A cat can breathe through its mouth, but most of the time, open-mouth breathing indicates an underlying health issue. Though exercise and heat both cause panting, most cats that exhibit this behavior need medical treatment from a veterinarian.
A cat breathes through its open mouth when breathing through the nose does not provide a sufficient amount of oxygen. Respiratory issues including nasal blockage and asthma prevent the lungs from receiving adequate amounts of air. Cardiovascular and hematological issues cause a dangerous decrease in the amount of oxygen transported by the blood. Neurological damage and environmental factors also cause panting. Open-mouth breathing can be an early indicator of significant health problems.