#18,415
Yesterday Japan's Light Horse Quarantine Council announced that country's first detection of equine (H3N8) influenza since 2008, affecting at least 3 horse breeding farms in Kumamoto Prefecture on the island of Kyūshū.
First, the (translated) text of that statement, after which we'll look at some of the history (and potential significance) of Equine Influenza.
Light Defense Association News Flash (Extra Edition)
April 10, 2025
Light Horse Quarantine Council Secretariat
(JRA Equestrian Affairs Department Quarantine Division)
Equine influenza outbreak in Kumamoto Prefecture
On April 8, 2025, outbreaks of equine influenza were confirmed at three heavy-breeding horse farms in Kumamoto Prefecture. Currently, Kumamoto Prefecture is taking steps to prevent the spread of the virus by isolating infected horses, restricting the movement of horses, providing vaccination guidance, and disinfecting facilities.
As this disease spreads rapidly through droplet infection, it is important to work to prevent outbreaks by thoroughly managing horse hygiene and encouraging vaccinations, as well as to prevent the spread of infection by early detection and diagnosis of infected horses, isolating them, refraining from moving them, and disinfecting facilities and equipment.
Managers should thoroughly monitor the health of their horses, and if they notice any symptoms that may be indicative of this disease, such as fever or respiratory symptoms, they should immediately contact a veterinarian. In addition, if this disease is suspected, the horse should be kept away from other horses.
This is the first case of the disease in Japan since 2008, and during the 2007 outbreak there were cases where the spread of infection was thought to have been due to human movement. Therefore, please take precautions such as not entering other horse breeding facilities carelessly, changing shoes and jackets when entering, and thoroughly disinfecting your hands and vehicles.For an overview of equine influenza, please click here.
・Equine Infectious Disease Series: Equine Influenza (4th Edition)
(Continue . . . )
Over the past few years China has confirmed at least 3 H3N8 infections in humans, causing severe illness and at least 1 death. While these were due to avian (AIV) H3N8, this influenza subtype can also be found in Canines (CIV), Equines (EIV), and even marine mammals.
There are subtle, genetic differences between the AIV, CIV, and EIV versions of this subtype, and while each pretty much stays in its own lane, spillovers have occurred.
Originally, Canine H3N8 evolved directly from Equine H3N8, when it abruptly mutated enough to adapt to a canine host, and rapidly began to spread among greyhounds at a Florida race track in 2004 (see EID Journal article Influenza A Virus (H3N8) in Dogs with Respiratory Disease, Florida).
While equine H3N8 isn't typically thought as a zoonotic disease, it has been shown experimentally capable of infecting both pigs (see J.Virol.: Experimental Infectivity Of H3N8 In Swine) and cats (see Equine influenza A(H3N8) virus infection in cats).
In 2019, an EID Journal Historical Review of Equine H3N8 warned:
Volume 25, Number 6—June 2019
Historical Review
Equine Influenza Virus—A Neglected, Reemergent Disease Threat
Alexandra Sack, Ann Cullinane, Ulziimaa Daramragchaa, Maitsetseg Chuluunbaatar, Battsetseg Gonchigoo, and Gregory C. Gray
Abstract
Equine influenza virus (EIV) is a common, highly contagious equid respiratory disease. Historically, EIV outbreaks have caused high levels of equine illness and economic damage. Outbreaks have occurred worldwide in the past decade. The risk for EIV infection is not limited to equids; dogs, cats, and humans are susceptible.
This is a topic we also looked at in 2018, in Equine H3N8: Looking At A long-shot In The Pandemic Sweepstakes and before that, in 2014's Study: Dogs As Potential `Mixing Vessels’ For Influenza.
A more recent (Feb 2025) review of Equine influenza viruses can be found in the journal Viruses:So, while the risks of human infection are believed to be very low - when we see outbreaks of H3N8 - we tend to pay attention.
Francesco Branda 1,*, Dong Keon Yon 2,3,4, Mattia Albanese 5,6, Erica Binetti 5,6, Marta Giovanetti 7,8,9, Alessandra Ciccozzi 10, Massimo Ciccozzi 1, Fabio Scarpa 10, Giancarlo Ceccarelli 5,11,12,*Editors: Stephane Pronost, Gang Lu, Romain Paillot Author informationPMCID: PMC11946173 PMID: 40143233AbstractEquine influenza (EI) is a highly contagious respiratory disease caused by the equine influenza virus (EIV), posing a significant threat to equine populations worldwide. EIV exhibits considerable antigenic variability due to its segmented genome, complicating long-term disease control efforts. Although infections are rarely fatal, EIV’s high transmissibility results in widespread outbreaks, leading to substantial morbidity and considerable economic impacts on veterinary care, quarantine, and equestrian activities.
The H3N8 subtype has undergone significant antigenic evolution, resulting in the emergence of distinct lineages, including Eurasian and American, with the Florida sublineage being particularly prevalent. Continuous genetic surveillance and regular updates to vaccine formulations are necessary to address antigenic drift and maintain vaccination efficacy.
Horses, we learned last December, may also be susceptible to HPAI H5N1 infection (see EID Journal: Evidence of Influenza A(H5N1) Spillover Infections in Horses, Mongolia), while in 2018 - in Transbound Emerg Dis: Serological Evidence Of Canine H3N2 In Horses - China - we saw evidence that the avian-origin canine H3N2 virus was capable of infecting equines.Additionally, rare cross-species transmissions have raised concerns regarding the zoonotic potential of EIV. This review provides a comprehensive overview of the epidemiology, pathogenesis, and prevention of EI, emphasizing vaccination strategies and addressing the socio-economic consequences of the disease in regions where the equine industry is vital.
In 2016, in Epizootics, Host Ranges, and Conventional Wisdom we looked at the history of equine epizootics - including the panzootic of 1872 - and at a study (see A Review of Evidence that Equine Influenza Viruses Are Zoonotic) that argued that human EIV infections occasionally occur.
While H3N8 remains pretty far down our zoonotic worry list, novel viruses have a long history of coming at us from out of left field. And given its continued evolution across multiple lineages (AIV, CIV, EIV), H3N8 is worthy of our attention.
