Arch Virol (2006) 151: 735–751
Modelling the spread of scrapie in a sheep ﬂock: evidence
for increased transmission during lambing seasons
, M. E. Chase-Topping
, L. Matthews
, D. Lajous
, N. Hunter
, J. D. Foster
, G. Simm
, and M. E. J. Woolhouse
INRA, Unit´e de Math´ematiques et Informatique Appliqu´ees, Jouy-en-Josas, France
Centre for Tropical Veterinary Medicine, University of Edinburgh, Midlothian, U.K.
INRA, Station d’Am´elioration G´en´etique des Animaux,
Institute for Animal Health, BBSRC Neuropathogenesis Unit, Edinburgh, U.K.
Scottish Agricultural College, Edinburgh, U.K.
Received January 22, 2004; accepted September 5, 2005
Published online November 23, 2005
Summary. Presence of scrapie infectivity in the placenta suggests the possibility
of increased transmission of scrapie during the lambing season. This hypothesis
was explored here using a mathematical model of scrapie transmission dynamics
which has previously been successfully used to study several scrapie outbreaks
in Scottish sheep ﬂocks. It was applied here to the Langlade experimental sheep
ﬂock (INRA Toulouse, France), in which a natural scrapie epidemic started in
1993. Extensive data were available, including pedigree, scrapie histopathological
diagnoses and PrP genotypes. Detailed simulations of the scrapie outbreak reveal
that the observed patterns of seasonality in incidence can not be accounted for by
seasonality in demography alone and provide strong support for the hypothesis of
increased transmission during lambing. Observations from several other scrapie
outbreaks also showing seasonal incidence patterns support these conclusions.
Scrapie is a naturally occurring transmissible spongiform encephalopathy of sheep.
The disease is associated with a conformationally abnormal form of the prion pro-
tein PrP. Polymorphisms of the PrP gene encoding for this protein largely control
the susceptibility and resistance of sheep to the disease . The epidemiology
of scrapie and particularly the transmission mechanisms are still incompletely
understood . Vertical transmission, i.e. maternal transmission to lamb, is
thought to occur although it is possible that contamination occurs post-natally