Epizootic hemorrhagic disease virus oral infection affects midge reproduction and is vertically transmitted to offspring in Culicoides sonorensis

Epizootic hemorrhagic disease virus (EHDV: Reoviridae: Orbivirus) is a Culicoides-borne pathogen that affects a variety of ruminants, causing significant economic losses and/or ecological impacts in animal agriculture/wildlife populations worldwide. In this study, we examined the effect of EHDV serotype-2 oral infection on the survival and reproduction of Culicoides sonorensis Wirth and Jones (a confirmed vector of EHDV in North America), and the potential vertical transmission of EHDV-2 (from infected female to its offspring) in this midge species. Culicoides sonorensis females were fed on defibrinated bovine blood mixed with EHDV-2 (5.5 log₁₀ PFU/ml) or without EHDV-2 (control). Adult survival/longevity, oviposition rates, number of eggs deposited, egg hatch rates (fertility), larval survival, larval stage duration, eclosion rates, and sex-ratios of the progeny were recorded and compared between the two groups. In addition, the progeny (eggs and F₁ generation adults) of EHDV-2 fed females were processed for viral detection through RT-qPCR and plaque assays. Survival/longevity of the blood-fed adults, oviposition rates, number of eggs deposited, larval stage duration, eclosion rates, and sex-ratios were not significantly different between the two groups. However, egg hatch rates were significantly lower in the EHDV-2 fed group (35.8 ± 5.2%) than the control group (74.5 ± 6.8%), but larval survival rates were higher in the EHDV-2 fed group (59.8 ± 4.9%) compared to the control group (34.1 ± 6.5%). EHDV-2 (Ct < 35) was detected in the eggs (3.4%, 1/29 females tested, Ct = 22.1 [4.9 log₁₀ PFUe/ml]) and F₁ adult progeny (1.7%, 1/58 adults tested, Ct = 23.5 [4.5 log₁₀ PFUe/ml]) of the orally exposed females through RT-qPCR as well as through plaque assays. Our findings suggest that EHDV-2 infection has no major impact on C. sonorensis survival/longevity or oviposition but has a significant negative effect on midge fecundity/fertility. Our study also provides evidence for the vertical transmission of EHDV-2 from an infected adult female to its offspring in C. sonorensis. However, salivary transmission of EHDV-2 from the vertically infected progeny and its significance in the epidemiology of hemorrhagic disease are currently unknown and remain to be examined in further studies. Overall, these findings collectively indicate that Orbivirus infection can negatively affect vector reproduction, and that vertical transmission is a probable mechanism of overwintering of EHDV in North America.