Lungs naturally do not contain fluid it only has air sacs. The presence of Fluid and pus in the lungs sacs are the major cause of Pneumonia. If you notice in the X-ray that your dog has fluid in the lungs, then this is the sign that your dog is suffering from pneumonia. If a pup has fluid in the lungs, he will develop the below signs and symptoms. Coughing. Difficulty breathing. Wheezing. Runny nose Nasal whistling Irregular breathing Open mouth breathing. Restlessness