Recently, the risk of tick-borne disease has been associated with exposure to ticks from increasing outdoor activity. This study was performed to detect and identify the tick-borne pathogens in ticks removed from tick-bitten humans. We classified 33 ticks into three species: A. testudinarium (20, 60.6%; 7 adults and 13 nymphs) was the most common followed by H. longicornis (9, 27.3%; 5 adults and 4 nymphs) and I. nipponensis (4, 12.1%; 3 adults and 1 nymph). According to a tick survey study conducted by the KCDC (Korea Centers for Disease Control and Prevention) from 2013 to 2015, H. longicornis was the most dominant species (88.9%), followed by H. flava, I. nipponensis, I. persulcatus, H. japonica, A. testudinarium, and I. granulatus when ticks were collected from the vegetation and forests in the ROK using dry-ice bait traps and a flagging method (14). Interestingly, our results showed that when ticks were collected from tick-bitten humans, A. testudinarium was the most common.
For the molecular detection of tick-borne pathogens, we performed pathogen-specific N-PCR to detect the DNA of the tick-borne pathogens, namely SFG Rickettsia, A. phagocytophilum, Borrelia spp., Bartonella spp., Babesia spp., and C. burnetii. Three tick samples (3 of 33, 9.1%) were positive for A. phagocytophilum DNA, 12 tick samples (12 of 33, 36.4%) were positive for R. monacensis, R. tamurae or Ca. R. jingxinensis DNA, and three ticks (3 of 33, 9.1%) were positive for B. gibsoni or B. microti DNA. Previous studies that investigated the prevalence of tick-borne infectious agents in ticks collected by dragging and flagging grass vegetation in the ROK showed that A. phagocytophilum was detected in 1.9% of H. longicornis ticks (15) and 0.1% of I. persulcatus ticks, and Rickettsia spp. were detected in 1.7% of H. longicornis ticks (16). One study reported that a pool of H. longicornis, H. flava, and I. nipponensis ticks collected by dragging vegetation in the ROK were positive for the Rickettsia spp. 17 kDa antigen (60/311, 19.3%) and ompA gene (53/311, 17.04%) (17). In the present study, the infection prevalence of Rickettsia species (R. monacensis, R. tamurae, and Ca. R. jingxinensis) and A. phagocytophilum in the ticks collected from humans was higher than that of ticks collected from the vegetation. Thus, we suggest that further study is needed to compare the infection prevalence of tick-borne pathogens, including Rickettsia spp., A. phagocytophilum, and Babesia between ticks isolated from humans and ticks collected from grass vegetation.
A. phagocytophilum infection was first reported with serological evidence from humans in 2002, and it is currently the most frequently reported tick-borne bacterial infection in the ROK (18). The detection of Anaplasma spp. in ticks from grazing cattle collected from all ROK provinces has been reported (19). Another study confirmed a human granulocytic anaplasmosis (HGA) with A. phagocytophilum in a patient from the ROK who had a history of tick bites, clinical symptoms, and positive laboratory findings (20). The present results showed that A. phagocytophilum was detected in A. testudinarium and I. nipponensis ticks. The amplicon sequences of the partial ankA gene in A. testudinarium (Tick 1) and I. nipponensis (Tick 29 and Tick 30) demonstrated more than 99% similarity. In the phylogenetic analysis, the sequences of the ankA gene from different types of ticks clustered together, showed > 99% similarity with A. phagocytophilum strains isolated from humans in the ROK (Fig. 1A).
The first isolation of R. monacensis from ticks in the ROK was reported in 2013 (21). A previous study from the ROK reported that I. nipponensis was infected with the human pathogen R. monacensis and that H. longicornis and H. flava were infected with unknown SFG Rickettsia pathogens (17). Our results confirmed the presence of R. monacensis in I. nipponensis ticks removed from humans. In addition, our results indicated that I. nipponensis ticks are most likely the vectors responsible for transmitting R. monacensis infections in the ROK. Therefore, further studies are needed to determine the role of I. nipponensis in the transmission of the R. monacensis pathogen to humans; the blood of patients bitten by I. nipponensis ticks and the ticks themselves should be investigated for the presence of R. monacensis.
R. tamurae was first isolated from A. testudinarium ticks collected in Japan in 1993. R. tamurae was formally identified as a novel species by genetic and phylogenetic analyses in 2006 (22). In 2011, the first case of human infection was confirmed using molecular and serological analyses in Japan (23). The presence of SFG Rickettsia including R. tamurae was found in Amblyomma and Dermacentor ticks in Thailand (24) and in Haemaphysalis ticks in Peninsular Malaysia (25). In addition, R. tamurae was found in Amblyomma ticks from an area endemic for Brazilian spotted fever in Brazil (26). Supporting these previous studies, our results showed the presence of R. tamurae in A. testudinarium ticks.
The presence of a potentially novel species of Ca. R. jingxinensis was proposed in H. longicornis nymphs from Jingxin in Northeastern China in 2016 (27) and was detected in H. longicornis ticks in Xi’an, China in 2017 (28). In the ROK, the pathogenicity of Ca. R. jingxinensis is not clear. Therefore, a further assessment of the potential pathogenicity in humans and animals is needed.
There have been no previous reports of R. tamurae or Ca. R. jingxinensis from ticks in the ROK; here, we report the first identification of R. tamurae and Ca. R. jingxinensis in ticks obtained from tick-bitten humans.
Babesia was first discovered in animals by Babes in 1988, and more than 100 species have been identified. In the ROK, Babesia spp. have been isolated from cattle and other mammals (raccoon, deer, and badger) since the 2000s (29–31). Babesia spp. are mainly carried by Ixodes ticks. Previous studies using ticks collected from grass and vegetation in the ROK reported that H. longicornis was the most common tick species infected with Babesia (16, 19). Our results showed that B. microti was found in both H. longicornis and A. testudinarium. In the USA, the primary vector for the transmission of B. microti to humans is the tick Ixodes scapularis in the nymphal stage (32). The present results suggest that further study is needed to determine the type of ticks that are the vectors for the transmission of B. microti to humans in the ROK.
B. gibsoni was first identified in nymphs of Rhipicephalus sanguineus ticks from infected dogs in Asia (33). B. gibsoni was detected in A. testudinarium ticks in this study. The first case of human babesiosis (KO1) was reported in 2007 in the ROK, and it was highly related to Chinese ovine Babesia spp. (12). Based on the phylogenetic analysis of the 18S rDNA gene in our study, the pathogen clustered with a group of Babesia spp., isolated from a tick in Japan, which was diverged from the KO1 strain (Fig. 1D). The present results indicate that Babesia spp. may vary based on their geographical distributions.
Further investigation is needed to determine the difference between pathogens found in ticks isolated from humans and ticks collected from grass vegetation. In addition, transmission studies should be conducted to determine whether the pathogens found in ticks are the same as those found in humans bitten by those ticks. To confirm the transmission of pathogens from ticks to humans, serological testing on the blood of tick-bitten patients and their ticks will be necessary. Further experiments and correlation analysis using the blood samples of tick-bitten humans and ticks isolated from them may help predict the transmission of tick-borne diseases.