Arch Virol (2001) 146: 1173–1186
Population dynamics of a scrapie outbreak
, P. G. Coen
, J. D. Foster
, N. Hunter
, and M. E. J. Woolhouse
Centre for Tropical Veterinary Medicine, University of Edinburgh, Midlothian, U.K.
Institute for Animal Health BBSRC Neuropathogenesis Unit, Edinburgh, U.K.
Accepted October 24, 2000
Summary. A detailed analysis of a scrapie outbreak in a ﬂock of Cheviot sheep
is described. A total of 33 cases of 1473 sheep born to the ﬂock were reported
between 1985 and 1994. The epidemiology of scrapie can only be understood
with reference to sheep demography, the population genetics of susceptibility to
scrapie, pathogenesisduring a long incubation period, and the rate of transmission
(by both horizontal and vertical routes), all of which interact in complex ways. In
recent work a mathematical model incorporating these elements was developed
and successfully reproduced key features of an earlier outbreak of scrapie in
this ﬂock. Here an application of the model to the second outbreak is described.
The model accurately reproduces observed allele frequencies and total numbers
of susceptible animals remaining at the end of the outbreak. A major difference
between the two outbreaks is the very much lower force of infection in the second
outbreak. This provided additional information which suggested two ways in
which our existing assumptions be reﬁned; ﬁrstly, older animals have reduced
susceptibilitytoscrapieandsecondly,homozygous andheterozygous susceptibles
have different incubation periods.
Scrapie is a transmissible spongiform encephalopathy which naturally infects
sheep. It is a fatal and incurable disease which is estimated to have occurred on
over10% of British sheep farms (and over 5% of farmsin the last 6 years) . The
epidemiology of scrapie is complex and remains incompletely understood .
There have, however, been several well studied outbreaks in individual ﬂocks ,
 and these provide data which are proving extremely valuable for developing
a quantitative understanding of the dynamics of this disease.
Current address: Department of Child Health, Royal London Hospital, Whitechapel,