Ticks are the most important vectors of human pathogens, causing increasing public health burdens worldwide. Ticks can cause a variety of diseases, such as tick-borne viral illness, Lyme disease, spotted fever group rickettsiosis, human anaplasmosis, and human babesiosis (Jia et al., 2020b). Ticks transmit infectious agents through continuous attachment to the host’s skin and blood feeding for days. The skin plays a key role in vector-borne diseases because it is the site where the arthropod co-inoculates pathogens and its saliva. It is becoming increasingly evident that the skin is not simply a physical barrier, but is also an organ important in innate immunity, as the first barrier met by the tick and the pathogen (Wikel, 2013; Boulanger and Wikel, 2021). The cutaneous response contributes to the tick-borne pathogen multiplying and sometimes persisting before disseminating, as in Lyme borreliosis (Bernard et al., 2020), spotted fever rickettsiosis (Heinze et al., 2014; Jia et al., 2020a), and Jingmen tick virus infection (Jia et al., 2019). Although various pathogens have been detected in skin lesions, the skin infectome after a tick bite remains largely unknown. This knowledge will undoubtedly provide a deep understanding of the risks presented by tick bites in humans.

In recent years, next-generation sequencing has been successfully used to characterize the complete tick microbiome, identify novel and unexpected pathogens, and elucidate tick microbiome–host interactions, with the goal of developing antivectorial control measures (Carpi et al., 2011; Hawlena et al., 2013; Nakao et al., 2013; Williams-Newkirk et al., 2014). We collected 14 skin lesions from patients in a sentinel hospital who had a history of a tick bite. The microbiome and virome of skin biopsy specimens were identified utilizing a meta-transcriptomics approach, and pathogens that could potentially be transmitted from skin to blood were analyzed, providing valuable data illustrating the horizontal transmission of pathogens from ticks to humans.

A total of 14 patients with a tick bite were enrolled in this study. Nine patients saw a doctor while the tick was still on their bodies (four Ixodes persulcatus, four Dermacentor silvarum, and one Haemaphysalis concinna). The ticks were most likely to be found on the scalp (4/9) or the back (2/9). The interval between the tick biting and sample collection ranged from 1 to 30 days, with a median of 8 days. The necrotic cells were distributed in the epidermis of eschars, while inflammatory infiltration by lymphocytes was identified in some skin lesions by hematoxylin and eosin (HE) staining ( Figure S1 ). The clinical symptoms varied from mild to severe and included local itching, headache, rash, lymphadenectasis, and fever, with laboratory tests showing that some patients had an abnormal liver function and neutropenia ( Table 1 ).

Advances in sequencing technologies have revealed that the diversity of tick microbiome and virome goes far beyond our previous understanding (Li et al., 2021; Narasimhan et al., 2021); it is, therefore, interesting to characterize the host skin infectome after a tick feeds. Transcriptome analysis of skin lesions in patients with a clear tick bite history revealed that four bacterial genera, Pseudomonas, Acinetobacter, Corynebacterium, and Propionibacterium, were common in ticks and patients’ skin. However, we cannot determine whether the detected bacteria were in the tick or on the tick, or whether pathogens or bacteria were transmitted from the tick’s surface. In addition, excluding known bacterial pathogens, such as the spotted fever group rickettsia C. burnetii (Huang et al., 2021; Fu et al., 2022), a potentially new pathogen, R. cervicalis, was detected in both the blood and skin samples of patients. Although more than 10 viral families were found in ticks by RNA-seq in our previous study (Li et al., 2021), only three viruses (i.e., JMTV, BLTV4, and DTMV) could be detected in skin biopsy specimens. This report not only identifies three new potential pathogens in humans but also shows that the skin barrier is vital in preventing horizontal transmissions of tick-associated bacteria or virus communities to the host.

The co-transmission of vector microbiomes and vector-borne pathogens has been previously suspected in insects (Finney et al., 2015). However, regarding the transmission of tick-borne pathogens, very few studies have been performed to investigate the potential transmission of microbiota during the process of a tick bite. In addition, very few studies have investigated the role of skin microbiota in pathogen transmission at the skin interface. At the skin interface, the microbiome also contributes to the regulation of inflammation and is likely to affect the host’s responses to the tick bite. The skin might play a key role in the process of tolerance as the first interface met by the tick and the pathogen (Bernard et al., 2020). Our study also implies that the duration of tick blood feeding is important for the successful passage of an infectious agent into the bite site and the subsequent establishment of infection, although a larger sample size is required to confirm our results. Furthermore, this observation has practical implications for disease prevention.

