Abstract
Bluetongue is primarily a disease of sheep in South Africa, while cattle and goats are mostly subclinically infected. The viraemia of bluetongue virus in cattle lasts much longer than in sheep and the role of cattle in the epidemiology of bluetongue in South Africa is poorly understood. Bluetongue virus has a segmented double-stranded ribonucleic acid genome and reassortment of genomes is a common feature. The aim of the study was to investigate whether reassortment occurs between vaccine and field strains when simultaneously administered to cattle. Six cattle between the ages of 6 and 12 months were infected with five strains of modified live vaccine bluetongue virus and a virulent field isolate of bluetongue virus 4. Blood samples were subsequently collected daily from these animals from day 1 to day 39 post-inoculation. Viruses were directly isolated during viraemia from the buffy coat on Vero cells using the plaque forming unit method. Analysis of plaques indicated that no reassortants between virulent field and vaccine strains occurred and the virulent bluetongue virus 4 was identified as the predominant virus strain. However, a reassortant virus between two bluetongue virus vaccine strains was isolated from the buffy coat. Whole genome sequences from the vaccine viruses were compared to the suspected reassortant and it was found that segment 8 exchanged between the bluetongue virus 8 and bluetongue virus 9 vaccine strains. The use of the live-attenuated bluetongue virus multivalent vaccine in South Africa causes circulation of different vaccine serotypes in Culicoides spp. and susceptible hosts and cattle might provide the ideal host for reassortment to occur.
Highlights
Bluetongue (BT) is a non-contagious viral disease of domestic and wild ruminants with serious socio-economic impacts
The analysis revealed that the virus had received its segment 10 (NS3/A) from an South Africa (SA) bluetongue virus (BTV) 2 modified live vaccine (MLV) strain (Maan et al 2010)
Blood samples were collected daily from the animals from day 1 to day 39 postinoculation and viraemia was detected between day 2 and day 32 in four of the animals and in the two other animals viraemia could be detected until 39 days post-inoculation using virus isolation
Summary
Bluetongue (BT) is a non-contagious viral disease of domestic and wild ruminants with serious socio-economic impacts. The outer layer of the virion consists of VP 2 encoded by segment 2. The function of VP 1, the RNA-dependent RNA polymerase encoded by segment 1, is transcription and replication (Boyce et al 2004). Structural VP 4 caps the newly synthesised messenger RNA (capping and trans-methylase enzyme, encoded by segment 4) (Ramadevi et al 1998) and VP 6 (encoded by segment 9) unwinds and re-anneals dsRNA during transcription and replication (RNA-dependent ATPase and helicase, encoded by segment 9) (Stauber et al 1997). The outer capsid proteins are involved in attachment (Hassan & Roy 1999) and release (VP 5) of the virus into the cytoplasm of mammalian cells (Hassan et al 2001)
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