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Reverse zoonosis of influenza A virus in companion animals; What it means to veterinary and human medicine

      Background: The 2009-2010 Influenza season globally was dominated by novel pH1N1 influenza. Quite uniquely, pH1N1 was detected in companion animals; initially ferrets and subsequently in greater than 10 domestic cats during this season causing severe disease and death. Affected animals originated in households with influenza-like illness in humans. These newly reported atypical influenza infections in companion animals of a presumed human adapted influenza virus were hypothesized to represent spill over infections following very high infection rates in humans during that flu season. The 2010-2011 season was extremely mild for humans and 1/3 of humans cases were Influenza B, remaining cases were pH1N1 and H2N3. Given the markedly reduced level of circulating influenza virus in 2011, we would anticipate a nominal seroprevalence in influenza A in companion animals during this time.
      Methods: To test that hypothesis, we conducted seroprevalence study testing of over 2000 dog and cat sera collected during late summer and fall of 2010 and spring 2011 for Influenza A antibodies utilizing a commercially available ELISA. Sera originated from animals from the Midwest United States.
      Results: We detected Influenza A antibodies in 9.0% of canine samples and 5.2% of feline samples. Moreover, February 2011, we and others diagnosed pH1N1 in 5 cats with clinical disease; two cases were fatal. Three cases were severe; two cats survived, one was euthanized. Moreover, in three households with a single feline mortality, in-contact cats had clinical disease suggestive of cat-to-cat transmission.
      Conclusion: Companion animal species living in very close proximity to humans and other domestic and wild animal species provide an exquisite environment for viral recombination, evolution and emergence. Every interspecies transmission event increases the probability of a high impact evolutionary event and contributes to a broadening diversity of influenza A subtypes and dynamics of viral transmission globally. Halting transmission events is the primary solution to the escalating risk of evolution of highly pathogenic emergent influenza A viruses and thus immense morbidity and mortality in multiple species on a global scale. Our current study to molecularly characterize viruses circulating in companion animals is the first step to monitor and ultimately control transmission.