Descriptive epidemiology of scrapie in Great Britain: results of a postal survey

Abstract

In 1998, a questionnaire was sent to 11,554 British sheep farmers to determine how many believed that scrapie cases had occurred in their flock; 61.4 per cent of them responded anonymously. The results indicated that 14.9 per cent of farmers with more than 30 breeding ewes thought that they had ever experienced scrapie in their flock and 2.7 per cent thought that they had had cases in the past 12 months. A comparison of these results with the number of farmers reporting suspect scrapie cases to MAFF, in accordance with the statutory requirement, suggests that only 13 per cent of farmers who suspect that they may have cases of scrapie are currently reporting them. Scrapie occurred in all regions of the country but there was an apparent regional variation. Larger farms and those with purebred sheep appeared to be at greater risk of having cases. Other differences between affected and unaffected farms included lambing practices and sheep purchasing policy. On the majority of farms the first case occurred in a purchased animal. The survey also revealed a need for the provision of further information about scrapie to farmers.

Epidemiological analysis of data for scrapie in Great Britain

“…Focusing therefore on results for somewhat higher reproduction numbers (say R 0 >1 . 25), we conclude that a substantial proportion of clinical cases must remain undetected -perhaps 80 % or more, in line with earlier suggestive estimates of under-reporting [17]. Figure 2a illustrates how scenarios combining low case ascertainment with selective breeding can match both the outbreak duration and annual incidence estimates obtained from the postal-survey data.…”

“…Figure 3 displays this quantity for the scenarios considered in Figure 2, calculated as explained in the 'Between-flock transmission model ' section. Inclusion of the prevalence arising from epidemics in bought-in animals only [17] adds y25 % to these numbers. Assuming that ascertainment is such that not more than half of the outbreaks remain undetected, the infection prevalence range thus found across all scenarios considered is in line with our previously published order-of-magnitude estimate of between 0 .…”

“…The postal survey [17] results provide information on (a) annual rates of scrapie-affected farms, (b) the fraction of farms affected over the period 1993-1998 and in 1998, and (c) on the annual incidence r, on affected farms. In order to calculate the mean duration of scrapie outbreaks from these data, we need to relate this mean duration to the number of farms that have experienced at least one scrapie case over a defined time period.…”

“…This value is based on multiplying the median annual incidence for farms with homebred cases as given in ref. [17] with the ratio between the mean and median flock sizes given in ref. [16].…”

Dynamics of a scrapie outbreak in a flock of Romanov sheep – estimation of transmission parameters

“…As a result of the epidemic of bovine spongiform encephalopathy (BSE) and its link with new-variant Creutzfeldt-Jakob disease (vCJD) in humans, control of scrapie [5] and possibly other TSEs in mammalian species has become a priority, and epidemiological research into scrapie has been intensified. In Britain, a postal survey was conducted in 1998 [6][7][8][9][10], and interim results of a large-scale survey of sheep genotypes and occurrence of scrapie have been reported by Baylis et al [11]. In 1998, a selective breeding programme for control and eventual eradication of scrapie has been started in The Netherlands.…”