Spatial and temporal distribution of bluetongue and its Culicoides vectors in Bulgaria

Abstract

Surveillance of Culicoides (Diptera: Ceratopogonidae) biting midges was carried out between 2001 and 2003, at 119 sites within a 50 x 50-km grid distributed across Bulgaria, using light trap collections around the time of peak adult midge abundance. Sentinel and ad hoc serum surveillance of hosts susceptible to bluetongue infection was carried out at around 300 sites between 1999 and 2003. Following the initial incursion of bluetongue virus 9 (BTV-9) into Bourgas province in 1999, affecting 85 villages along the southern border, a further 76 villages were affected along the western border in 2001, with outbreaks extending as far north as 43.6 degrees N. The BTV-9 strain in circulation was found to have a low pathogenicity for Bulgarian sheep populations, with less than 2% of susceptible individuals becoming sick and seroconversions detected up to 30 km from recorded outbreaks in the south. The major Old World vector Culicoides imicola Kieffer was not detected among over 70,000 Culicoides identified in summer collections, suggesting that BTV-9 transmission in Bulgaria was primarily carried out by indigenous European vectors. The most likely candidates, the Palaearctic species complexes - the Culicoides obsoletus Meigen and C. pulicaris L. complexes - were widespread and abundant across the whole country. The C. obsoletus complex represented 75% of all individuals trapped in summer and occurred in high catch sizes (up to 15,000 individuals per night) but was not found across all outbreak sites, indicating that both Palearctic complexes probably played a role in transmission. Within the C. pulicaris complex, only C. pulicaris s.s., C. punctatus Meigen and C. newsteadi Austen were sufficiently abundant and prevalent to have been widely involved in transmission, whilst within the C. obsoletus complex most trapped males were C. obsoletus s.s. Adult vectors were found to be largely absent from sites in west Bulgaria for a period of at least 3 months over winter, which, taken along with the spatiotemporal pattern of outbreaks in the region between years, indicates the virus may be overwintering here by an alternative mechanism - either by covert persistence in the vertebrate host or possibly by persistence in larval stages of the vector.

A survey of biting midges of the genus Culicoides Latreille, 1809 (Diptera: Ceratopogonidae) in NE Bulgaria, with respect to transmission of avian haemosporidians

“…Two epizooties of bluetongue virus (serotype 9) were observed in Bulgaria in 1999 and 2001 (Purse et al 2006). The following species are related to the bluetongue disease: Culicoides pulicaris complex, C. obsoletus complex and Monoculicoides group (see Appendix 1) (Nedelchev 2008).…”

“…The following species are related to the bluetongue disease: Culicoides pulicaris complex, C. obsoletus complex and Monoculicoides group (see Appendix 1) (Nedelchev 2008). A survey focused on the culicoid vectors of BTV in Bulgaria (Purse et al 2006) revealed C. obsoletus complex as the most abundant followed by C. pulicaris complex; these two complexes represented 75% and 16%, respectively, of all individuals trapped, whereas, the major Old World vector, C. imicola, was not found. In our study, the representatives of subgenus Monoculicoides (C. puncticollis, C. nubeculosus and probably C. riethi) were the most abundant (33.48%) followed by the ornithophilic C. circumscriptus representing 31.30% of the insects sampled.…”

“…All these species have been recorded on the basis of morphological identification. All previous studies of the genus Culicoides in Bulgaria were focused on their role in the transmission of the bluetongue virus (BTV), a disease of major economical importance (Purse et al 2006;Nedelchev 2008).…”

The phenology and population dynamics of Culicoides spp. in different ecosystems in The Netherlands

“…More than 88% of all collections consisted of members of this sibling group, including C. obsoletus s.s. and C. dewulfi. Nine percent consisted of C. pulicaris, another species suspected as BTV vector Purse et al, 2006).…”

“…Since then, BTV has spread into Mediterranean islands such as Corsica, Sicily and Sardinia. Recent BTV outbreaks in the Balkans are thought to have arrived from Asian Turkey and may be vectored by local vector species (Purse et al, 2006). Model predictions suggest a widespread distribution of BTV across the Mediterranean basin and into central Europe as far as southern Germany and Austria (Tatem et al, 2003;Wittmann et al, 2001).…”