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Until about 15 years ago, dogs were thought largely immune to influenza A infection. That changed in 2004 when an equine H3N8 influenza virus mutated enough to adapt to a canine host, and began to spread among greyhounds at a Florida race track (see EID Journal article Influenza A Virus (H3N8) in Dogs with Respiratory Disease, Florida).
A 96% match to the equine H3N8 virus, this canine H3N8 was believed to have jumped directly from horses to dogs without any reassortment.Since then we've seen several more flu viruses turn up in dogs (and cats), including H5N1, H5N8, H1N1, H6N1, and H7N2.
But the most successful canine flu to date has been an avian H3N2 virus, which was first reported in South Korea in 2007 (see 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.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
Today, we've a report suggesting that the avian-origin canine H3N2 virus has jumped from dogs to horses in China. While the full article is behind a pay wall, we get the gist from the following abstract.
Transbound Emerg Dis. 2018 Dec 19. doi: 10.1111/tbed.13104. [Epub ahead of print]
Serological Evidence of H3N2 Canine Influenza Virus Infection among Horses with Dog Exposure.
Zhou P1,2,3, Luo A1,2,3, Xiao X1,2,3, Hu X1,2,3, Shen D4, Li J1,2,3, Wu X1, Xian X1,2,3, Wei C1,2,3, Fu C1,2,3, Zhang G1, Sun L1,2,3, Li S1,2,3.
Abstract
Currently, Canine influenza virus (CIV) H3N2 is continuously circulating in dog populations in China, Korea, and the United States (US). Both influenza SA-α-2,3-Gal and SA-α-2,6-Gal receptors have been observed in the respiratory tracts of both horses and dogs. Hence, the increasing number of CIV H3N2 cases in the world indicates a potential risk for transspecies transmission to horses with dog exposure.
Here, a seroepidemiological survey of CIV H3N2 infections in horses was conducted using hemagglutination inhibition (HI), microneutralization (MN) and the chicken embryo neutralization test (CENT). From April 2014 to November 2016, 399 sera from race horses were collected in Guangzhou, Dongguan, Huizhou, and Shenzhen in China.
Nine specimens (2.2%, 9/399) were positive for CIV H3N2 with HI titers ≥ 1:20, MN titers ≥ 1:80 and CENT titers ≥ 1:80. Furthermore, these positive horses showed significant correlation with dog exposure, and some dogs (20%, 3/15) from the same riding clubs as the positive horses also possessed antibodies against CIV H3N2.
This study is the first to provide seroepidemiological evidence of CIV H3N2 infection in horses with exposure to dogs. Based on these findings, continuous serological and virological surveillance of CIV H3N2 infection among horses is imperative, and further animal experiments should be performed. This article is protected by copyright. All rights reserved.
While long-shots in the influenza pandemic derby, canine and equine flu are of particular interest because the H3N8 and H3N2 subtypes they carry are similar to pandemic strains of the past (see chart below).
There is still a lot of debate over these pre-1900 influenza pandemics, with conflicting views over whether the 1890-93 `Russian flu’ was due to the H2N2, H3N2, or H3N8 virus, but with most attributing the 1900 outbreak to H3N8 (see Transmissibility and geographic spread of the 1889 influenza pandemic).
Although it has never been shown to infect humans, 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 pandemic potential.
And anytime we see a novel flu virus continue to expand its host range, we need to pay attention.
