The American Society for the Prevention of Cruelty to Animals instructs owners not to worry if a dog is coughing up foamy white phlegm as this is a common sign of kennel cough. Kennel cough is described as a relatively benign canine equivalent of the common cold.
The ASPCA notes that if the white foam is the result of kennel cough, the dog has a persistent dry cough that is accompanied by a honking sound. Most dogs do not show any signs of kennel cough other than the respiratory symptoms of this bacterial and viral infection. The organization explains that while some dogs cough to produce the white foam, others may simply gag. Fever, while not likely, is normal.
WebVet also notes that white foam is the most iconic sign of kennel cough and isn't cause for alarm in most healthy dogs. The website explains that kennel cough is highly contagious in places like veterinary offices, dog kennels and dog parks due to its airborne nature. To prevent the spread of the illness, owners ideally keep their affected dogs at home until they are no longer exhibiting symptoms. WebVet also recommends using a harness rather than a leash until the coughing subsides.