Use of a preclinical test in the control of classical scrapie

Abstract

Scrapie control in Great Britain (GB) was originally based on the National Scrapie Plan's Ram Genotyping scheme aimed at reducing the susceptibility of the national flock. The current official strategy to control scrapie in the national flock involves culling susceptible genotypes in individual, known affected flocks (compulsory scrapie flock scheme or CSFS). However, the recent development of preclinical test candidates means that a strategy based on disease detection may now be feasible. Here, a deterministic within-flock model was used to demonstrate that only large flocks with many home-bred ewes are likely to be a significant risk for flock-to-flock transmission of scrapie. For most other flocks, it was found that the CSFS could be replaced by a strategy using a currently available live test without excessive risk to other farmers, even if the proportion of susceptible genotypes in the flock is unusually large. Even for flocks that represent a high risk of harbouring a high prevalence of infection, there would be limited probability of onward transmission if scrapie is detected soon after disease introduction (typically less than 5 years). However, if detection of disease is delayed, the existing CSFS strategy may be the most appropriate control measure in these cases.

Fig. 1.

Summary of the flock parameters used in the model and description of the potential number of infected breeding sheep sold to other farms by each flock type if scrapie is present in the flock over a 15 year period. The number of flocks in each flock category reported in the postal survey data are described in each bubble. Pure-bred flocks with small proportions of home-bred sheep were not examined in this model as these flock types are extremely rare. The percentage of the flock subgroup, major flock category (commercial or pure-bred) and total flocks (commercial and pure-bred) are reported, respectively, in parentheses. For example, in the high-risk category, 83 % (91/109) of large pure-bred flocks with a large percentage of home-bred sheep sold more than one infected breeding sheep each year in a 15 year epidemic. This high-risk group comprised 14 % (91/635) of all pure-bred flocks and 2 % [91/(635+3832)] of all pure-bred and commercial flocks.

Fig. 2.

Violin plot showing the distribution of flocks with respect to the average number of infected breeding sheep sold to other farms each year by each flock type in the first year of an epidemic. The Violin plot is similar to a box plot, except that it also shows the probability density of the data at different values. The line=1 represents one infected sheep sold on per year and denotes the cut-off point between low-risk and moderate-risk flocks.

Fig. 3.

Mean number of infected breeding sheep sold by different flock types per year (in each year of an epidemic) after implementation of live-test strategies. The CSFS strategy reduced the number of infected sheep sold to zero in all cases (and is not shown on these graphs). The upper bars indicate the potential maximum number of infected breeding sheep sold per year (per 100 sheep in a flock). The minimum number of infected sheep sold per year (per 100 sheep in a flock) is zero in all years. The black line represents one infected sheep sold.