Abstract:
Several studies have demonstrated the ability of certain viruses to overwinter in arthropod
vectors. The over-wintering mechanism of bluetongue virus (BTV) is unknown. One
hypothesis is over-wintering within adult Culicoides midges (Diptera; Ceratopogonidae) that
survive mild winters where temperatures seldom drop below 10 °C. The reduced activity of
midges and the absence of outbreaks during winter may create the impression that the virus
has disappeared from an area. Light traps were used in close association with horses to collect
Culicoides midges from July 2010 to September 2011 in the Onderstepoort area, in Gauteng
Province, South Africa. More than 500 000 Culicoides midges were collected from 88 collections
and sorted to species level, revealing 26 different Culicoides species. Culicoides midges were
present throughout the 15 month study. Nine Culicoides species potentially capable of
transmitting BTV were present during the winter months. Midges were screened for the
presence of BTV ribonucleic acid (RNA) with the aid of a real-time quantitative reverse
transcription polymerase chain reaction (RT-qPCR) assay. In total 91.2% of midge pools tested
positive for BTV RNA. PCR results were compared with previous virus isolation results (VI)
that demonstrated the presence of viruses in summer and autumn months. The results
indicate that BTV-infected Culicoides vectors are present throughout the year in the study
area. Viral RNA-positive midges were also found throughout the year with VI positive midge
pools only in summer and early autumn. Midges that survive mild winter temperatures
could therefore harbour BTV but with a decreased vector capacity. When the population size,
biting rate and viral replication decrease, it could stop BTV transmission. Over-wintering of
BTV in the Onderstepoort region could therefore result in re-emergence because of increased
vector activity rather than reintroduction from outside the region.
