Bronchitis

• If the cough is ending in swallowing, gagging or choking, the dog is coughing up mucus and then swallowing it. The mucus production increases when a dog has CB. When suffering from bronchitis, the excessive and thick mucus may be edematous, which means that there is an accumulation of serous (resembling serum) and it can span the space of the dog's airway or come together as a mucus plug.
  
  

Respiratory Infection

• When a dog releases substantial mucus from its mouth, this indicates a respiratory infection or other health problems. Mucus is produced in reaction to an infection or virus. Sometimes the condition will clear up on its own in a week or two, but thick mucus can be particularly troubling for a little dog that has small airways. Some dogs are plagued with thick mucus because they suffer from chronic bronchial infections or allergies.
  
  

Kennel Cough

• Kennel cough, so named because dogs in kennels often contract this condition, can result in thick mucus and a hacking cough, although not all dogs that have kennel cough develop excessive mucus.
  
  

Allergies

• Severe allergies can cause thick mucus. Giving your dog cod liver oil may help boost your dog's immunity and make it less susceptible to allergies. Vitamin C may also help it fend off allergies. You can also give your dog an expectorant, which relieves the build up of mucus. The herb's fennel and anise seeds also reduce mucus build-up.
  
  

Cold

• Cold like symptoms in a dog, including sneezing, may mean that it has inflammation or infection of the nose, which is called rhinitis -- or sinusitis, if it involves sinuses. This can result in a thick mucus nasal discharge that may be green or yellow in color.
  
  

Secondary Infection

• If you have your dog treated and it is still coughing, it may have some kind of secondary bacterial infection.