Until early in the last decade, cats and dogs were generally thought to be immune to influenza viruses. That perception changed in 2003 when avian HPAI H5N1 began to spill over into cats in Southeast Asia.
The following comes from a World Health Organization GAR report from 2006.
H5N1 avian influenza in domestic cats
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.
A year later (2004), equine H3N8 jumped from horses to dogs and began to spread globally (see EID Journal article Influenza A Virus (H3N8) in Dogs with Respiratory Disease, Florida).
Three years after that (2007), a second canine flu virus emerged - this time evolved from an avian H3N2 virus - and began to spread in Asia. (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.
In 2015, Canine H3N2 arrived in North America (see CDC’s Key Facts On The New H3N2 Canine Flu), and has now spread across most of the United States.While avian-origin canine H3N2 has not been shown to infect humans, numerous reports have come out of China and Korea suggesting it may be adapting to other hosts, and that it continues to reassort with other avian and human flu viruses. Including:
A Canine H3N2 Virus With PA Gene From Avian H9N2 - KoreaPerhaps most significantly, canine H3N2 was observed jumping from dogs to cats both in the field, and in the lab (see Interspecies Transmission Of Canine H3N2 In The Laboratory), leading researchers to write:
Canine H3N2 Reassortant With pH1N1 Matrix Gene
Virology J: Human-like H3N2 Influenza Viruses In Dogs - Guangxi, China
Conclusion Cats appear to be another susceptible host of CIV H3N2, whereas ferrets are not likely natural hosts. The molecular-based mechanism of interspecies and intraspecies transmission of CIV H3N2 should be further studied.Over the past five years, we've learned a good deal more about the susceptibility of dogs and cats to a variety of avian, swine, equine, and human flu strains. A few recent on-topic blogs include:
The last example above demonstrates just how quickly a novel (in this case, avian H7N2) virus can spread among a population of cats in an animal shelter, as more than 400 cats were affected.
Additionally, at least two people were infected, and 5 others showed low titers, suggestive of infection.All provide evidence that cats (and dogs) can serve as intermediate hosts, and transmit novel flu viruses on to other species, including humans.
The difference between dogs and cats is that so far, we've not seen any feline specific flu subtype become fixed in cats, while two canine specific subtypes (H3N2 and H3N8) continue to circulate in dogs.Today, we've a new study published in the journal PLoS, that finds that H3N2 canine flu viruses - when serially passaged in feline (CRFK) cells - acquired mutations that might allow the virus to become better adapted to cats.
The full open-access study is available at the link below, and I've only posted the highlights. When you return, I'll have a postscript.
Adaptation of H3N2 canine influenza virus to feline cell culture
Haruhiko Kamiki, Hiromichi Matsugo, Hiroho Ishida, Tomoya Kobayashi-Kitamura, Wataru Sekine, Akiko Takenaka-Uema, Shin Murakami, Taisuke Horimoto
H3N2 canine influenza viruses are prevalent in Asian and North American countries. During circulation of the viruses in dogs, these viruses are occasionally transmitted to cats. If this canine virus causes an epidemic in cats too, sporadic infections may occur in humans because of the close contact between these companion animals and humans, possibly triggering an emergence of mutant viruses with a pandemic potential.
In this study, we aimed to gain an insight into the mutations responsible for inter-species transmission of H3N2 virus from dogs to cats. We found that feline CRFK cell-adapted viruses acquired several mutations in multiple genome segments.
Among them, HA1-K299R, HA2-T107I, NA-L35R, and M2-W41C mutations individually increased virus growth in CRFK cells. With a combination of these mutations, virus growth further increased not only in CRFK cells but also in other feline fcwf-4 cells. Both HA1-K299R and HA2-T107I mutations increased thermal resistance of the viruses. In addition, HA2-T107I increased the pH requirement for membrane fusion.
These findings suggest that the mutations, especially the two HA mutations, identified in this study, might be responsible for adaptation of H3N2 canine influenza viruses in cats.(SNIP)
In conclusion, we identified multiple mutations required for adaptation of H3N2 CIV to feline cells. Although a study with experimental infection of feline cell-adapted viruses in cats is required, the mutations found in these viruses may contribute to understand the mechanism on cross-species transmission of CIVs and risk assessment for pandemic preparedness.(Continue . . . )
Although currently regarded as a relatively low-risk virus - two years ago 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.
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).As companion animals, both dogs and cats have unique access to humans, and so we watch the evolution of influenza viruses they can carry closely. For more on on the carriage of novel flu viruses by pets and companion animals, you may wish to revisit:
EID Journal: HPAI H5N6 In Domestic Cats - Korea, 2016
Influenza A(H6N1) In Dogs, Taiwan
Report: Skunks and Rabbits Can Catch And Shed Avian Flu