In dogs as in humans, influenza attacks the upper respiratory tract, is highly contagious and typically presents with a persistent cough. It spreads in respiratory secretions — the stuff that comes out in sneezes and coughs — and the severity of the illness can vary significantly. About 20 percent of infected dogs show no symptoms at all, while others are obviously sick [source: VCA].
All dogs, regardless of breed or age, are susceptible to the flu — the virus hasn't been around long enough for any natural (innate) immunity to develop, says Landolt. Even dogs who have had CIV can get it again, because the acquired immunity that comes with exposure to the virus doesn't last forever [source: South Loop Animal Hospital].
"The exact duration of protection is unknown," adds Landolt.
CIV transmission thrives on close contact [source: ASPCA]. This means it doesn't spread particularly well in the general population, but in areas packed with dogs, it's like wildfire [source: Iowa State]. At shelters, doggie day cares, boarding facilities, dog shows and dog parks, all it takes is one infected dog to cough on, sneeze on or touch noses with another dog, and you're looking at a possible outbreak. With no natural immunity in the population, pretty much every dog exposed to the virus contracts it [source: VCA].
There are two general types of flu sickness. Most dogs have the mild form. They display the hallmark cough and often have nasal discharge (a runny nose) and a low fever. They might be sneezing, have runny eyes and lose their appetites [source: ASPCA]. The nasal discharge might turn into a thick, yellow-green mucous if they've developed a secondary bacterial infection of the upper respiratory tract. Dogs with the mild form usually recover easily [source: Iowa State].
In the severe form of canine flu, the situation is more serious. Here, CIV infection is complicated by pneumonia. Dogs who develop this type of illness are typically ones with weaker immune systems due to medical conditions or very young or old age [sources: Lewis, Iowa State]. In severe flu, fevers are high, coughing is more intense and breathing might be labored and/or fast [source: Landolt]. They might be coughing up blood [source: PetMD]. Severe canine influenza requires hospitalization [source: PetMD].
Despite knowing what to look for, identifying dog flu at home is unlikely. Bordetella bronchiseptica, adenovirus type-2 and parainfluenza virus all can cause similar symptoms to CIV [source: Landolt]. Accurate diagnosis requires a veterinarian.