Nikhil Prasad Fact checked by:Thailand Medical News Team Jun 18, 2026 1 hour, 26 minutes ago
Medical News: A little-known virus that was silently circulating in parts of East Asia for years is now drawing increasing concern from scientists and public health experts. Known as the Yezo virus (YEZV), this emerging tick-borne pathogen has been detected in humans, ticks, and wildlife across Japan, China, and Russia, raising fears that it could become a growing health threat in regions where tick populations are expanding.
Newly discovered Yezo virus spreads across Japan, China, and Russia as scientists warn of its growing public health
threat and expanding geographic reach.
Although the virus was only officially identified in 2021, researchers have since discovered that it had likely been infecting people long before it was recognized. With evidence of ongoing spread, genetic evolution, and the potential to cause serious illness, the Yezo virus is rapidly becoming one of the most closely watched emerging infectious diseases in Asia.
Discovery of a Previously Unknown Human Virus
The Yezo virus was first brought to scientific attention after two patients in Hokkaido, Japan, developed unusual febrile illnesses following tick bites in 2019 and 2020. Scientists investigating the cases isolated a previously unknown virus from their blood samples and subsequently identified it as a new member of the Orthonairovirus genus within the Nairoviridae family.
The virus was named after "Yezo," the historical Ainu name for Hokkaido, where the first recognized human infections occurred.
What made the discovery particularly concerning was that retrospective investigations revealed that the virus had likely been infecting humans for years before it was officially identified. Stored patient samples showed evidence of infections dating back to at least 2014, suggesting that the pathogen had remained undetected while quietly circulating among humans, wildlife, and ticks.
Researchers from Hokkaido University, the National Institute of Infectious Diseases of Japan, the University of Tokyo, the Heilongjiang Center for Disease Control and Prevention in China, the Harbin Veterinary Research Institute of the Chinese Academy of Agricultural Sciences, and research institutions affiliated with the Russian Academy of Sciences have all played important roles in investigating the virus and its spread.
What Exactly Is the Yezo Virus?
YEZV is an enveloped, negative-sense, single-stranded RNA virus. Its genome consists of three segments known as the Large (L), Medium (M), and Small (S) segments. These genetic components encode important viral proteins including the RNA-dependent RNA polymerase, glycoproteins, and nucleocapsid proteins that allow the virus to infect cells and replicate within the body.
Genetic studies revealed that the virus belongs to the Sulina genogroup of orthonairoviruses and is most closely related to Sulina virus, which was previously detected in Ixodes ricinus ticks in Romania. This relationship suggests that related viruses may be distributed much more widely across Eurasia than previously believed.
Unlike some of its more dangerous relatives, such as Crimean-Congo hemorrhagic fever virus, the Yezo virus does
not typically cause severe hemorrhagic disease. However, it can still trigger significant illness and potentially serious complications.
Ticks Are the Main Culprits
Current evidence indicates that the virus is primarily transmitted through the bite of infected hard ticks, especially Ixodes persulcatus, commonly known as the taiga tick. This tick species is widespread across northern Asia and is already known to carry several pathogens capable of infecting humans.
Investigations in Hokkaido detected YEZV RNA in local tick populations, confirming that the virus was actively circulating within tick ecosystems.
Scientists also found antibodies against the virus in wild animals including deer and raccoons. These findings suggest that wildlife species act as natural reservoirs, allowing the virus to persist in nature while ticks transmit it between animal hosts and occasionally to humans.
The Virus Expands Beyond Japan
Initially, researchers believed the virus might be confined to northern Japan. However, that assumption quickly proved incorrect.
A major surveillance study conducted in Heilongjiang Province in northeastern China screened 988 patients who sought medical care after experiencing tick bites. Investigators identified 18 previously undetected YEZV infections, representing approximately 2 percent of those tested.
Virus isolation from patient blood samples confirmed active infections. The findings demonstrated that the virus was already circulating in human populations outside Japan.
The situation became even more concerning when researchers identified the virus in Russia. Multiple YEZV isolates were recovered from Ixodes persulcatus ticks collected in West Siberia and the Russian Far East. Although the minimum infection rate among ticks was approximately 0.12 percent, genetic analysis revealed substantial diversity among Russian strains.
Scientists identified multiple viral lineages, indicating that the pathogen had likely been circulating across a broad geographic area for a considerable period.
Migratory Birds Could Spread the Virus Further
One of the most alarming discoveries involves migratory birds. Researchers studying bird migration routes in Hokkaido identified YEZV-positive ticks attached to migratory birds. Since many bird species travel thousands of kilometers along international flyways, they may serve as vehicles for transporting infected ticks across national borders.
This raises concerns that the virus could spread into new regions where suitable tick species already exist. Scientists fear that migratory birds may facilitate movement of the virus throughout northern Asia and potentially into parts of Europe.
Climate change may further worsen the situation. Rising temperatures are allowing tick populations to expand into previously unsuitable habitats, increasing opportunities for human exposure.
Symptoms Often Begin Like the Flu
Human infections typically develop between four and seventeen days after a tick bite.
Most patients experience a sudden onset of high fever, often exceeding 39 degrees Celsius. Other common symptoms include severe fatigue, headaches, dizziness, muscle pain, and general weakness.
Pain in the legs appears to be particularly common among infected individuals.
Around half of patients also experience nausea, vomiting, abdominal discomfort, or other gastrointestinal symptoms. Skin rashes, including urticarial and maculopapular eruptions, have also been reported.
