Culicoides biting midges (Diptera: Ceratopogonidae) are the smallest haematophagous insects, which transmit the fatal virus to ruminants [1]. More than 1,400 species of Culicoides have been described and present on all land with the exception of Antarctica and New Zealand [1,2]. A size of Culicoides is around 1–3 mm, and they have a complete metamorphosis life cycle going through the egg, larva, pupa, and adult stages [1,3]. Females preparing to spawn after mating with males suck up blood of human or animal to get animal protein at dawn or dusk [1,3]. Breeding sites of Culicoides are watery and nutritious streams that can help them grow and develop larvae [4]. Adult Culicoides have different wing patterns based on species, which helps in easy identification of different species [5].

Arthropod-borne viruses (arboviruses) are major pathogens in the veterinary field and are transmitted by arthropods such as Culicoides biting midges [1]. Arboviruses infected with ruminants cause abortion, stillbirth, and congenital malformation [6,7]. Akabane virus (AKAV) and Aino virus (AINOV) belong to the family Peribunyaviridae [1,8]. Sporadic outbreaks of AKAV have been reported widely in Japan, Southeast Asia and the Middle East, including Korea [9–12]. In Japan, JaGAr39, the prototype of AKAV, was first isolated in 1959 [9]. In Korea, a case of AKAV was first reported in 1980 [12]. In 2010, meningitis associated with AKAV infection occurred in a large-scale in South Korea [13]. More than 500 cattle exhibited symptoms such as movement disorder, trembling, and in severe cases, the cattle exhibited an inability to stand. Chuzan virus (CHUV) and Ibaraki virus (IBAV) are members of the family Reoviridae [1,14]. CHUV was first emerged in Japan, 1985, and the clinical signs are neurological abnormalities and hydranencephaly [15]. IBAV infection is characterized by fever, salivation, anorexia, and a deglutitive disorder in cattle. Symptoms except deglutitive disorder with dysphagia are usually mild. Bovine ephemeral fever virus (BEFV) grouped in the family Rhabdoviridae is similar to the IBAV, which is characterized by acute fever [14]. High fever, nasal discharges, salivation, and inability to stand appear to the principal signs of cattle infected with BEFV.

The surface temperature of the earth is rising continuously due to global warming, which is expected to rise further [3]. Temperature is one of the environmental factors that affect the activity of poikilothermic Culicoides [16,17]. Viruses that spread faster due to increased vector activity in warmer atmospheric temperatures result in severe economic losses in the industrial animal field. The aim of this study was to investigate the Culicoides distribution in different cattle farms in the area where encephalomyelitis occurred on a large-scale in 2010, and environmental conditions which affect the Culicoides abundance.

Transmission of arthropod-borne viral disease is difficult to prevent as they cannot be visually observed and spread quickly and widely. Surveillance has steadily been conducted in several countries to investigate the distribution of Culicoides that is a vector of arboviruses, and to test against target viruses for predicting the outbreak [2,22,23]. As these investigations have been carried out for a long time, haematophagous vector collection methods have been developed on a variety of occasions, such as collection means using UV light with a wide range of wavelengths [24], and traps in which carbon dioxide (CO₂) is released at diverse concentrations to attract Culicoides [25]. They showed that the standard CDC UV light trap collected the Culicoides significantly more than the normal LED trap, and CO₂ trap releasing decanal or phenol collected highly compared to trap with CO₂ alone. These results will be a useful source to collect Culicoides more effectively. In addition, new species of Culicoides as a vector of arboviruses are continuously being discovered [25–27]. In Korea, new Culicoides species were discovered in 2013 [28]. Therefore, epidemiological studies on arthropod-borne are expected to experience continual growth.

In 2016 and 2017, a total number of 15,655 Culicoides were collected during the collection period. The species were classified as C. arakawae, C. punctatus, C. nipponensis and C. tainanus. In this study, C. arakawae was the extensively collected species while C. arakawae which is a transvector of leucocytozoon disease in Japan was generally collected from the chicken farm [29]. Previous studies showed that C. arakawae was rarely found in ruminant farms and is blood-sucking midges which parasitize chicken [29,30]. There were no chicken farms around the 5 cattle farms where Culicoides were collected in this study. The detection of antigens to AKAV, CHUV, and BEFV in C. arakawae suggests that C. arakawae can hold arboviruses antigen although it is not principal vector that transmit arboviruses to ruminants.

When compared with the antigen detection results against arboviruses in Culicoides, the antigen test in the bovine blood did not show any correlation with the type of virus and region. During the study period, there were no reported damages associated with arboviruses, and the appearance and epidemic of 5 arboviruses seemed to be rare. On the other hand, antibody test in the bovine serum, antibody to BEFV was detected and the antibody titer was high in one farm (Wanju). As this farm was vaccinated only against AKAV, there is a possibility that the farm might get infected with BEFV. However, there were no virus-related clinical symptoms and the antibody titer can be considered as a maternal antibody. Serological tests using blood were not initially designed and there was a lack of sample number and collection period. In future studies, a large number of blood samples from various regions will be employed to perform the more specific serological examination. In this study, because blood samples were obtained from randomly selected cattle, it was difficult to determine the exact timing of the virus infection. In order to supplement these shortcomings, surveillance study using sentinel cattle that confirmed no-virus infection before investigated year should be conducted.

Previous studies have reported that Culicoides are highly influenced by various environmental factors like temperature, humidity, and wind speed [1,31]. Generally, infections caused by arboviruses and activity rates of Culicoides increase in proportion to rising temperature and are interrupted by wind speed. As a result of our study, we could not confirm the dependent effect of the wind speed. However, the measured values of all the environmental factors had a narrow range and it was difficult to confirm uniformity in results. Therefore, a more frequent sampling will be needed in future studies. Additionally, with respect to the collection of Culicoides from the farm, more Culicoides were collected from outside than inside at low altitude farms (50 m), while more Culicoides were collected from inside than outside at high altitude farms (300, 500, and 600 m). It appeared that Culicoides were attracted by the warmth of the cattle and gathered inside the farm due to lowering of temperatures at higher altitude. However, there was no significant difference in measured temperature inside and outside the farms during the collection period, so the close causal relationship between altitude and mobility of Culicoides was not clearly revealed in this study. In South Africa, more Culicoides were collected from old stable than new stable, and more Culicoides were collected from outside than inside the stable [31]. Published study reports about an experiment conducted to compare the sampling rates of females and males Culicoides from inside and outside the farm in Eastern Slovakia [32]. However, there exist not enough studies based on how altitude is related to the abundance of Culicoides. It is not easy to conclude the relationship between the number of collected Culicoides and the environmental factors because altitude alone exactly affects environmental conditions. Therefore, it is necessary to study whether more various conditions affect behavior of Culicoides. Furthermore, through environmental studies on farms that can attract or avoid Culicoides, we may suggest the manuals that farmers can prevent the occurrence of Culicoides related diseases.