Rosemonas is thought to be opportunistically pathogenic to humans, particularly immunocompromised patients with underlying diseases such as acute leukemia, cancer, or rheumatoid arthritis (Dé et al., 2004). A previous study found that Rosemonas sp. (R. cervicalis) could be isolated from adult Dermacentor nuttalli and its larval progeny, and obtained evidence of transovarial transmission (Liu et al., 2010). This study further demonstrates the role of the tick in transmitting R. cervicalis to humans.

Tick-associated DTMV was also detected in the skin of case 4, but not from the same patient’s blood sample. It showed high similarity with DTMV strain China/HLJ/2015, which was identified in I. persulcatus in the same province as our patients, and with Lesnoe mivirus, which is from the same tick species but is found in Russia. Another horizontally transmitted tick virus is BLTV4. It is particularly noteworthy that we detected BLTV4 in both skin and blood samples from two patients. BLTV4 was first identified in a metaviromic study carried out in China in 2012 and was subsequently reported in Hyalomma asiaticum, Hyalomma truncatum, Dermacentor reticulatus, and Rhipicephalus microplus collected in Romania and Kenya (Shi et al., 2016; Zhang et al., 2021). We cannot provide evidence of the pathogenicity of these viruses; however, considering their wide distribution in terms of both tick species and geographic location, we stress the importance of continuous active surveillance of patients with a tick bite history.

JMTV was first identified in R. microplus ticks sampled from Jingmen in Hubei Province and Wenzhou in Zhejiang Province, China, in 2010 (Qin et al., 2014). The genome of JMTV comprises four segments of single-stranded positive-sense RNA. Two segments exhibit homology with the non-structural protein NS3 and NS5 sequences of flaviviruses, whereas the other two segments are unique and have no known homologs (Qin et al., 2014). Since its initial identification, JMTV has been identified in several regions of China as well as in Brazil, Kosovo, Uganda, Turkey, the French Antilles, and Trinidad and Tobago (Ladner et al., 2016; Emmerich et al., 2018; Souza et al., 2018; Dinçer et al., 2019; Pascoal et al., 2019; Sameroff et al., 2019; Temmam et al., 2019; Guo et al., 2020). JMTV can infect a broad range of animal hosts, including cattle, rodents, bats, and primates (Souza et al., 2018; Sameroff et al., 2019; Guo et al., 2020). It has been reported as the causative agent of febrile disease in China and Kosovo (Emmerich et al., 2018; Jia et al., 2019). In our study, among 10 patients with JMTV infection, seven had been co-infected with either bacteria or other viruses in the skin. More in-depth laboratory studies and long-term epidemiological studies are needed to clarify the pathogenicity of JMTV in humans.

We found that some pathogenic bacteria could infest patients very quickly after a tick bite, as exemplified by CRT and R. cervicalis (e.g., cases 4 and 6). However, it appears that viruses took several days (i.e., 8–30 days) to successfully cause viremia (e.g., JMTV and BLTV4 in cases 7, 8, and 10). This may be related to the difference in the abundance of bacteria and viruses in ticks, as well as to differences in humans’ immune responses to bacteria and viruses. These are possible directions for future study.

This work is limited by the small sample size. In addition, the pathogenicity of new agents deserves further verification. The detection of seroconversion or a fourfold increase in antibodies in reaction to these bacteria and viruses would be useful to clarify their pathogenicity. Nevertheless, this work is the first research on patients’ skin infectome after a tick bite and demonstrates that more attention should be paid to the cutaneous response to prevent tick-borne illness.

The data presented in the study are deposited in the Gene Expression Omnibus data repository, accession number GSE141235.

The studies involving human participants were reviewed and approved by Mudanjiang Forestry Central Hospital. The patients/participants provided their written informed consent to participate in this study.

NJ contributed to the conception and design of the study. XBN performed the statistical analysis. NJ, X-BN, and T-YL reviewed the draft. JZ wrote the first draft of the manuscript. X-MC, R-RJ, and RW investigated the hospital. X-MC, J-FJ, J-LS, J-LY, FW, YG, M-CC, J-GZ, WW, JL, T-TY, Q-BH, N-NY, CB, Y-LC, and Y-CZ collected the samples. H-BL and LZ contributed to the methodology. L-FL organized the database. QW contributed to the data curation. All authors contributed to the article and approved the submitted version.