Vector Competence of Culicoides sonorensis (Diptera: Ceratopogonidae) for Epizootic Hemorrhagic Disease Virus Serotype 2 Strains from Canada and Florida

Abstract

Epizootic hemorrhagic disease virus (EHDV), an Orbivirus transmitted by Culicoides spp. vectors, is represented by seven serotypes and numerous strains worldwide. While studies comparing vector competence between serotypes exist, studies between viral strains are lacking. In this study, we examined the rates of infection, dissemination, and transmission of two strains of EHDV-2 orally fed to the known vector, Culicoides sonorensis Wirth & Jones. Culicoides sonorensis cohorts were fed an infectious blood meal containing EHDV-2 strains from either Alberta, Canada (Can-Alberta) or Florida (5.5 log₁₀ PFUe/mL) and tested for the vector's susceptibility to infection and dissemination. In addition, transmission rates of the virus were assessed and compared using capillary tube and honey card methods. Our results show that the Florida strain had higher infection and dissemination rates than the Can-Alberta strain in spite of the Florida strain having significantly lower viral titers in C. sonorensis bodies, legs, and saliva than the Can-Alberta strain. Overall transmission rates were not significantly different between the two strains but varied significantly between the methods used. These findings suggest that the consequences of EHDV infection in C. sonorensis vary between virus strains and have huge implications in future vector competence studies involving Culicoides species and Orbiviruses.

Keywords: Culicoides sonorensis; EHDV-2; dissemination; epizootic hemorrhagic disease virus; infection; strain; transmission; vector competence.

Figure 1

Viral titers in the bodies, legs, and saliva of Culicoides sonorensis infected with Can-Alberta or Florida strain of EHDV-2. Overall viral titers, viral titers during the first 3–7 days post-feeding (early), and viral titers during the last 8–13 days post-feeding (late) are shown for both strains. Error bars indicate the standard error of the mean.

Figure 2

Viral titer relationships between bodies, legs, and saliva of C. sonorensis infected with two strains of EHDV-2. (a) Body by leg titer correlation for Can-Alberta, (b) body by leg titer correlation for Florida strain, (c) body by leg titer correlation for Can-Alberta, (d) body by leg titer correlation for Florida, (e) leg by saliva correlation for Can-Alberta, (f) leg by saliva correlation for Florida. Figure 2a,b show correlations during the early period (days 3–7 post-feeding) and Figure 2c–f show correlations during the late period (days 8–13 post-feeding).

Figure 3

Virus detection by saliva collection method for two strains of EHDV-2 in Culicoides sonorensis as a percentage of total salivary assays completed. Consensus between the results of capillary assays and honey cards was reached for 52.8% of samples. Chi-square results indicated that positive detections were significantly dependent upon method, although this result was confounded by strain. Greater detections were made by capillary assays for the Can-Alberta strain and by honey cards for Florida strain infected midges.

Figure 4

Mean viral titers of Can-Alberta and Florida strains of EHDV-2 in the saliva of C. sonorensis collected using honey card or capillary tube assays. Saliva samples were collected daily from days 7–13 post-feeding. Culicoides sonorensis individuals were initially fed 5.5 log₁₀PFUe/mL of each strain through a blood meal. Error bars show the standard deviation.