We retrospectively investigated the presence of YEZV RNA in 310 blood samples from tick bite patients. The samples included 144 samples from patients hospitalized in the Kemerovo region of Russia during 2024 and 2 sets of samples from the Khabarovsk region, 60 samples from patients hospitalized in 2015 and 106 samples from patients hospitalized in 2023–2024. We used an in-house real-time reverse transcription PCR (qRT-PCR) for YEZV RNA detection (Appendix Table 1). We identified 2 positive samples: 1 from the Kemerovo region of Russia and 1 from the Khabarovsk region of Russia.

The patient from the Khabarovsk region, Far East, Russia, was a 68-year-old woman who was bitten by a tick in 2015; the species of tick was not determined. Five days after the bite, tick bite fever developed, accompanied by headache, malaise, myalgia, lumbago, and nausea. Two days after fever onset, the patient was hospitalized. A physical examination at admission revealed no meningeal symptoms or rash (Table 1). An eschar, ≈4 mm in diameter and consistent with a tick bite site, was observed. Laboratory testing did not detect thrombocytopenia or leukopenia. Serum levels of liver aminotransferases were unremarkable. While hospitalized, the patient received unidox solutab (100 mg 2×/d for 8 d) (Appendix Table 2). The patient fully recovered on day 9 of hospitalization and was discharged. Molecular investigation of the patient’s blood samples revealed negative results for common tickborne pathogens. However, positive results were obtained for Rickettsia heilongjiangensis and YEZV (Table 1). qRT-PCR analysis detected YEZV RNA at a concentration of 10⁶ copies/mL in the patient’s serum collected on day 1 after fever onset.

The second clinical case occurred in the Kemerovo region, West Siberia, Russia, in 2024. The patient, a 69-year-old man, was admitted to the hospital with a diagnosis of tickborne fever of moderate severity. His clinical manifestation included fever, malaise, and myalgia (Table 1). The patient reported a tick bite 3 days earlier; however, the tick was not available for examination. Platelet and leukocyte counts and serum levels of liver aminotransferases were unremarkable. While hospitalized, the patient received prophylactic intramuscular ceftriaxone (1 g 2×/d for 9 d) (Appendix Table 2). He was discharged on day 9 after a full recovery. Molecular analysis of his blood sample for tickborne pathogens yielded negative results (Table 1). qRT-PCR analysis detected YEZV RNA at a concentration of 5 × 10⁵ copies/mL in serum collected on the first day after fever onset.

To assess the epidemiologic situation in Russia, we tested a total of 2,497 ticks, consisting of 3 species (I. persulcatus, Haemaphysalis concinna, and H. japonica), collected during 2006–2023 in the Kemerovo region and during 2023 from the Khabarovsk region of Russia (Table 2). We tested for YEZV RNA testing by using qRT-PCR. We amplified positive samples by using in-house primer pairs (Appendix Table 3) and then performed sequencing.

Figure

Figure. Geographic distribution of Yezo virus (YEZV) detection in ticks from Russia. Red diamonds indicate sampling locations of YEZV-positive Ixodes persulcatusticks. A) Distribution across Russia. YEZV prevalence (%) and...

We detected YEZV RNA exclusively in I. persulcatus ticks from both the Kemerovo and Khabarovsk regions of Russia (Figure). The prevalence of YEZV infection in ticks was determined to be 0.5% in the Khabarovsk region and ranged from 0.3% to 0.9% in the Kemerovo region in different sampling years (Table 2).

We obtained 12 nucleoprotein gene segment, 12 glycoprotein precursor gene segment, and 10 RNA-dependent RNA polymerase gene segment sequences of YEZV. In addition to the obtained sequences, we included sequences from China, Japan, and Russia, accessed from GenBank, in the analysis. We selected Sulina virus, the closest known relative of YEZV, as the outgroup. Our analysis revealed the obtained sequences formed 3 distinct clades across all 3 genome segments: 1 from the Khabarovsk region (clade A) and 2 from the Kemerovo region (clades B and C) (Appendix Figure 1). Khabarovsk variants exhibited close phylogenetic proximity to sequences from China and Japan. Nucleotide differences between sequences from clades B and C (Kemerovo region) were 7.8% for the nucleoprotein gene segments, 9.0% for the glycoprotein precursor gene segments, and 9.7% for the RNA-dependent RNA polymerase gene segments (Appendix Figure 2). Of note, the division of viruses from Kemerovo sequence into 2 distinct clades correlated with their specific tick sampling locations. All viral genome sequences from this study have been deposited into GenBank (accession nos. PV770287–98, PX898217–28, PX904390–9).

We identified 3 genomic variants of YEZV within Russia. The variant from Khabarovsk is closely related to viruses previously reported in China and Japan. We detected 2 genomic variants in the Kemerovo region. This genetic diversity, which correlates with sampling location, might indicate an association between different YEZV variants and distinct I. persulcatus tick populations (8). Reassortment events in the YEZV genome have previously been reported (strains BT-2135 and T-HLJ02) (Appendix Figure 1 (9); however, we did not detect reassortant strains among the obtained sequences. Further investigation is needed to determine the causes of high diversity of YEZV in West Siberia, Russia, compared with Japan and China, and to identify possible novel genetic variants of the YEZV by studying more western regions of the I. persulcatus tick habitat.

We identified 2 patients with YEZV infection after tick bites: 1 from the Kemerovo region with monoinfection and the other from the Khabarovsk region with co-infection with R. heilongjiangensis. Both patients’ clinical manifestations included nonspecific symptoms common to many infections. Because neither patient had traveled abroad, the cases represent autochthonous YEZV transmission within Russia. All blood samples were tested for the presence of YEZV RNA retrospectively, preventing the investigation of blood parameter dynamics. Previous studies have reported that, in addition to general clinical manifestations such as fever and malaise, YEZV infection might be accompanied by elevated serum levels of liver aminotransferases (alanine aminotransferase and aspartate aminotransferase), thrombocytopenia, and leukopenia (5–7,10). In both cases we identified, platelet and leukocyte counts and liver aminotransferase levels were unremarkable (Appendix Table 2).

Specific clinical manifestations of YEZV infection that would enable reliable clinical diagnosis are difficult to define. Other common tickborne diseases, such as tick-borne encephalitis, borreliosis, and rickettsiosis, do not always occur with specific symptoms, which can lead to misdiagnosis. In those cases, qPCR or qRT-PCR is used to confirm the etiologic agent. The emergence of YEZV as a novel tickborne pathogen highlights the need to update diagnostic panels for routine testing of patients with unclear clinical manifestations after tick bites.