A dog coughing and spitting out foaming mucus may be caused by improper diet, gastrointestinal irritation, food poisoning, respiratory infection, etc. The dog needs to be observed for other complications. If it is caused by improper diet, the dog needs to be deprived of food and water, and some probiotics should be fed to help digestion. If it is accompanied by other symptoms, such as poor energy, loss of appetite, diarrhea, etc., it needs to be sent to the hospital for examination and treatment in time.