Characterizing the Epidemiology of Bluetongue and Epizootic Hemorrhagic Disease Virus in Louisiana
Bluetongue virus (BTV) and Epizootic Hemorrhagic Disease Virus (EHDV) are arthropod-borne viruses (Reoviridae; Orbivirus) transmitted by biting midges in the genus Culicoides. These viruses cause hemorrhagic disease and induce serious morbidity and mortality rates in wild and captive deer. Cattle are normally asymptomatic but serve as the reservoir/amplifying host for these viruses and play an important role in the transmission cycle. The objectives of this study were to characterize the epidemiology of these viruses as they relate to white-tailed deer and cattle, evaluate different trap types for collecting Culicoides midges, and determine if these viruses can be detected in bone marrow of white-tailed deer. Using CDC black light traps baited with dry ice, we identified 15 species of Culicoides during a 7 year study at the Idlewild Research Station in Clinton, La which experienced several outbreaks of hemorrhagic disease in the white-tailed deer herd. Real-time quantitative PCR was performed to screen for BTV and EHDV in 1711 pools representing 24,859 specimens collected at the farm. The results showed 5 species (C. debilipalpis, C.stellifer, C. venustus, C. haematopotus, crepuscularis) to be positive for BTV and EHDV. The overall results showed a distinct positive correlation with collected specimens of C. debilipalpis and C. stellifer and the number of BTV/EHDV confirmed deer deaths. These 2 species accounted for over 60% of BTV positive pools and supports evidence that they are the probable BTV vectors in Louisiana. This was the first report of EHDV PCR positive field collected specimens from these species in Louisiana. Agar gel immunodiffusion test results showed a significant increase in BTV/EHDV antibodies in cattle and white-tailed deer from before the vector season versus after the season for each year. Notably, PCR results showed cows and deer to be positive for EHDV-6 and BTV-12, which was the first time these serotypes have been reported in Louisiana. We compared the number of specimens and species captured using 3 animal baited traps, CDC traps with and without black light, and a rotator trap which collected at different time periods. Animal-baited traps did not capture any different species of Culicoides than the CDC black light traps. The most active time period for Culicoides midges was 0600-0800h in which most specimens were captured, especially for C. debilipalpis. The minimum infection rate for BTV was higher in midges collected in traps without light but not for EHDV. Bone marrow samples collected from white-tailed deer were PCR positive up to 4 months postmortem for BTV and 3 months for EHDV. This can be a useful tool for determining serotype and probable cause of death during epizootics, especially for wildlife agents and deer farmers.