Cornell University, the Wisconsin Veterinary Diagnostic Laboratory and the National Veterinary Services Laboratory has indicated that the large, ongoing canine flu outbreak in the midwest US is being caused by an H3N2 influenza strain, not the expected H3N8 canine flu strain. Molecularly, the strain is closely related to H3N2 strains that are circulating in dogs in China and South Korea. H3N2 canine flu emerged in that region in the mid 2000s and is widely circulating in some areas. This raises a few questions: 1) How did it get here? The importation issue comes up again, but potential sources need to be investigated. 2) Will the canine H3N8 vaccine provide any protection? I suspect no. 3) Does this change the response? Not really. Identifying potentially infected dogs and keeping them away from other dogs is still a key control measure. Vaccination is unlikely to be effective but still isn’t a bad idea, in case it provides some limited protection and/or if there is also H3N8 circulating in the region. 4) Does this explain why the outbreak is so big and seems to be expanding? Maybe. I’ve been a bit surprised at the scope of this outbreak given what we know about H3N8 canine flu. This strain might be more transmissible, shed for longer periods of time or have other differences that make it spread more easily in the dog population. The Asian H3N2 strain has been shown to be highly transmissible and able to cause severe disease (Kang et al Vet Res 2013). 5) Who else can get infected? Asian H3N2 has been shown to be able to infect cats (Song et al, J Gen Virol 2011). There is currently no evidence of human risk, as far as I know, but this needs to be investigated since flu viruses are unpredictable. Given the large number of infected dogs, it should be possible to determine whether there are some associated human cases. The risk is pretty low but it’s wise to look.