Studies on overwintering of bluetongue viruses in insects

Abstract

Bluetongue viruses (BTVs) are economically important arboviruses that affect sheep and cattle. The overwintering mechanism of BTVs in temperate climates has eluded researchers for many years. Many arboviruses overwinter in their invertebrate vectors. To test the hypothesis that BTVs overwinter in their vertically infected insect vectors, Culicoides sonorensis larvae were collected from long-term study sites in northern Colorado, USA, and assayed for the presence of BTV RNA by nested RT-PCR. Sequences from BTV RNA segment 7 were detected in 30 % (17/56) of pools composed of larvae and pupae collected in 1998 and in 10 % (31/319) of pools composed of adults reared from larvae collected in 1996. BTV was not isolated from the insects. Additionally, Culicoides cell-culture lines derived from material collected at one of the sites, or derived from insect samples collected during a BTV outbreak, contained BTV RNA segment 7. In contrast, segment 2 RNA was detected at half the rate of segment 7 RNA in the field-collected larvae and was only detected in the Culicoides cell lines with one of two primer sets. These data suggest that BTVs could overwinter in the insect vector and that there is reduced expression of the outer capsid genes during persistent infection.

Bluetongue virus: virology, pathogenesis and immunity

“…However, conventional models for the transmission of BTV suggest that if adverse winter conditions last for more than 100 days, the virus should be unable to survive from one year to the next. Persistence of BTV in the larvae of vector Culicoides is considered to be highly improbable [61] although it cannot be ruled out [101]. Some authors have postulated that persistently infected T cells, recruited by midge biting, could facilitate the transmission of BTV for periods as long as nine weeks post infection in Dorset sheep [96].…”

Detection of Persistent Chikungunya Virus RNA but not Infectious Virus in Experimental Vertical Transmission in Aedes aegypti from Malaysia

“…Similarly, bluetongue virus RNA but not infectious virus was detected in Culicoides midge larvae from the field. 28 Segments of the viral RNA encoding the outer capsid genes, required for growth in insect cells, were downregulated; it was hypothesized that this allows bluetongue virus to survive as a persistent, noncytopathic infection of the insect host which is inactive over winter, but with the ability to reactivate later. Another alphavirus, Sindbis virus, has been shown to persist in a nonproductive RNA form in immunodeficient mouse brains, and can be reactivated as infectious virus after several months.…”

Epizootiology of the Schmallenberg virus in Poland

“…Zdolność SBV, jak również BTV do przetrwania zimy przy braku aktywnego wektora i powodowania nowych ognisk chorobowych w tych samych miejscach wiosną pozostaje zagadką (15,52). Stwierdzenie transowarialnej transmisji arbowirusów u owadów jest trudne ze względu na to, że częstotliwość jej występowania jest zazwyczaj niska.…”