Summary
Bluetongue
(BT) is an orbiviral disease of ruminant livestock. In Europe, the
insect vectors implicated in BT virus transmission are Culicoides
imicola, C. obsoletus, C. scoticus, C. pulicaris
and C. dewulfi. C. imicola an Afro-Asiatic
species is believed to account for approximately 90% of BT
virus transmission; the latter four species are endemic to the Palaearctic
region and are becoming increasingly important as the zone of BT virus
infection in Europe moves northwards. As Culicoides distribution
and ecology is critical in the spread of BT, it is essential to map
their geographic ranges and to determine the ecological and climatic
factors influencing their occurrence. Since 2000, more than 50 000
light trap collections have been made across Italy and the presence/absence
and abundance of C. imicola ascertained; 3000 of
these collections were also screened for two species (C. obsoletus,
C. scoticus) of the Obsoletus Complex. To gain further
insights into vector breeding habitats, spatial analysis techniques
and statistical methodologies were used to investigate the association
between the trap collection (municipality [polygons] and geographic
point level) data and environmental and climatic variables. For the
first dataset, 100 municipalities with the largest collections
of C. imicola and 100 of the Obsoletus Complex, were
selected and the average values of six independent climatic/environmental
variables were calculated for each polygon. Factor analysis identified
two principal factors explaining 84% of the total variability in climatic
and environmental variables; 87.5% of trap collections were correctly
classified by a discriminant analysis model. C. imicola was
found to predominate in more sparsely vegetated habitats. Obsoletus
Complex species were associated with a more densely vegetated habitat.
For the second dataset, the presence/ absence of C. imicola
at 172 sites in the region of Calabria (southern Italy) was investigated
using 13 climatic, environmental and pedological variables. Factor
analysis identified four factors; 89.5% of trap collections were correctly
classified by a discriminant analysis model. Proximity to water-holding,
clay soils was the most important factor explaining the markedly disjoint
distribution of C. imicola in the toe of Italy.
Soil type may explain the patchy distribution of C. imicola
elsewhere in Mediterranean Europe.
Keywords
Bluetongue,
Clay, Culicoides imicola, Forest, Habitat, Italy, Obsoletus
Complex, Shade, Soil, Sunlight, Vector.
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