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Bluetongue virus (BTV) is an arthropod-borne, segmented double-stranded RNA virus that can cause severe disease in both wild and domestic ruminants. BTV evolves via several key mechanisms, including the accumulation of mutations over time and the reassortment of genome segments.Additionally, BTV must maintain fitness in two disparate hosts, the insect vector and the ruminant. The specific features of viral adaptation in each host that permit host-switching are poorly characterized. Limited field studies and experimental work have alluded to the presence of these phenomena at work, but our understanding of the factors that drive or constrain BTV's genetic diversification remains incomplete. Current research leveraging novel approaches and whole genome sequencing applications promises to improve our understanding of BTV's evolution, ultimately contributing to the development of better predictive models and management strategies to reduce future impacts of bluetongue epizootics.

The seasonal abundance of Culicoides midges, the vector of Bluetongue and African horse sickness viruses (BTV/AHSV) and the presence of viruses in midges were determined in 3 geographic areas in South Africa. In the Onderstepoort area, more than 500,000 Culicoides midges belonging to 27 species were collected. Eighteen midge species were collected throughout Winter and the presence of AHSV and BTV RNA in midges was detected using real time reverse transcription quantitative polymerase chain reaction. The nucleic acid of AHSV was found in 12 pools out of total pools of 35 Culicoides. Twenty‑five Culicoides species were detected in the Mnisi area. The RNA of BTV was detected in 75.9% of the midge pools collected during Winter and 51.2% of those collected during Autumn. Antibodies for BTV were detected in 95% of cattle sampled using a competitive enzyme‑linked immunosorbent assay (cELISA). The dominant species in these 2 areas was Culicoides imicola. Eight Culicoides species were collected in Namaqualand. Culicoides imicola represented the 0.9% and Culicoides bolitinos the 1.5% of total catches, respectively. Antibodies for AHSV were detected in 4.4% of 874 equines tested using an indirect ELISA. Results showed that transmission of AHSV and BTV can carry on throughout Winter and the outbreak may begin as soon as Culicoides populations reach a certain critical level.

Within Northern California, Culicoides sonorensis is the major vector of Bluetongue virus (BTV) and annual infection of livestock is distinctly seasonal (typically July‑November). Our recent studies compare the population dynamics of C. sonorensis midges with occurrence of BTV infection of C. sonorensis and sentinel dairy cattle throughout both the seasonal and interseasonal ('overwintering') periods of BTV activity. Spring emergence and seasonal abundance of adult C. sonorensis on the sampled farms coincided with rising vernal temperature. Intensive surveillance confirmed widespread infection of both sentinel cattle and vector midges during the August‑November period of seasonal BTV transmission. Bluetongue virus infection of parous female midges captured in traps set during daylight hours was also detected during the interseasonal period of virus activity, whereas there was no concurrent active infection of sentinel cattle during the overwintering period. The finding of BTV‑infected vector midges during mid‑Winter suggests that BTV can overwinter in Northern California by infection of long‑lived female C. sonorensis midges that were infected during the prior seasonal period of virus transmission and which, then, entered a quiescence in the fall (Autumn) and re‑emerged sporadically during the overwintering period. Notably, vertical transmission of BTV was not detected among progeny of midges infected in the laboratory nor in field‑collected larvae. In addition to defining the mechanism of BTV over‑wintering in a temperate region, the studies reviewed in this article also provide precise documentation of temporal changes in the annual abundance, dispersal and dynamics of BTV infection of Culicoides midges. Collectively these findings are critical to the creation of accurate predictive models of BTV infection in livestock and to development of sound abatement strategies.