The 'Culicoides obsoletus group' in Italy: relative abundance, geographic range, and role as vector for Bluetongue virus.

Abstract

As Bluetongue virus (BTV) spread in Italy following its first incursion in 2000, it soon became apparent that, besides Culicoides imicola, additional species of the subgenus Avaritia were involved as vectors, namely one or more of the species that belong to the so-called 'Culicoides obsoletus group', which comprises C. dewulfi, C. chiopterus, C. obsoletus sensu stricto, C. scoticus and C. montanus; the three last named species are considered generally as forming the Obsoletus complex. This study presents the findings made over the last decade and more, within the Italian entomological surveillance program for Bluetongue. It describes the integrated morphological and molecular approach used to identify the species of the 'C. obsoletus group', maps in detail their relative abundances and geographic ranges in Italy, clarifies the hitherto unknown comparative seasonal abundances of C. obsoletus s.s. and C. scoticus in a site in Central Italy, and provides further details on the potential vector status of five species of the 'C. obsoletus group', with emphasis on C. obsoletus s.s., C. scoticus and C. montanus. Unlike the situation in Northern Europe, Culicoides dewulfi and C. chiopterus are uncommon to rare in Italy. In contrast, the Obsoletus complex occurs abundantly throughout Italy, with C. obsoletus s.s. being the most prevalent and ecologically adaptive of the three species making up the complex. A longitudinal study conducted at a site in Central Italy revealed that: (i) species of the Obsoletus complex prefer horses to sheep; (ii) their parity rates range from 10% (March) to 56% (November); (iii) throughout the year C. scoticus is consistently more abundant than C. obsoletus s.s.; (iv) abundances in both, C. obsoletus s.s. and C. scoticus, peak in May-June, with the peak of the latter species being more evident. Bluetongue virus was first isolated from wild caught midges of the Obsoletus complex in 2002. Thereafter, pools of selected parous midges collected across Italy, and during multiple outbreaks of BT, have been found consistently PCR-positive for the virus. More recently, viral RNA has been detected in field specimens of C. dewulfi, C. obsoletus s.s., C. scoticus and C. montanus.