Live attenuated bluetongue vaccine viruses in Dorset Poll sheep, before and after passage in vector midges (Diptera: Ceratopogonidae)

Design of primers and use of RT-PCR assays for typing European bluetongue virus isolates: differentiation of field and vaccine strains

Bluetongue virus (BTV) is the causative agent of bluetongue, a disease of ruminant livestock that occurs almost worldwide between latitudes 3 degrees S and 5 degrees N. There are 24 serotypes of BTV (currently identified by serum neutralization assays). Since 1998, eight strains of six BTV serotypes (1, 2, 4, 8, 9 and 16) have invaded Europe. The most variable BTV protein is major outer-capsid component VP2, encoded by segment 2 (Seg-2) of the double-stranded RNA virus genome. VP2 represents the major target for neutralizing (and protective) antibodies that are generated in response to BTV infection, and is therefore the primary determinant of virus serotype. RT-PCR primers and assays targeting Seg-2 have been developed for rapid identification (within 24 h) of the six European BTV types. These assays are sensitive, specific and show perfect agreement with the results of conventional virus-neutralization methods. Previous studies have identified sequence variations in individual BTV genome segments that allow different isolates to be grouped on the basis of their geographical origins (topotypes). The assays described in this paper can detect any of the BTV isolates of the homologous serotype that were tested from different geographical origins (different Seg-2 topotypes). Primers were also identified that could be used to distinguish members of these different Seg-2 topotypes, as well as field and vaccine strains of most of the European BTV serotypes. The serotype-specific assays (and primers) showed no cross-amplification when they were evaluated with multiple isolates of the most closely related BTV types or with reference strains of the remaining 24 serotypes. Primers developed in this study will be updated periodically to maintain their relevance to current BTV distribution and epidemiology (http://www.iah.bbsrc.ac.uk/dsRNA_virus_proteins/ReoID/rt-pcr-primers.htm).

Replication of live-attenuated vaccine strains of bluetongue virus in orally infected South African Culicoides species

Field-collected South African Culicoides (Diptera, Ceratopogonidae) were fed on sheep blood containing 16 live-attenuated vaccine strains of bluetongue virus (BTV) comprising serotypes -1, -2, -3, -4, -5, -6, -7, -8, -9, -10, -11, -12, -13, -14, -16 and -19. After 10 days extrinsic incubation at 23.5 degrees C, 11 and seven of the 16 BTV serotypes used were recovered from Culicoides (Avaritia) imicola Kieffer and Culicoides (A.) bolitinos Meiswinkel, respectively. One serotype was also recovered from Culicoides (Remmia) enderleini Cornet & Brunhes. Bluetongue virus recovery rates and the mean titres for most serotypes were significantly higher in C. bolitinos than in C. imicola. Significant differences were found in virus recovery rates from Culicoides species fed on blood containing similar or identical virus titres of different BTV serotypes. In addition, we demonstrated that a single passage of live-attenuated BTV-1, -2, -4, -9 and -16 through the insect vector, followed by passaging in insect cells, did not alter its infectivity for C. imicola and that the oral susceptibility of C. imicola to the attenuated vaccine strains of BTV-1, -4, -9 and -16 remained similar for at least three consecutive seasons.

Recombinant canarypox virus vaccine co-expressing genes encoding the VP2 and VP5 outer capsid proteins of bluetongue virus induces high level protection in sheep

We describe the development and preliminary characterization of a recombinant canarypox virus vectored vaccine for protective immunization of ruminants against bluetongue virus (BTV) infection. Sheep (n=6) immunized with recombinant canarypox virus vector (BTV-CP) co-expressing synthetic genes encoding the two outer capsid proteins (VP2 and VP5) of BTV serotype 17 (BTV-17) developed high titers (40-160) of virus-specific neutralizing antibodies and were resistant to challenge with a field strain of BTV-17. In contrast, sheep (n=5) immunized with a commercial recombinant canarypox virus vector expressing the E and preM genes of West Nile virus were seronegative to BTV and developed pyrexia, lymphopenia, and extended, high-titered viremias following challenge exposure to the field strain of BTV-17. These data confirm that the BTV-CP vaccine may be useful for the protective immunization of ruminants against bluetongue, and it may avoid the problems inherent to live-attenuated (LA) BTV vaccines.

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

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.