Submitted to: American Society for Virology Meeting
Publication Type: Abstract Only
Publication Acceptance Date: 4/11/2002
Publication Date: 7/21/2002
Citation: White, D.M., Wilson, W.C., Blair, C.D., Beaty, B.J. 2002. Possible overwintering mechanism of bluetongue virus. American Society for Virology Meeting. P11-1 Interpretive Summary: Bluetongue virus (BTV) costs U.S. cattle producers up to $125 million per year because of international import/export restrictions. The insect that transmits BTV in the U.S. is the biting gnat or midge, _Culicoides sonorensis_. Focusing viral control methods to a specific, vulnerable point in the life cycle of the virus could alleviate a significant portion of the economic loss due to the presence of the virus. Arthropod-borne viruses (arboviruses) are viruses that are completely dependent upon a blood-feeding insect host (arthropod vector) to be transmitted from animal to animal. Consequently, arboviruses must learn to survive periods of insect inactivity in areas of the world where there are winters (temperate areas) or long dry seasons (tropical areas). This survival mechanism is commonly called "overwintering" or "transeasonality". The overwintering mechanism represents a vulnerable bottleneck in the life cycle of any arbovirus. The overwintering mechanism of BTV has eluded researchers for many years. However, it has been shown that several arboviruses overwinter in their insect hosts. Overwintering midge larvae were collected from long-term study sites in northern Colorado and examined for the presence of BTV genetic material. BTV-specific genetic material was detected in 5 of 26 (19%) pools collected in 1998, and in 26 of 319 (8%) pools collected in 1996. Additionally, cultured cells derived from larval gnats collected at the same site were positive for BTV genetic material. Finally, evidence was found for the fact that the virus may overwinter in a different form from that which infects the animal host. This could explain the relative difficulty of virus isolation from insects versus animals, and could have implications for the detection and control of the disease.
Technical Abstract: The overwintering mechanism of bluetongue virus (BTV) has eluded researchers for many years. While overwintering in the vertebrate host has been the favored hypothesis, it has been shown that several arboviruses overwinter in their invertebrate vectors. Overwintering _Culicoides sonorensis_ larvae were collected from long-term study sites in northern Colorado and examined for the presence of BTV nucleic acid by RT-nested PCR. Sequences from the S7 segment of BTV RNA were detected in 5 of 26 (19%) pools comprised of larvae and pupae collected in 1998, and in 26 of 319 (8%) pools comprised of adults reared from larvae collected in 1996. BTV was not isolated from the adult pools. Additionally, cell lines derived from culicoid larvae collected at the same site were positive for BTV nucleic acid. Finally, BTV RNA sequences from a cell line derived from larvae collected in the midst of a BTV outbreak were also positive for BTV nucleic acid. Interestingly, in contrast to the S7 segment, the L2 RNA segment could never be detected in any of the field-collected larvae, or the cell lines derived from larvae. These data suggest that BTV may not require abundant expression of the outer coat genes to persist in the insect vector. This could also explain the low rate of isolation of virus from insects. If the vertebrate cell receptor ligand VP2 (which is encoded by L2) is expressed at very low levels in the insect, traditional vertebrate cell-based isolation methods would be inefficient until the virus had amplified itself sufficiently to express all virus genes required for vertebrate cell infection.