Until about a decade ago, it was widely (and erroneously) believed that dogs and cats were not generally susceptible to influenza A infections.
That perception began to change in 2004 with two unrelated events; the jump of equine H3N8 influenza from horses to Florida greyhounds, and the infection by avian H5N1 of tigers fed infected chickens in Thailand.
While not considered major players (yet) in the spread of human or novel influenza viruses, their role as companion animals make dogs and cats of particular interest to influenza researchers.
First a look back at some of the evidence on dogs & cats susceptibility to influenza – then I’ll have a couple of new studies that shed additional light on their ability to contract, and spread, certain subtypes of flu.
In 2004, the H3N8 equine influenza – a strain that has been around in horses nearly a half century – was discovered to have jumped, and adapted to dogs, creating a new dog-specific (canine) lineage of H3N8.
Since then, H3N8 has continued to spread among dogs both in North America and around the globe.
While we’ve yet to see any evidence that this equine/canine H3N8 virus can infect humans, there are a number of different H3N8 lineages out there, including the equine, canine, avian, and even a recently discovered Mammalian Adapted H3N8 In Seals.
And a related H3N8 virus is thought to have sparked the 1900 influenza pandemic, giving it a track record in humans, and is considered likely to return someday (see Are Influenza Pandemic Viruses Members Of An Exclusive Club?).
Daesub Song, Bokyu Kang, Chulseung Lee, Kwonil Jung, Gunwoo Ha, Dongseok Kang, Seongjun Park, Bongkyun Park, and Jinsik Oh
In South Korea, where avian influenza virus subtypes H3N2, H5N1, H6N1, and H9N2 circulate or have been detected, 3 genetically similar canine influenza virus (H3N2) strains of avian origin (A/canine/Korea/01/2007, A/canine/Korea/02/2007, and A/canine/Korea/03/2007) were isolated from dogs exhibiting severe respiratory disease.
In late 2012, in China: Avian-Origin Canine H3N2 Prevalence In Farmed Dogs, we saw a study that found more than 12% of farmed dogs tested in Guangdong province carried a strain of canine H3N2 similar to that seen in Korea.
During the 2009 H1N1 pandemic we saw reports of dogs infected, and in the middle of the last decade we saw several reports indicating that dogs were susceptible to the H5N1 bird flu virus (see Study: Dogs And H5N1).
Cats, too, were infected during the 2009 H1N1 pandemic (see Companion Animals And Novel H1N1 & EID Journal: Pandemic H1N1 Infection In Cats), and In 2011, it was announced that Korea’s canine H3N2 had jumped to cats (see Korea: Interspecies Transmission of Canine H3N2).
Previously we’d seen reports of cats infected with the H5N1 virus after consuming infected birds. The following comes from a World Health Organization GAR report from 2006.
28 February 2006
Several published studies have demonstrated H5N1 infection in large cats kept in captivity. In December 2003, two tigers and two leopards, fed on fresh chicken carcasses, died unexpectedly at a zoo in Thailand. Subsequent investigation identified H5N1 in tissue samples.
In February 2004, the virus was detected in a clouded leopard that died at a zoo near Bangkok. A white tiger died from infection with the virus at the same zoo in March 2004.
In October 2004, captive tigers fed on fresh chicken carcasses began dying in large numbers at a zoo in Thailand. Altogether 147 tigers out of 441 died of infection or were euthanized. Subsequent investigation determined that at least some tiger-to-tiger transmission of the virus occurred.
In 2006, Dr. C.A. Nidom demonstrated that of 500 cats he tested in and around Jakarta, 20% had antibodies for the bird flu virus. In 2007 the FAO warned that: Avian influenza in cats should be closely monitored, and in 2012 the OIE reported on Cats Infected With H5N1 in Israel, although so far no sustained virus transmission in cats or from cats to humans has been observed.
Contrary to the prevailing scientific opinion until the early 2000’s, dogs and cats are obviously both susceptible to a variety of influenza A viruses. All of which proves that you never know what you are apt to find until you actually start looking for it.
