The Spreading of the Bluetongue Virus Throughout Europe

Conte et al. (2009) write that "the midge Culicoides imicola is the principal vector of bluetongue virus (BTV) that causes an infectious disease of domestic and wild ruminants," and that "over the last ten years, BTV has invaded Mediterranean countries and much of Northern Europe," inducing several scientists (and others!) to contend that the BTV vector had expanded its range northward "because of rising temperatures," as suggested by the work of Mellor (2004), Purse et al. (2005) and Mellor et al. (2008). However, a careful examination of Culicoides population data in Italy prior to 2000 was made by Goffredo et al. (2003); and they determined that "trapping conditions of previous collections would have had very little chance of catching C. imicola," or detecting its presence, suggesting there was insufficient evidence to make the case for a warming-induced northward expansion of the BTV vector, due to the fact that it may have already been present there but undetected.

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.

Additional References
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.