Yanbian is located at the junction of China, North Korea, and Russia, and has a long border. Strengthening ecological and environmental protection in China means that the species distribution along the border has gradually diversified; the numbers and species of ticks are thus constantly changing, and their activity is increasing. Ticks and other vectors in the border zone can migrate to each other using various methods, increasing the risk of tick-borne diseases. In this study, 2673 ticks collected from eight counties and cities in Yanbian were classified and analyzed. H. longicornis and I. persulcatus were the dominant tick species in Yanbian. H. longicornis has strong reproductive ability and environmental adaptability, and is widely distributed throughout Asia and the Pacific, including China, Russia, South Korea, Japan, Australia, New Zealand, and the South Pacific islands. It is often parasitic in medium and large wild and domestic animals, whereas humans are accidental hosts. H. longicornis spreads a variety of pathogens that can affect wild animals and livestock, as well as human health.
Ticks can be infected with viruses, bacteria, including Rickettsia and spirochetes, and other pathogens. In addition, ticks can act as both vectors and hosts in the process of infectious disease transmission. The main research into tick-borne pathogen co-infections is currently focused on Borrelia burgdofferi, Babesia microti, Ehrlichia, and Anaplasma phagocytophilum [16]. Previous studies confirmed that one-third of patients with a CRT infection had neurological symptoms that differed from other SFGR infections [17], and were associated with a higher case-fatality rate when co-infected with SFTSV [18]. More attention should thus be paid to SFTSV transmission through both tick bites and close contact with infected cases [19]. In this study, we confirmed the existence of CRT/SFTSV co-infection in I. persulcatus (n = 13), H. japonica (n = 5), and D. silvarum (n = 1) in Yanbian. I. persulcatus is a common dominant tick species in Yanbian, and is especially widely distributed in Hunchun, Yanji, Helong, and other regions, resulting in a high risk of CRT and SFTSV infection via tick bites in these regions.
SFGR forms a long-lasting infection cycle between ticks and mammals and can also be transmittedvertically through tick eggs, making ticks the main host and vector of SFGR. Liu et al. [20] identified the new SFGR genotype Candidatus Rickettsia jingxinensis in H. longicornis in Northeast China, and Jiang et al. [12] reported the new SFGR genotype Candidatus Rickettsia longicornii in H. longicornis in Korea. The new SFGR genotypes Candidatus Rickettsia longicornii ompA, ompB, sca4, and rrs were detected in H. longicornis, I. persulcatus, and H. japonica in Yanbian. The SFGR Candidatus Rickettsia longicornii gene sequence detected in ticks displayed high homology with the newly discovered Candidatus Rickettsia longicornii genotype (ROK-HL727)-related gene sequence in South Korea, and also had a high homology with the ompA gene and ompB gene of an unknown SFGR genotype found in H. longicornis from Dandong, China (border between China and North Korea). This suggests that the new SFGR Candidatus Rickettsia longicornii genotypes are widely distributed in the border region between China and North Korea. Although there have been no reports of infections caused by the new SFGR genotype, the ompA gene sequence of Candidatus Rickettsia longicornii found in Yanbian was highly homologous to an unknown species of Rickettsia detected in mouse spleen tissue in South Korea. These findings indicate that the Candidatus Rickettsia longicornii identified in this study was likely to be infectious in mammalian hosts and even in humans. It is therefore necessary to strengthen the surveillance for the SFGR Candidatus Rickettsia longicornii genotypes in ticks and their hosts in the border regions of China, North Korea, and Russia, as well as in other areas with a concentrated distribution of H. longicornis, to prevent cross-border transmission and an epidemic of tick-borne diseases affecting human health and animal husbandry.
SFTS is a novel infectious disease that was first discovered in China. Its main clinical manifestations consist of acute fever, thrombocytopenia, and leukopenia. The pathogen was isolated from a patient’s serum and termed SFTSV [21]. At present, most ticks detected and isolated from cases of SFTS were H. longicornis. The first report of the disease occurred when SFTSV was detected in H. longicornis on the skin of sheep in Henan Province, China [22], and SFTSV was isolated from H. longicornis on sheep in the SFTS epidemic area in Shandong Province, China [23]. SFTSV was first isolated from H. longicornis in Korea [24]. In the current study, the SFTSV Small, Medium, and Large gene sequences were obtained by gene amplification, and homology analysis indicated 98%-99% homology with the SFTSV gene sequence found in South KoreaPhylogenetic analysis showed that the SFTSV Small, Medium, and Large gene sequences were in the same clade as isolates from Jilin and Jiangsu, respectively, and were closely related to SFTSV isolated from Zhejiang and South Korea. This may be related to the parasitism of migratory birds by ticks in the east or SFTSV transmission by migratory birds themselves during cross-sea migration. Korean researchers suggested that migratory birds may play an important role in the spread of SFTSV [25]. The above results suggest that the border area of China, North Korea, and Russia is a key region for preventing tick-borne SFTSV, and should thus be considered in the prevention and control of imported infectious diseases in the border region.
T. orientalis is a protozoon that infects cattle and buffalo, and which is generally transmitted by ticks of the genus Haemaphysalis [26]. T. sinensis was originally isolated from naturally infected cattle by Bai Qi and others in Gansu Province of China. Chinese researchers investigated the taxonomic status of this undetermined species by comparing it with other bovineTheileria worms via morphological comparison, and inoculation transmission and host specificity tests. It was finally termed T. sinensis [27]. T. orientalis is transmitted by H. longicornis, H. concinna, and H. japonica. However, the vector tick species differ between regions; for example, H. japonica is the main vector of Oriental Taylor disease in Russia, followed by H. concinna, while H. concinna is also the main vector in Korea, whereas H. longicornis is the main vector in China and Japan. In this investigation, we detected T. orientalis in H. longicornis but failed to detect T. sinensis in any ticks. This may be related to the collection area and small numbers of its vector, H. japonica.