Which brings us to a pair of recently published studies. The first being on the virulence (or lack, thereof) of H5N1 in dogs and cats, and what that might portend as far as transmission is concerned.
Arch Virol. 2014 Nov 22.
Greater virulence of highly pathogenic H5N1 influenza virus in cats than in dogs.
Kim HM1, Park EH, Yum J, Kim HS, Seo SH.
Highly pathogenic H5N1 influenza virus continues to infect animals and humans. We compared the infectivity and pathogenesis of H5N1 virus in domestic cats and dogs to find out which animal is more susceptible to H5N1 influenza virus. When cats and dogs were infected with the H5N1 virus, cats suffered from severe outcomes including death, whereas dogs did not show any mortality.
Viruses were shed in the nose and rectum of cats and in the nose of dogs. Viruses were detected in brain, lung, kidney, intestine, liver, and serum in the infected cats, but only in the lung in the infected dogs. Genes encoding inflammatory cytokines and chemokines, Toll-like receptors, and apoptotic factors were more highly expressed in the lungs of cats than in those of dogs.
Our results suggest that the intensive monitoring of dogs is necessary to prevent human infection by H5N1 influenza virus, since infected dogs may not show clear clinical signs, in contrast to infected cats.
An interesting result, considering that last spring Korea Detected H5N8 Antibodies In asymptomatic Farm Dogs.
The second study, which appears in the December issue of the EID Journal, looks at the ability of cats to contract, and spread a contemporary strain of the equine/canine H3N8 virus.
Shuo Su1, Lifang Wang1, Xinliang Fu, Shuyi He, Malin Hong, Pei Zhou, Alexander Lai , Gregory Gray, and Shoujun Li
Interspecies transmission of equine influenza A(H3N8) virus has resulted in establishment of a canine influenza virus. To determine if something similar could happen with cats, we experimentally infected 14 cats with the equine influenza A(H3N8) virus. All showed clinical signs, shed virus, and transmitted the virus to a contact cohort.
That cats are susceptible to EIV by direct inoculation is not surprising because infection of cats with various influenza A viruses has been reported. Feline respiratory tract epithelial cells contain sialic acid α-2,3-galactose β-1,3-N-acetyl galactosamine (SA α2,3 gal) receptors for avian and equine influenza viruses and SA α2,6 gal receptors for mammalian influenza virus (13).
However, our finding of horizontal transmission of EIV among cats is significant. If transmission occurs outside the laboratory, and if the basic reproduction rate is higher than 1.0, then EIV could potentially establish itself and circulate in this new host species. Why it has not yet happened naturally, as it did for canine influenza virus (H3N8), remains to be determined. Possibilities include lower transmission efficiency, lower probability of horse–cat contact, less virus shedding in a laboratory, or feline behavior (less social contact than dogs).
These researchers repeated this experiment with an older strain of the equine H3N8 virus, and while some of the cats seroconverted, they all remained asymptomatic. Illustrating the variance of virulence one often finds between clades or strains of the same influenza A subtype.
In just over a decade we’ve gone from believing that dogs and cats aren’t really susceptible to flu to viewing them as Potential `Mixing Vessels’ For Influenza.
WASHINGTON, DC – June 19, 2014 – Equine influenza viruses from the early 2000s can easily infect the respiratory tracts of dogs, while those from the 1960s are only barely able to, according to research published ahead of print in the Journal of Virology. The research also suggests that canine and human influenza viruses can mix, and generate new influenza viruses.
Although it’s true that pigs and birds are considered far superior biological `flu factories’, any jump of a novel flu virus to a new species is viewed with concern, because it affords the virus new opportunities to acquire host adaptations – or reassort with other viruses – and thereby increases its chances of becoming a human health threat.
While the future role of dogs and cats in the evolution of influenza is subject to debate, for now, your pet is at far greater risk of catching the flu from you, than you are from it (see Companion Animals & Reverse Zoonosis).