Genetic and serological analyses of equine influenza viruses isolated in Kumamoto and Hokkaido, Japan in 2025.
Abstract: In April and May 2025, outbreaks of equine influenza occurred for the first time in 17 years in Japan. Equine influenza virus (EIV) of the H3N8 subtype was mainly detected in heavy draft horse populations in Kumamoto Prefecture and the Tokachi area of Hokkaido. In total, 10 EIVs were isolated from infected horses and then were used for genetic and serological analyses. Phylogenetic analysis of all eight genes revealed that all Japanese isolates were clustered with the Florida sublineage clade 1 (Fc1) viruses and were closely related to North American Fc1 viruses detected in 2024-2025. The results suggest that the epidemic virus was introduced from North America to Japan. The high sequence identity across all eight genes in these Japanese isolates indicates that EIVs from a common origin spread in Kumamoto Prefecture and the Tokachi area of Hokkaido. These two places are separated by a straight-line distance of approximately 1500 km, but draft horses are regularly transported between the two places and infected horses may have moved before movement restrictions were imposed. The virus neutralization assay showed that the horse antisera against A/equine/South Africa/4/2003 (a Fc1 vaccine strain recommended by the World Organisation for Animal Health) and A/equine/Ibaraki/1/2007 (a Fc1 vaccine strain in Japan) cross-neutralized well with two viruses isolated in Kumamoto Prefecture and the Tokachi area of Hokkaido in 2025. This suggests that the current Fc1 vaccine strains are effective against the epidemic Fc1 isolates in Japan in 2025.
Copyright © 2025 Elsevier B.V. All rights reserved.
Publication Date: 2025-08-30 PubMed ID: 40902312DOI: 10.1016/j.vetmic.2025.110701Google Scholar: Lookup
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Summary
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Genetic and serological analyses of equine influenza viruses isolated in Kumamoto and Hokkaido, Japan in 2025 reveal that the outbreak viruses belong to the Florida sublineage clade 1 (Fc1) closely related to strains from North America, and current Fc1 vaccine strains are effective against them.
Background of Equine Influenza Outbreak in Japan 2025
- Equine influenza outbreaks occurred in Japan in April and May 2025 after a 17-year gap.
- The virus subtype identified was H3N8, mainly detected in heavy draft horse populations.
- The affected areas were Kumamoto Prefecture and the Tokachi area of Hokkaido, which are approximately 1500 km apart.
- The outbreaks represent the first equine influenza cases in nearly two decades in Japan.
Isolation and Genetic Analysis of Virus Strains
- A total of 10 equine influenza virus (EIV) isolates were collected from infected horses during the outbreaks.
- Whole-genome sequencing was performed to analyze all eight viral genes from these isolates.
- Phylogenetic analysis showed that all isolates clustered within the Florida sublineage clade 1 (Fc1) group of H3N8 EIVs.
- The Japanese strains were closely related to Fc1 viruses detected in North America in 2024-2025.
- This indicates that the virus likely originated from North America and was introduced into Japan.
- High genetic similarity across all eight genes among isolates suggests a common origin for the viral spread within Japan.
- The geographical separation between Kumamoto and Tokachi suggests infected horses or draft horse transportation led to virus dissemination before movement restrictions.
Serological Analysis and Vaccine Effectiveness
- Virus neutralization assays measured the ability of horse antisera to neutralize the 2025 outbreak viruses.
- Two vaccine strains, A/equine/South Africa/4/2003 (an Fc1 strain recommended by World Organisation for Animal Health) and A/equine/Ibaraki/1/2007 (a Japanese Fc1 strain), were tested.
- Antisera against these vaccine strains cross-neutralized well with the isolated EIVs from Kumamoto and Tokachi.
- This demonstrates that current Fc1 vaccine strains remain effective in providing immunity against the outbreak viruses in Japan.
Implications of the Study
- The study provides genetic evidence that the 2025 equine influenza outbreaks in Japan were caused by Fc1 viruses imported from North America.
- It highlights the role of horse transportation in the spread of the virus across distant Japanese regions.
- Findings confirm the appropriateness of the current Fc1-based vaccines for controlling such outbreaks in Japan.
- Enhanced surveillance and movement controls could be critical to preventing future introductions and spread of EIV.
Cite This Article
APA
Nemoto M, Kawanishi N, Kamei R, Furusho K, Kawauchi K, Yabuuchi Y, Oue Y, Uchida Y, Nishiura H, Reedy SE, Chambers TM, Li F, Bannai H, Yamanaka T, Tsujimura K.
(2025).
Genetic and serological analyses of equine influenza viruses isolated in Kumamoto and Hokkaido, Japan in 2025.
Vet Microbiol, 310, 110701.
https://doi.org/10.1016/j.vetmic.2025.110701 Publication
Researcher Affiliations
- Equine Research Institute, Japan Racing Association, Tochigi, Shimotsuke, Japan. Electronic address: nemoto_manabu@equinst.go.jp.
- Equine Research Institute, Japan Racing Association, Tochigi, Shimotsuke, Japan.
- Kumamoto Chuo Livestock Hygiene Service Center, Kumamoto, Japan.
- Kumamoto Chuo Livestock Hygiene Service Center, Kumamoto, Japan.
- Hokkaido Tokachi Livestock Hygiene Service Center, Obihiro, Hokkaido, Japan.
- Hokkaido Tokachi Livestock Hygiene Service Center, Obihiro, Hokkaido, Japan.
- Hokkaido Tokachi Livestock Hygiene Service Center, Obihiro, Hokkaido, Japan.
- Emerging Virus Group, Division of Zoonosis Research, National Institute of Animal Health, National Agriculture and Food Research Organization, Tsukuba, Ibaraki, Japan.
- Emerging Virus Group, Division of Zoonosis Research, National Institute of Animal Health, National Agriculture and Food Research Organization, Tsukuba, Ibaraki, Japan.
- Department of Veterinary Science, Gluck Equine Research Center, University of Kentucky, Lexington, KY, USA.
- Department of Veterinary Science, Gluck Equine Research Center, University of Kentucky, Lexington, KY, USA.
- Department of Veterinary Science, Gluck Equine Research Center, University of Kentucky, Lexington, KY, USA.
- Equine Research Institute, Japan Racing Association, Tochigi, Shimotsuke, Japan.
- Equine Research Institute, Japan Racing Association, Tochigi, Shimotsuke, Japan.
- Equine Research Institute, Japan Racing Association, Tochigi, Shimotsuke, Japan.
MeSH Terms
- Animals
- Horses
- Japan / epidemiology
- Influenza A Virus, H3N8 Subtype / genetics
- Influenza A Virus, H3N8 Subtype / isolation & purification
- Influenza A Virus, H3N8 Subtype / immunology
- Influenza A Virus, H3N8 Subtype / classification
- Horse Diseases / virology
- Horse Diseases / epidemiology
- Orthomyxoviridae Infections / veterinary
- Orthomyxoviridae Infections / epidemiology
- Orthomyxoviridae Infections / virology
- Phylogeny
- Antibodies, Viral / blood
- Disease Outbreaks / veterinary
- Neutralization Tests
Conflict of Interest Statement
Declaration of Competing Interest The authors declare no conflicts of interest.
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