Vertical transmission in the culicoid vector as a possible mechanism for bluetongue virus transeasonality

Abstract

Bluetongue virus (BTV) is transmitted between vertebrate hosts by arthropod vectors in the genus Culicoides. In tropical and sub-tropical climates the virus is maintained in a continual cycle between the culicoid vector and the vertebrate host; however, bluetongue disease is also enzootic in temperate regions, such as northeastern Colorado. The mechanism by which BTV overwinters in these regions, where the adult culicoid vector does not survive cold winter months, is unknown. The primary objective of this study was to determine if vertical transmission of BTV in the culicoid vector is the mechanism for BTV transeasonality in regions of temperate climate.Culicoid larvae were collected from two sites near Brighton, Colorado, where BT disease is known to be enzootic. A light-trap was also set at one of these sites for the collection of adult culicoid flies.Processed pools of field-collected culicoid larvae, culicoid flies allowed to emerge in the laboratory from collected larvae and culicoid flies collected in the light-trap were first assayed for the presence of BTV antigen using an antigen capture enzyme-linked immunosorbent assay (ELISA). A total of 49,521 larvae, 11,221 emergent flies, and 901 light-trap collected flies were assayed. The resulting minimum field infection rates (MFIRs), based on the presence of BTV antigen, were 0. 20% for larvae, 0.59% for emergent flies and 1.56% for light-trap collected flies (P<0.001).Isolation of infectious BTV was then attempted from the putatively positive emergent and light-trap collected fly pools. Three different isolation systems were used: 1) intravascular inoculation of embryonated chicken eggs (ECE ), 2) baby hamster kidney (BHK-21) cell culture and 3) intrathoracic inoculation of Aedes triseriatus mosquitoes.The culicoid pools putatively positive for BTV by ELISA yielded a total of 12 BTV isolates. The MFIRs for emergent flies and light-trap collected flies, based on virus isolation, were 0.10% and 0.11%, respectively (P=0.66). All of the isolates were found to be BTV serotype 17, except for one isolate from an emergent fly pool, which was determined to be BTV serotype 11.C. variipennis, the only known vector for BTV in the United States, was not the only vector from which BTV was isolated. BTV isolates were obtained from pools composed of C. variipennis (MFIR=0.0 7%), C. crepuscularis (MFIR=0.18%) and culicoids belonging to the selfia group (MFIR=0.22%). No statistically significant differences (P<0.05) were observed among the three species.The results of this study support the hypothesis that BTV is vertically transmitted in the culicoid vector and that this may be the mechanism by which the virus overwinters in temperate regions. Additionally, it appears that C. variipennis is not the only culicoid species in which the virus is maintained. C. variipennis is the epidemic vector for the disease, but other culicoid species, C. crepuscularis and members of the selfia group in particular, may be endemic vectors and reservoir hosts for the virus.