. We explore the flock level cost-effectiveness of scrapie control using a deterministic transmission model and industry estimates of costs associated with genotype testing, pre-clinical tests and the value of a sheep culled. Benefit was measured in terms of the reduction in the number of infected sheep sold on, compared to a baseline strategy of doing nothing, using Incremental Cost Effectiveness analysis to compare across strategies. As market data was not available for pre-clinical testing, a threshold analysis was used to set a unit-cost giving equal costs for CSFS and multiple pre-clinical testing (MT, one test each year for three consecutive years). Assuming a 40% within-flock proportion of susceptible genotypes and a test sensitivity of 90%, a single test (ST) was cheaper but less effective than either the CSFS or MT strategies (30 infected-sales-averted over the lifetime of the average epidemic). The MT strategy was slightly less effective than the CSFS and would be a dominated strategy unless preclinical testing was cheaper than the threshold price of £6.28, but may be appropriate for flocks with particularly valuable livestock. Though the ST is not currently recommended, the proportion of susceptible genotypes in the national flock is likely to continue to decrease; this may eventually make it a cost-effective alternative to the MT or CSFS.

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Quantifying the risk from ovine BSE and the impact of control strategies

Cited by 11 publications

References 33 publications

Trends in genotype frequency resulting from breeding for resistance to classical scrapie in Belgium (2006~2011)

Trends in genotype frequency resulting from breeding for resistance to classical scrapie in Belgium (2006~2011)

Protective effect of the T112 PrP variant in sheep challenged with bovine spongiform encephalopathy

An Economic Evaluation of Preclinical Testing Strategies Compared to the Compulsory Scrapie Flock Scheme in the Control of Classical Scrapie

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