Thursday, May 11, 2023

China: Emergence of a Novel Reassortant H3N6 Canine Influenza Virus

Dogs as potential `mixing vessels' for Influenza


For more than a decade we've been monitoring the emergence and spread of an avian H3N2 virus which jumped to dogs - first in South Korea, then later in China - about 15 years ago (see 2008's EID Journal article  Transmission of Avian Influenza Virus (H3N2) to Dogs).

Since then we've seen numerous reports coming out of China and Korea suggesting the canine H3N2 may be adapting to other hosts, and continues to reassort with other avian and human flu viruses.  Including:

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

Canine H3N2 entered the United States, and began spreading widely in 2015 (see Midwest Canine Influenza Outbreak Due To `New’ Korean H3N2 Virus). Since then it has co-circulated with canine H3N8 in dogs, sparking outbreaks in dozens of states.

Since it differs genetically from both the equine and avian H3N2 viruses, H3N2 was enough of a public health concern that the CDC ordered a risk analysis in March of 2016 (see PDF report).  

While the risks of human infection were viewed as low, canine H3N2 was added to the CDC's IRAT List of zoonotic flu viruses with pandemic potential in the summer of 2017.

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).

While there are no known instances of human infection with canine H3N2 or H3N8, these viruses continue to evolve, and the detection of H3N2 in cats is proof they can expand their host range.  

Just last month, in Increased Public Health Threat of Avian-origin H3N2 Influenza Virus During Evolution in Dogs (Revisited), we looked at a report from Chinese scientists on potential for canine H3N2 to jump from dogs to humans.  They wrote:

We found that, during adaptation in dogs, H3N2 CIVs became able to recognize the human-like SAα2,6-Gal receptor, showed gradually increased hemagglutination (HA) acid stability and replication ability in human airway epithelial cells, and acquired a 100% transmission rate via respiratory droplets in a ferret model. We also found that human populations lack immunity to H3N2 CIVs, and even preexisting immunity derived from the present human seasonal influenza viruses cannot provide protection against H3N2 CIVs.

Today, we've another report from China, this time describing the detection of a canine-avian flu reassortant virus - H3N6 - which was isolated multiple times from dogs in Liaoning Province, China. These scientists (including Hualan Chen), report this H3N6 reassortment continues to evolve `at an incredible speed', making this virus well worth our attention.

I've only posted the link, Abstract, and some excerpts from a much longer report. Follow the link to read it in its entirety. 

Front. Microbiol., 11 May 2023
Sec. Virology
Volume 14 - 2023 |

Emergence of a novel reassortant H3N6 canine influenza virus

Bo Meng1†, Hailing Li1†, Chong Feng1, Weiwe Guo1, Yali Feng1, Dawei Zhu2, Hualan Chen3* and Ying Zhang1*1

Although the natural hosts of avian influenza viruses (AIVs) are wild birds, multiple subtypes of AIVs have established epidemics in numerous mammals due to their cross-species spillover. Replication and evolution in intermedia mammalian hosts may facilitate AIV adaptation in humans. Because of their large population and intimacy with humans, dogs could act as such an intermedia host. 

To monitor the epidemiology of canine influenza viruses (CIVs) in Liaoning, China, we performed three surveillances in November 2018, March 2019, and April 2019. Five H3N2 and seven novel H3N6 CIVs had been isolated

Since the N6 neuraminidase (NA) genes were clustered with the H5N6 AIV, there is a high possibility that these H3N6 CIVs were generated from a H3N2 CIVs and H5N6 AIVs reassortment case

In addition, the H3N6 CIV showed increased mammalian adaptation ability compared to all the H3N2 strains in both in vitro and in vivo studies. Even though isolated 3 months later, the March 2019 isolated H3N2 viruses replicated more efficiently than the November 2018 isolated viruses. Our study indicated that H3 CIVs were undergoing an evolution process, through both genetic mutations and gene reassortment, at an incredible speed.


Liaoning Province has the largest pet dog breeding and trading base in China. Numerous dogs are distributed daily from Liaoning to other provinces of China. In this study, we investigated the epidemic situation of CIVs and analyzed the biological properties of different isolates in Liaoning.

4. Discussion

In this study, we performed active CIV surveillance in Liaoning Province for the 2018 to 2019 flu season. Twelve CIVs were isolated, including five H3N2 and seven H3N6 viruses. We systematically analyzed the genetic and biological properties of these CIVs. The phylogenetic analysis of the HA genes of 12 CIVs demonstrated that isolates in this study were all clustered into the antigenic variants emerging in 2016 (Lyu et al., 2019). NA genes of the H3N6 CIVs were clustered into H5N6 AIV groups, which were isolated in China from 2017 to 2018. There is a high possibility that the novel H3N6 CIVs are generated from H3N2 CIV and H5N6 AIV reassortment cases. H5N6 AIVs have circulated in China since 2014, and trans-species infections and reassortant cases have been reported when the H5N6 circulated in domestic poultry (Yu et al., 2015; Gu et al., 2022).
Previous studies demonstrated that novel reassortant H3N6 viruses had been isolated from migratory birds and domestic birds, which contained some gene segments from the dominant H5N6 viruses (Li et al., 2020). To our knowledge, this is the first report that avian H5N6 viruses can recombine with H3N2 CIVs.

This study indicated that CIVs are evolving and adapting to mammalian hosts at an incredible speed.
The H3N2 CIV isolated in November 2018 showed limited replicative ability in vitro and in vivo, but 3 months later, increased replication was detected in March 2019 H3N2 CIV isolates. Moreover, 1 month later, novel reassortant H3N6 CIVs appeared in April 2019. The H3N6 reassortant possessed enhanced mammalian adaption ability compared with H3N2 CIVs.

Genetic mutation and reassortment were both observed among our CIVs isolates. There were only five amino acid differences between 6068/18 and 15001/19, PB2-A292T, PB2-K578E, PA-I561M, HA-G271D (H3 numbering), and NS1-G142E. The effects of these substitutions on the biological properties of these viruses require further exploration. The alignment of the N6 NA stalk showed that there was an 11 amino acid deletion at positions 58 to 68, which had been identified in H5N6 AIVs. According to the in vitro and in vivo replication study results, it seemed that the H3N2 HA and H5N6 NA genes of the novel CIV are more functionally and optimally cooperative than the HA and NA genes of H3N2 CIVs. Genome reassortment between different subtypes of flu strains is an important driving force for the evolution of influenza viruses and the sources of potential pandemic strains. Three out of four human influenza pandemics of the last century were caused by reassortant viruses (Li and Chen, 2014).

As companion animals, dogs are closely related to humans, and their potential threat to public health deserves high attention. In this study, we found that H3 CIVs were undergoing an evolution process at an incredible speed and provided important information about the evolutionary status of H3 CIVs. Further surveillance and risk analysis of CIVs should be performed.

The obvious good news is that these detections were made in 2018 and 2019, and so far, we've not seen a spillover into humans. The bad news is, these viruses may be a lot closer to having that ability today, than they were 3 years ago. 

While we obsess over H5N1, the truth is there are other potential pandemic contenders out there, and its anyone's guess which one will stumble across the right combination of genetic changes to spark the next global health crisis.