#19,129
Twenty years ago most veterinarians would have told you that dogs were not particularly susceptible to influenza A viruses, but that notion changed in 2004 when we saw the jump of equine H3N8-like influenza to Florida greyhounds.
In 2008, an EID Journal article reported:Influenza A Virus (H3N8) in Dogs with Respiratory Disease, FloridaLike its equine counterpart (which has been around at least a half century) - canine H3N8 has not been shown to infect humans. The CDC considered this to be a dog-specific lineage of H3N8; worth watching but not an imminent threat.
Sunchai Payungporn*, P. Cynda Crawford†, Theodore S. Kouo*, Li-mei Chen*, Justine Pompey*, William L. Castleman†, Edward J. Dubovi‡, Jacqueline M. Katz*, and Ruben O. Donis*
Abstract
In 2004, canine influenza virus subtype H3N8 emerged in greyhounds in the United States. Subsequent serologic evidence indicated virus circulation in dog breeds other than greyhounds, but the virus had not been isolated from affected animals. In 2005, we conducted virologic investigation of 7 nongreyhound dogs that died from respiratory disease in Florida and isolated influenza subtype H3N8 virus.
In 2007 we saw an avian H3N2 virus jump to dogs in South Korea (see 2008 EID report Transmission of Avian Influenza Virus (H3N2) to Dogs). Analysis showed that the HA and NA genes of the A/canine/Korea/01/2007 (H3N2) isolate were closely related to those identified in 2003 from chickens and doves in South Korea.
Unlike canine H3N8 - which has remained relatively stable - H3N2 has shown an affinity to reassort with other influenza viruses. We've seen numerous reports coming out of both China and Korea suggesting canine H3N2 may be adapting to other hosts, and continues to reassort with other avian and human flu viruses. A few examples include:
In 2015 this Asian canine H3N2 virus finally turned up in North America (see CDC Statement On H3N2 Canine Influenza In Chicago Region), and since then has spread across much of the United States.A Canine H3N2 Virus With PA Gene From Avian H9N2 - Korea
Canine H3N2 Reassortant With pH1N1 Matrix Gene
Virology J: Human-like H3N2 Influenza Viruses In Dogs - Guangxi, China
Interspecies Transmission Of Canine H3N2 In The Laboratory
While quantified as a relatively low-risk virus, in 2017 the CDC added Canine H3N2 to their IRAT (Influenza Risk Assessment Tool) listing of novel flu subtypes/strains that circulate in non-human hosts and are believed to possess some degree of zoonotic potential. Their evaluation reads:
H3N2: [A/canine/Illinois/12191/2015]
The H3N2 canine influenza virus is an avian flu virus that adapted to infect dogs. This virus is different from human seasonal H3N2 viruses. Canine influenza A H3N2 virus was first detected in dogs in South Korea in 2007 and has since been reported in China and Thailand. It was first detected in dogs in the United States in April 2015. H3N2 canine influenza has reportedly infected some cats as well as dogs. There have been no reports of human cases.
Summary: The average summary risk score for the virus to achieve sustained human-to-human transmission was low risk (less than 4). The average summary risk score for the virus to significantly impact public health if it were to achieve sustained human-to-human transmission was in the low risk range (less than 4).
Last year (2025) waw two concerning reports come out of China:
Preprint: Fatal infection of a novel canine/human reassortant H3N2 influenza A virus in the zoo-housed golden monkeys
Although we've yet to see any evidence of human infection with canine H3N2, this virus continues to reassort and evolve, and we've evidence of spillover to felines, primates, and even spillback into birds.
While most of the research we've seen has come out of Asia, today we've a preprint on the detection of a new (reassorted) strain of Canine H3N2 in Alabama in 2022.
The Evolution and Transmission Dynamics of H3N2 Influenza Virus Identified in A Respiratory Disease Outbreak in Companion Dogs in Alabama, USA
Shakiba Kazemian, Sana Tamim, Peter Neasham, Samiah Kanwar, Kevin Zhong, and 7 more
This is a preprint; it has not been peer reviewed by a journal.
https://doi.org/10.21203/rs.3.rs-9053609/v1
This work is licensed under a CC BY 4.0 License
Abstract
Canine Influenza Virus H3N2, initially of avian origin, emerged in Asia around 2005–2007 and subsequently spread globally, reaching the United States in 2015. Despite initial containment efforts, H3N2 CIV re-emerged in several US states in 2023. This study investigates the evolution and transmission dynamics of H3N2 CIV identified in companion dogs during an outbreak in Alabama, USA, between August and October 2022.We performed whole-genome sequencing on five H3N2 CIV isolates and conducted phylodynamic modeling, incorporating 100 global H3N2 CIV genomes per segment. Phylogenetic reconstruction showed the Alabama strains formed distinct, tightly clustered groups, suggesting recent common ancestry and localized evolution. Phylogeographic inference revealed discordant origins for different gene segments. The HA, MP, NP, NS, and PA genes traced ancestry to California strains, the NA gene to Florida strains, and the PB1 and PB2 genes to Illinois strains.This genomic incongruence strongly indicates that the Alabama outbreak strain emerged through a reassortment event involving viruses from these three geographically distinct regions.
The findings highlight reassortment as a crucial mechanism driving CIV H3N2 evolution and spread, likely facilitated by the movement of infected dogs. Low canine vaccination requires continued surveillance and shelter-focused vaccination to control CIV outbreaks and monitor adaptation.
(SNIP)
The findings presented here highlight the importance of continued surveillance for CIV, particularly in settings where dogs interact (dog parks) or are housed in close proximity,such as shelters and breeding facilities, which poses vulnerability for dog populations.Shelters and stray dog populations are a particular challenge since densely populated environments and frequent dog-to-dog contact can act as hotspots for viral transmission and potentially facilitate further reassortment events and contribute to the ongoing adaptation and evolution of the virus.
Our study primarily focused on the genetic analysis of the viral strains circulating during the Alabama outbreak. Future investigations could benefit from incorporating additional data sources and information on dog movement patterns, to provide a more comprehensive understanding of the outbreak dynamics. The introduction of influenza strains from geographically distinct locations coupled with the lack of widespread vaccination in the canine population, created conditions conducive to reassortment and generation of new viral diversity.
In conclusion, the findings from the Alabama H3N2 CIV outbreak strongly suggest that reassortment events play a critical role in the emergence and spread of the virus. This case study underscores the importance of continued vaccination and surveillance for influenza virus evolution in canines and the potential public health implications of reassortment events in zoonotic influenza viruses.
While exact numbers are hard to come by, veterinarians report that uptake of canine flu vaccination remains low. Not all that surprising, given that CIV is not considered a `core antigen', the hassle/expense of scheduling 2 vet visits, and the public's growing distrust of vaccines.
But companion animals (mostly dogs and cats) are uniquely positioned to serve as a bridge to introduce zoonotic diseases to humans.
Which makes it important that we increase surveillance, and take reasonable steps - where possible - to limit the spread of emerging pathogens, including canine influenza.