Abstract
Since 1998 there have been significant changes in the global distribution of bluetongue virus (BTV). Ten previously exotic BTV serotypes have been detected in Europe, causing severe disease outbreaks in naïve ruminant populations. Previously exotic BTV serotypes were also identified in the USA, Israel, Australia and India. BTV is transmitted by biting midges (Culicoides spp.) and changes in the distribution of vector species, climate change, increased international travel and trade are thought to have contributed to these events. Thirteen BTV serotypes have been isolated in India since first reports of the disease in the country during 1964. Efficient methods for preparation of viral dsRNA and cDNA synthesis, have facilitated full-genome sequencing of BTV strains from the region. These studies introduce a new approach for BTV characterization, based on full-genome sequencing and phylogenetic analyses, facilitating the identification of BTV serotype, topotype and reassortant strains. Phylogenetic analyses show that most of the equivalent genome-segments of Indian BTV strains are closely related, clustering within a major eastern BTV ‘topotype’. However, genome-segment 5 (Seg-5) encoding NS1, from multiple post 1982 Indian isolates, originated from a western BTV topotype. All ten genome-segments of BTV-2 isolates (IND2003/01, IND2003/02 and IND2003/03) are closely related (>99% identity) to a South African BTV-2 vaccine-strain (western topotype). Similarly BTV-10 isolates (IND2003/06; IND2005/04) show >99% identity in all genome segments, to the prototype BTV-10 (CA-8) strain from the USA. These data suggest repeated introductions of western BTV field and/or vaccine-strains into India, potentially linked to animal or vector-insect movements, or unauthorised use of ‘live’ South African or American BTV-vaccines in the country. The data presented will help improve nucleic acid based diagnostics for Indian serotypes/topotypes, as part of control strategies.
Highlights
Bluetongue (BT) is a non-contagious arboviral disease of domestic and wild ruminants that is transmitted via the bites of adult females of certain Culicoides spp. (Diptera, Ceratopogonidae) [1,2,3,4,5]
Isolates of bluetongue virus were obtained from the Orbivirus Reference Collection (ORC) at The Pirbright Institute (TPI)
Nucleotide substitution models obtained for different Bluetongue virus (BTV) genome-segment, using Bayesian Information Criterion (BIC), were: TN93+G+I (Seg-1, -2, -6 and -8); T92+G+I (Seg-3, -4 and -5) and T92+G (Seg-7); GTR+G+I (Seg-9) and GTR+G (Seg-10)
Summary
Bluetongue (BT) is a non-contagious arboviral disease of domestic and wild ruminants that is transmitted via the bites of adult females of certain Culicoides spp. (Diptera, Ceratopogonidae) [1,2,3,4,5]. Bluetongue (BT) is a non-contagious arboviral disease of domestic and wild ruminants that is transmitted via the bites of adult females of certain Culicoides spp. Bluetongue virus (BTV) is occasionally transmitted ‘vertically’ across the placenta, in seminal fluid, by an oral route [6, 7], or directly through direct contact [8]. The BTV genome is composed of ten linear segments of dsRNA (Seg-1 to Seg-10 in order of decreasing molecular weight), packaged within a triple layered icosahedral protein capsid that is approximately 90 nm in diameter [14,15,16,17]. The BTV genome segments encode seven structural proteins (VP1 to VP7) and four non-structural proteins (NS1, NS2, NS3/NS3a, and NS4). BTV is closely related to several other economically important orbiviruses, including African horse sickness virus (AHSV) and Epizootic haemorrhagic disease virus (EHDV), with which it shares physico-chemical characteristics and vector species, differing in host range [2]
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