More concerningly, approximately 28 percent of patients develop neurological symptoms, including altered mental status and other nervous system abnormalities.
Distinctive Laboratory Findings
The virus produces a characteristic pattern of abnormalities in blood tests.
Many patients develop leukopenia, meaning abnormally low white blood cell counts, often accompanied by lymphocytopenia. Thrombocytopenia, or low platelet counts, is also frequently observed.
Researchers consistently found elevated liver enzymes including AST and ALT, indicating liver inflammation or injury. Levels of lactate dehydrogenase, creatine kinase, and ferritin are often significantly elevated as well.
Abnormal blood clotting markers have also been reported. Patients frequently exhibit prolonged activated partial thromboplastin time and elevated D-dimer levels, indicating disturbances in coagulation pathways.
Some Patients Develop Severe Disease
Although many patients recover with supportive treatment, some cases can become severe.
One of the first documented Japanese patients, a 41-year-old man, experienced dramatic elevations in liver enzymes. His AST level reached an astonishing 3,703 U/L, reflecting significant liver injury. His platelet count fell to approximately 75,000 per microliter, while viral levels in his blood reached 7.3 × 10⁷ copies per microliter.
Another patient experienced similar abnormalities and was simultaneously infected with Borrelia miyamotoi, a bacterial pathogen also transmitted by ticks.
In the Chinese study, eight of the eighteen confirmed patients required hospitalization. Elevated liver enzymes were observed in approximately 72 percent of cases, while inflammatory markers were frequently increased.
Complications can include hepatitis, rhabdomyolysis, blood clotting abnormalities, prolonged recovery, and potentially more severe systemic disease.
Animal Studies Reveal a Dangerous Potential
Laboratory studies have revealed that the virus may possess greater pathogenic potential than currently appreciated.
Experiments involving AG129 interferon receptor-deficient mice showed that YEZV could cause severe hepatitis, dramatic weight loss, and death within six days of infection. Researchers found particularly high viral loads in the liver, confirming that the organ is a major target of infection.
Scientists later developed a mouse-adapted strain known as MA-YEZV. Studies using this strain in immunocompetent C57BL/6J mice demonstrated widespread infection throughout multiple organs and tissues.
The virus triggered intense inflammatory responses characterized by elevated levels of interferon-gamma, tumor necrosis factor-alpha, and interleukin-6. Researchers also observed activation of inflammasome pathways associated with severe disease.
In the middle of growing international concern, this
Medical News report highlights how these animal studies suggest the virus possesses the biological capability to cause far more severe disease under certain circumstances than has been observed in most human cases to date.
Mutations Raise Future Concerns
Like many RNA viruses, YEZV continuously evolves through mutation.
Russian and Chinese isolates already show evidence of genetic diversity. Scientists have identified variations affecting important viral proteins that may influence replication, host adaptation, and immune system interactions.
Particularly concerning was the discovery that serial passage of the virus in laboratory animals produced 31 non-synonymous mutations. Several of these mutations enhanced viral replication and virulence.
Researchers identified changes affecting the OTU-like protease, which helps the virus evade immune defenses, as well as mutations in the RNA-dependent RNA polymerase that could influence replication efficiency.
Although no evidence currently suggests efficient human-to-human transmission, scientists are closely monitoring the virus for signs of further adaptation.
Treatment Options Show Promise
There are currently no approved antiviral therapies specifically designed for YEZV infection.
Treatment remains largely supportive and focuses on symptom management, monitoring liver function, maintaining hydration, and treating any co-infections.
However, several experimental therapies have produced encouraging results. Favipiravir successfully protected infected AG129 mice from lethal disease.
Ribavirin also demonstrated potent antiviral activity in laboratory studies, significantly reducing viral replication, preventing mortality, and limiting tissue damage. Researchers reported an IC50 value of approximately 18.2 μM.
Interestingly, remdesivir failed to show meaningful effectiveness against the virus in experimental studies.
These findings provide hope that future clinical trials may identify effective treatment strategies.
Conclusion
The discovery of the Yezo virus highlights how new infectious disease threats can emerge quietly and remain undetected for years before attracting scientific attention. Although officially identified only in 2021, evidence now shows that the virus had likely been infecting humans since at least 2014 while circulating among ticks and wildlife across parts of East Asia. The growing number of detections in Japan, China, and Russia, combined with evidence that migratory birds may transport infected ticks across international borders, underscores the potential for further geographic expansion. Climate change, expanding tick habitats, and increased human exposure to wildlife environments may further amplify transmission risks. While most infections currently appear manageable, the virus has demonstrated the ability to cause significant liver injury, blood abnormalities, neurological symptoms, and severe disease in experimental models. Continued surveillance, improved diagnostics, development of antiviral treatments, and research into vaccines will be essential to prevent this emerging pathogen from becoming a much larger public health challenge in the future.
References:
https://www.nature.com/articles/s41467-021-25857-0
https://academic.oup.com/jid/article-abstract/233/6/1036/8468645?redirectedFrom=fulltext
https://link.springer.com/article/10.1186/s12866-025-04624-x
https://www.cidrap.umn.edu/tick-borne-disease/surveillance-turns-18-previously-undetected-cases-tick-borne-yezo-virus-china
https://wwwnc.cdc.gov/eid/article/32/5/25-1620_article
https://www.nature.com/articles/s41467-026-70851-z
https://journals.plos.org/plospathogens/article?id=10.1371/journal.ppat.1012101
https://www.mdpi.com/1999-4915/17/8/1125
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