Mouth breathing in cats is not normal and may be a sign of a serious medical condition. Cats typically breathe through their nose with a smooth and even breath. Air travels through the windpipe and into the lungs, where oxygen enters the blood stream. Carbon dioxide waste leaves the blood stream and is exhaled through the nose.
Exercise and Stress
After strenuous exercise, cats pant or breathe through their mouth. This is normal and allows your cat to cool off through the evaporation of moisture through the tongue, mouth and lungs. Some cats breath with an open mouth if they feel stressed or frightened. If breathing appears labored or rapid, or your cat appears anxious, consult a veterinarian as your cat may be experiencing heatstroke.
Feline asthma is caused by the inflammation of the passageways in the lungs. During an asthma attack, a cat may breathe through his mouth, cough, wheeze and extend his neck and hunch his shoulders. His lips and tongue may turn blue as he is not getting enough oxygen. Asthma may be triggered by allergens, stress, obesity, parasites or heart conditions.
Pleural effusion is a condition in which fluid accumulates around the lungs. The fluid compresses the lungs, preventing them from expanding and allowing sufficient air to enter. Symptoms of the condition include open-mouth, labored and rapid breathing, lethargy, loss of appetite and blue skin and mucous membranes. Pleural effusion has many possible causes including infections in the lung, cancer, abnormal lymph drainage and blood clots in the lungs.
Upper Respiratory Infections
In addition to open-mouth breathing, cats with upper respiratory infections have symptoms such as sneezing, congestion, cough, fever, runny nose and depression. Most upper respiratory infections are caused by viruses, such as the highly contagious feline calicivirus and feline herpes viruses. Bacterial infections including chlamydia and Bordetella also cause upper respiratory infections. These infections are often highly contagious; cats should be isolated as a part of their medical treatment.