Overlapping clinical signs and lesions make it challenging to distinguish between epizootic hemorrhagic disease (EHD) and Bluetongue (BT) affecting wild ruminants in the USA. Therefore, the syndrome caused by EHD and BT viruses is referred to as Hemorrhagic Disease (HD).
To date, seven serotypes for EHDV and 27 serotypes for BTV have been confirmed  including two genetically distinct BTVs identified from healthy goats in Italy (X ITL2015)  and goats and sheep in China (XJ1407) . There are genetically different strains of the virus throughout the world, and the field strains may differ on phenotypic properties like virulence and transmission potential . Mutation (a change in DNA sequence) and reassortment (the genetic recombination between different virus serotypes/ strains co-infecting a host cell) of RNA viruses contribute to the genetic diversity among field strains, changing transmissibility, pathogenicity, and causing altered virulence in susceptible ruminant hosts .
Moreover, the diversity of EHDV/BTV is associated with the reassortment of segments of the viral gene during the co-infections (with more than one virus strain) in either ruminant or vector host cells that lead to genetic change . Furthermore, the appearance of antigenically new viruses or genetic variants through mutations in either insect or animal hosts can develop due to genetic drift (mutation of individual genes) during virus replication . Worldwide, the evolution of different field strains is driven by genetic drift, genetic reassortment between viruses within each genus or serogroup, intragenic recombination, and the selective evolutionary pressure to establish genetically distinct virus strains in diverse epidemiological systems .
For livestock, mortality rates for BT and EHD may range between 0–100% depending on the ruminant host and prior infection, affecting the regions and countries’ economies depending on the severity of the outbreak . However, morbidity cost is related to the care for sick animals (e.g., veterinary cost and animal support) and reduced productivity of affected animals in livestock operations (e.g., weight loss, reduced milk yield, and abortion).
There is no cure or treatment for BTV and EHDV; therefore, the goal of BT and EHD management is to prevent virus spread into unaffected areas and clinical disease in ruminant hosts . For example, BT has been listed as a notifiable animal disease by the Office of International Epizootics . Restrictions in the trading and movement from enzootic regions of livestock and their products—including those that could be vertically transmitted, such as fetal bovine serum and fetal tissue—are suggested to avoid introducing the viruses to new places .
Historically, EHDV/BTV has occurred in the southeastern, central, and western USA. However, during the last 20 years, these viruses have spread northward within the USA territories, with cases reported across the upper Midwest and the northeastern USA . The distribution of EHD/BT depends on the occurrence of competent Culicoides vector species. The home range of Culicoides midges was historically maintained between latitude 35° S and 40° N , however, changes in the global range of vectors and distribution of BT and EHD have shown a northward expansion where the diseases are maintained between latitudes 35° S and 50° N .
Wittmann et al. (2002) found that increasing temperatures reduced the extrinsic incubation period for both BTV and EHDV, which can, in turn, facilitate the transmission of orbiviruses; although, rising temperatures also reduce vector survival . In addition, differences in vector competence between EHDV and BTV were described as higher temperatures (e.g., 27–30 °C) increase vector competence for EHDV (serotype 1) but not for BTV (serotypes 10 and 16) . When temperature and humidity were evaluated together, both high humidity/temperature and low humidity/temperature were detrimental for vector longevity .
While C. sonorensis and C. insignis are confirmed competent vectors for EHDV/BTV in the USA, there is strong evidence that suggests that other species can be involved in the transmission of EHDV/BTV viruses. However, in order to elevate other Culicoides spp. to confirmed vectors, a set of criteria needs to be fulfilled. There are four criteria defined by the World Health Organization (WHO 1967)  that are used as guidelines for incriminating an insect as a vector of a disease agent or pathogen: (1) repeated recovery of the pathogen from blood free, wild-caught insects; (2) demonstration in a control environment that the insect can become infected via a blood meal from a viremic vertebrate host or an artificial substitute; (3) demonstrate the transmission of the pathogen to a susceptible vertebrate host after the bite of an infected insect vector; and (4) demonstration on the field of the contact of the insect vector and susceptible vertebrate host populations . In North America, C. sonorensis has been reported as the primary vector of EHDV and BTV, as studies have demonstrated its implication as a vector in the field (via isolation of the pathogen from field-collected vectors and demonstrated contact between vector-ruminant host) and the laboratory (by the demonstration of vector infection after a blood meal from an infected host and subsequent transmission of the pathogen to a susceptible host) . Moreover, C. sonorensis complies with the four criteria that demonstrate its status as the primary insect vector for BTV/EHDV in the USA.
Vector competence studies are essential to establish the presence of primary vectors (critical to maintenance of the viruses in the host population) and secondary vectors (candidate vectors that may contribute to virus dissemination, helping to explain changes in distribution/geographic expansion of BT and EHD). Based on the four guidelines defined by WHO, vector competence—the capability of an insect vector species to become infected after a blood meal from a viremic/infected vertebrate host—can be estimated by using the vector implication criteria 2 and 3 of the of the World Health Organization . In search of other potential Culicoides vectors, accurate verification of ruminant host–Culicoides vector contact and subsequent blood meal analysis of the vector is needed. For example, C. debilipalpis (formerly C. lahillei) are considered candidate vectors for EHDV as laboratory studies have demonstrated that they can become infected after a blood meal and do come in contact with susceptible host ruminants in the field . Furthermore, laboratory studies have shown that C. venustus can get infected with the pathogen after a blood meal, while isolates of the pathogen from blood-free field-collected C. mohave have also been reported . Other Culicoides species regarded as potential vectors in Florida include C. stellifer, C. debilipalpis, C. venustus, C. pallidicornis, and C. biguttatus, as these species have been demonstrated to come into contact with susceptible ruminant species in the field . However, all four criteria from the World Health Organization need to be met to elevate vector species to competent vectors of BTV and EHDV.