A postal survey of British sheep farmers provided information on the propor
tion of farms that experienced their first case of scrapie in each year bet
ween 1962 and 1998. We found no evidence of a large increase in the proport
ion of scrapie-affected farms prior to, during or following the epidemic of
BSE in British cattle. After correcting for between-farm heterogeneity in
the probability of acquiring scrapie, we estimated the yearly between-flock
force of infection since 1962. The current force of infection is estimated
at approximately 0.0045 per farm per year and combined with a simple model
of scrapie spread provides an estimate of the average duration of a scrapi
e outbreak on an individual farm. Considering all farms, the average outbre
ak lasts for five years, but if only those farms that have cases in animals
born on the farm are considered, it lasts 15 years. We use these parameter
estimates to compare the proportion of farms with scrapie in time periods
of different lengths. In the survey 2.7% of farms had a case in 1998. The 5
.3% of farms reporting having a case between 1993 and 1997 is consistent wi
th the hypothesis that the scrapie force of infection remained constant ove
r this period.