Modelling the distribution of outbreaks and Culicoides vectors in Sicily: towards predictive risk maps for Italy.
Vector (911 light-trap catches from 269 sites) and serological surveillance data were obtained during recent bluetongue (BT) outbreaks in Sicily. The distributions of Culicoides vectors are compared with that of bluetongue virus (BTV) to determine the relative role of different vectors in BTV transmission in Sicily. The 'best' climatic predictors of distribution for each vector species were selected from 40 remotely-sensed variables and altitude at a 1 km spatial resolution using discriminant analysis. These models were used to predict species presence in unsampled pixels across Italy. Although Culicoides imicola, the main European vector, was found in only 12% of sites, there was close correspondence between its spatial distribution and that of the 2000 and 2001 outbreaks. All three candidate vectors C. pulicaris, C. newsteadi and C. obsoletus group were widespread across 2002 outbreak sites but C. newsteadi was significantly less prevalent in outbreak versus non-outbreak sites in Messina and BTV has been isolated from wild-caught adults of both C. pulicaris and C. obsoletus in Italy. The yearly distribution and intensity of outbreaks is attributable to the distribution and abundance of the vectors operating in each year. Outbreaks were few and coastal in 2000 and 2001 due to the low abundance and prevalence of the vector, C. imicola. They were numerous and widespread in 2002, following hand-over of the virus to more prevalent and abundant novel vector species, C. pulicaris and C. obsoletus. Climatic determinants of distribution were species-specific, with those of C. obsoletus group and C. newsteadi predicted by temperature variables, and those of C. pulicaris and C. imicola determined mainly by normalised difference vegetation index (NDVI), a variable correlated with soil moisture, vegetation biomass and productivity. The predicted continuous presence of C. pulicaris along the Appenine mountains, from north to south Italy, suggests BTV transmission may be possible in a large proportion of this region and that seasonal transhumance between C. imicola-free areas should not generally be considered safe. Future distribution models for C. imicola in Sicily should include non-climatic environmental variables that may influence breeding site suitability such as soil type.
Purse
B V
BV
Institute for Animal Health, Surrey, United Kingdom.
Caracappa
S
S
Marino
A M F
AM
Tatem
A J
AJ
Rogers
D J
DJ
Mellor
P S
PS
Baylis
M
M
Torina
A
A