Transmission efficiency of Culicoides sonorensis for epizootic hemorrhagic disease virus serotype 2 in white-tailed deer

Epizootic hemorrhagic disease virus (EHDV) is transmitted by Culicoides biting midges (Diptera: Ceratopogonidae) to numerous wild and domestic ruminants and is a significant pathogen of white-tailed deer (WTD; Odocoileus... [ view full abstract ]

Epizootic hemorrhagic disease virus (EHDV) is transmitted by Culicoides biting midges (Diptera: Ceratopogonidae) to numerous wild and domestic ruminants and is a significant pathogen of white-tailed deer (WTD; Odocoileus virginianus). In North America, the only confirmed vector of EHDV is C. sonorensis, although other Culicoides species are suspected vectors. Previous experimental infections have demonstrated Culicoides-to-WTD transmission of EHDV, but little is known about the minimum number of infective Culicoides bites required. Our objective was to determine if the bite from as few as five EHDV-infected C. sonorensis is sufficient to infect a WTD. Colonized C. sonorensis were intrathoracically inoculated with EHDV-2, held for 10 days at 26 °C to allow for virus replication, and were then given the opportunity to feed on susceptible WTD (n=2): deer A, 10 midges; deer B, 5 midges. After feeding on deer, Culicoides were processed individually for virus titration by a standard TCID50 assay. One midge from deer B was virus isolation negative, suggesting the inoculation failed. All remaining Culicoides were virus positive (geometric mean titer: deer A midges, 105.4 TCID50/midge; deer B midges, 105.8 TCID50/midge). After feeding, deer were observed twice daily and sedated on 0, 4, 7, 11, and 14 days post feeding (dpf) for physical examination and venipuncture (serology, virology, clinical pathology). Culicoides feeding was variable: deer A, 1/10 midges partially blood-fed but eight probe sites were observed on the skin (evidenced by focal erythema); deer B, 5/5 midges blood fed (one partial, four to repletion). Both deer developed viremia that peaked 7 dpf and developed mild clinical disease (decreased activity, decreased appetite, fever, erythema of non-haired skin, hyperemia of mucous membranes, and lymphopenia). In this preliminary study, infection of susceptible WTD with EHDV-2 was achieved with as few as 4 bites from infected C. sonorensis, demonstrating the efficiency of transmission.