A mysterious illness struck racing greyhounds at a Florida track in 2004. They were coughing and had runny noses and fevers. Many of them were dying [source: Iowa State]. The symptoms said "viral respiratory infection," but the source was unknown [source: Payungporn et al.].
Within eight months, the illness had spread to 14 greyhound tracks in six states [source: AVMA]. By mid-2005, it was in a dozen states and had moved beyond the track, showing up in shelters, boarding facilities and veterinary clinics, and affecting multiple breeds [source: AVMA]. By then, authorities had identified the pathogen: the H3N8 virus, a form of influenza type A.
Somehow, these dogs had come down with the flu.
It was the world's first known occurrence of canine influenza [source: AVMA]. More than a decade later, it's not exactly common, but it's around [source: Bowen]. A few Asian countries and most U.S. states have reported cases, with about 2,100 confirmed infections in America between January 2005 and March 2015 [sources: Landolt, VCA, Cornell]. The U.S. Department of Agriculture approved an H3N8 vaccine in 2009 [source: AVMA].
But then, in April 2015, a massive outbreak of dog flu in the U.S. Midwest gave authorities another surprise: These dogs had a different strain of the virus, one derived from avian flu that had never been seen outside of Asia [source: AVMA].
"As this  outbreak demonstrated, influenza is a very unpredictable virus," says Gabriele Landolt, D.V.M., an assistant professor at Colorado State University School of Veterinary Medicine. "Influenza A viruses can change quite easily."
The original, H3N8 canine influenza virus (CIV) is a mutation of the H3N8 equine influenza virus; it jumped from horses to dogs [sources: VCA, Iowa State]. The newer strain, H3N2, jumped from birds to dogs [source: Iowa State]. It was discovered in 2007 in South Korea and may prove more problematic than H3N8. "It appears that the H3N2 strain might spread more efficiently between dogs," says Landolt. It can also infect cats [source: Iowa State].
The two CIV strains are distinct, but you can't tell the difference without lab analysis. They're "the flu." And if you've ever had the flu, you know just what these dogs are going through.