AHS, BT bluetongue and SBV are economically important Culicoides-borne viral diseases affecting equids and ruminants. The importance of these arboviral diseases derives from their very wide geographic distribution, potential for rapid spread, and large economic impact 1,2. A considerable number of studies reported widespread occurrence of these diseases in Ethiopia. Demissie 40 reported AHS from Gamo Gofa, Wolaita and Hadiya zones of SNNPR. Zeleke et al 41 reported the occurrence of AHS in southern (Awassa, Hossana, Wondogenet, and Hagereselam), western (Jimma, Bedelle, Nekemte, Horroguduru, and Chaliya), and central (Bishoftu, Meki, Zeway, Filtimo, and Bekejo) Ethiopia using virus neutralization tests. Ayelet et al 42 reported AHS from Ada’a, Bahir Dar, Mecha, Dangla, Jimma and Sodo and Aklilu et al 11 reported AHS from central Ethiopia using reverse transcription polymerase chain reaction (RT-PCR) and virus isolation. Gizaw et al 13 reported the presence of group-specific antibodies against bluetongue virus using competitive enzyme-linked immunosorbent assay (c-ELISA) from Adami Tulu, Amibara, Areka, Arsi Negelle, Bene Tsemay, Doyo Gena, G/Mekeda, Fafan, and Jinka. Abera et al 14 reported the presence of bluetongue antibodies from Jimma, Bonga and Bedelle using c-ELISA and Gulima 43 reported the presence of bluetongue antibodies from Amhara regional state in northern Ethiopia. There is only one study on the sero-prevalence of SBV in Ethiopia. The author reported a very high apparent seroprevalence of 56.6% 10.
These diseases are transmitted by females of several species of midges belonging to the large genus Culicoides (Diptera: Ceratopogonidae) (which includes more than 1,300 described species worldwide 44,45. In this study, morphological identification confirmed the presence of seven species in Ethiopia. In the current study, various Culicoides species were collected, of which C. imicola was the largest number 4830 (52.8%). Similar to these results, entomological surveys carried out in many sub-Saharan African countries show that C. imicola is the dominant species 22,23, 46–48. A previous study by Mulatu and Hailu 30, reported the presence of C. imicola, C. milnei, C. neavei, C. zuluensis, C. fulvithorax and C. isioloensis in western parts of Ethiopia and Khamala and Kettle 31 reported the presence of C. fuscicaudae.
Poisson regression models were used to model the relationship between various environmental and climatic factors and the distribution of C. imicola and C. kingi catch. For both species, significantly higher catch was obtained in the subhumid agro-ecological zone. This finding suggests that Culicoides species require breeding habitat with high relative humidity and high temperatures 49. In the current study, higher numbers of Culicoides were caught in traps placed near animal pens. This result is consistent with Riddin et al 50 that reported high Culicoides catches near horse barns. The abundance of Culicoides near animal pens is mainly due to the presence of suitable breeding sites represented by moist soil sites, leaking animal watering troughs, and pond edges contaminated with feces 51.
The present study shows that mean annual minimum temperature, mean annual maximum temperature, and elevation are unfavorably related to C. imicola and C. kingi catches. Increases in mean annual minimum temperature, mean annual maximum temperature, and altitude/elevation significantly decreased the number of catches. However, higher catch numbers were positively associated with an increase in mean annual precipitation. Gusmão et al 21 et al suggests that persistent or heavy rain can create conditions for biting midges to proliferate but it can also be a barrier to the activity of adult (winged) biting midges and prevent them from flying. Rain can prevent adults from leaving their shelters. Culicoides vector activity generally declines or even ceases at low temperatures, and high temperatures (≥ 40˚C) are also lethal 36. Foxi et al 51 also reported relatively poor tolerance of Culicoides to lower temperatures.
Various climatic and environmental factors were used to model the species distribution of C. imicola and C. kingi. Soil type, altitude/elevation, livestock distribution, solar radiation, and mean minimum annual temperature were the most important variables for the C. imicola model. Wind speed, soil type, altitude/elevation, and vapor pressure were the variables that contributed most to the model for C. kingi. Although there is no previous information on the climatic requirements of C. kingi, numerous studies have examined the role of various climatic and environmental factors on the distribution and abundance of C. imicola. Global ensemble modeling of C. imicola by Leta et al 29 reported temperature covariates contributing 64% to their model. This is supported by Veronesi et al 52 which showed that temperature can affect fecundity, hatching, and survival of C. imicola. When reared at a higher temperature (28°C), C. imicola exhibited higher variability in fecundity and lower hatch rates, and the mean emergence rate from pupae was highest at 20°C. The distribution of C. imicola is probably directly limited by its relatively low tolerance to lower temperatures 53. As temperatures rise, adults hatch and populations gradually increase to reach a peak in abundance in spring or summer, depending on the site, which is a function of spring temperatures and summer drought. Because temperature shortens larval development time and the time between two blood meals, thus increasing laying frequency, which has a positive effect on population dynamics (and their growth), we expected that temperature would have a significant effect on abundance 24.
Our study also showed that solar radiation and livestock distribution are influential variables in the spread of C. imicola. According to Conte et al 54, intense solar radiation on C. imicola larval habitat combined with high nighttime temperatures accelerates larval development, resulting in multiple generations/season. The importance of livestock as a source of blood meals for C. imicola is well established 55. C. imicola is a bloodsucking insect that tends to feed on blood and breed near livestock and humans. The frequency of contact between Culicoides and vertebrate hosts is closely related to the multiplication of the pathogen and the risk of transmission 55,56.
In this study, C. imicola was found to have a larger suitable range compared to C. kingi. Globally, C. imicola is widespread in tropical and subtropical regions of Africa, the southern part of Europe, and some parts of Asia 57. The model describes that all regions have a small to large range of suitable areas, with Oromia and SNNPR regions having a larger range of suitable areas. High suitability for C. imicola was also demonstrated in the Amhara region, particularly adjacent to the Blue Nile basin and Lake Tana, in southern Afar, and in areas of the Somali region adjacent to Oromia. The results of the current model overlap in many ways with the previously published model of Leta et al 29. The current model emphasizes at national level by elaborating the distribution of C. imicola and C. kingi suitability in different regions of the country.
In conclusion, the entomological study shows the occurrence of C. imicola and C. king in different parts of Ethiopia, with C. imicola predominating. The widespread occurrence of these species indicates a higher risk of SBV, BT and AHS in different parts of Ethiopia. The models could help to understand the risk of introduction and spread of SBV, BT, and AHS.