The Spreading of the Bluetongue Virus Throughout Europe
Conte, A., Gilbert, M. and Goffredo, M. 2009. Eight years of entomological surveillance in Italy show no evidence of Culicoides imicola geographical range expansion. Journal of Applied Ecology 46: 1332-1339.
In response to even earlier fears of a potential BTV invasion, a national surveillance program for C. imicola had been established in Italy in the year 2000, where 70,000 light-trap collections were made at about 3800 different sites. Using the first year of data obtained from this program, Conte et al. defined the spatial distributions of three different C. imicola infection zones: zone I (endemicity), zone II (transition), and zone III (absence). Then, using data from 2002-2007, they quantified how C. imicola populations evolved through time in these three zones, working under the logical assumption that "a species that is undergoing geographical range expansion should have a population that remains stable over time in zone I and increases in zones II and III."
The three researchers say their results indicated there had been "no detectable range expansion of C. imicola population in Italy over the past six years." In fact, they report that "a weak, but significant reduction was observed in the transition zone [italics added]." Conte et al. therefore conclude that their data "support the hypothesis that the spread of BTV in Italy is not because of the geographical expansion of its main vector, but to a modification of the interaction between the virus, the vector and the environment, as may also have been the case in northern Europe." As for the future, they say their results highlight the fact that "precautions should be taken when inferring range progression for species requiring highly targeted forms of sampling and for which a constant probability of detection over time should be established," demonstrating once again that it is easy to blame global warming for the poleward expansion of a vector-spread disease, but that it is quite another thing to prove that such is truly the case.
Goffredo, M., Conte, A., Cocciolito, R. and Meiswinkel, R. 2003. Distribution and abundance of Culicoides imicola in Italy. Veterinaria Italiana 47: 22-32.
Mellor, P.S. 2004. Infection of the vectors and bluetongue epidemiology in Europe. Veterinaria Italiana 40: 167-174.
Mellor, P.S., Carpenter, S., Harrup, L, Baylis, M. and Mertens, P.P.C. 2008. Bluetongue in Europe and the Mediterranean Basin: History of occurrence prior to 2006. Preventive Veterinary Medicine 87: 4-20.
Purse, B.V., Mellor, P.S., Rogers, D.J., Samuel, A.R., Mertens, P.P.C. and Baylis, M. 2005. Climate change and the recent emergence of bluetongue in Europe. Nature Reviews Microbiology 3: 171-181.