The Pneumocystis fungus tends to infect felines with immune mediated disease such as those infected with the feline leukemia virus. The fungal infection will cause the feline to develop tachycardia, dyspnea, and a dry cough, paired with symptoms of the primary contracted condition. This fungal infection of the lungs promotes similar symptoms mimicking other common feline respiratory diseases and, therefore, requires a veterinary diagnosis for proper treatment. 

Fungal infection of the lungs in cats, or pneumocystosis, is caused by the agent Pneumocystis. Found worldwide, this etiological organism can infect virtually any mammal as its primary habitat is in the lungs causing opportunistic pneumonia. Pneumocystosis is believed to be contracted through airborne transmission, as a suspected healthy feline will be reported ill after being housed with a known infected species of mammal. Upon inhalation of the Pneumocystis agent, the fungi colonize within the lower respiratory tract of the feline, clustering the alveolar spaces and eventually blocking the alveolar capillaries, which prevents the act of gaseous exchange. In general, pneumocystosis does not cause clinical disease in felines unless the feline’s immune system is compromised. 

As the fungal spores colonize in the lungs and block off the alveolar capillaries of the bronchioles, the feline’s ability to exchange oxygen and carbon dioxide will be limited. The inability to properly exchange gases provokes the feline to display orthopnea type behavior. Orthopnea is the act of increasing lung capacity by standing with the elbows away from the body, focusing on bringing air into the lungs. Inadequate blood oxygen levels will cause the condition known as cyanosis, or the blue tinge of the feline’s mucous membranes. Over time, the feline affected by pneumocystosis may develop symptoms including: 

Pneumocystosis in cats is caused by the inhalation of spores present in the environment or exhaled from an infected mammal species. Pneumocystosis often infects animals during warmer months of the year, as freezing temperatures isolate the fungus. Mammal to mammal contraction is usually the result of overcrowding or stress commonly found in kennel, boarding, or animal shelter facilities. Felines with immunosuppressive conditions are prone to contracting the pneumocystosis fungus, whereas other felines may be inflicted with the fungus but do not show clinical signs of disease. 

As pneumocystosis in cats mimics other common feline respiratory ailments, a differential diagnosis must be completed. Blood tests will reveal the presence of other respiratory diseases, such as distemper, pneumonia, and lung parasites, which may be present in combination with the Pneumocystis fungus. In order to pinpoint a fungal infection, the veterinarian may choose to run a radiograph on the cat’s thoracic area with or without fluorescent dye to view the affected the lower respiratory tract. The alveolar capillaries will appear swollen and glow on the imagery, indicating an abnormality. In order to determine the nature of this found abnormality, the vet will be required to conduct a cytology on a sample which would be collected using fine-needle aspiration. In order to collect a sample from inside the cat’s lung, the feline will need to be placed under anesthesia and a small, flexible camera will be used to guide the vet to the abnormality. 

If the feline is experiencing severe trouble breathing, the initial step in treating pneumocystosis is stabilizing the cat through oxygen therapy. The feline may receive supplemented oxygen through the use of a mask, oxygen tube or oxygen chamber. Once the feline is stable, the veterinarian will select an appropriate antifungal medication, such as fluconazole or itraconazole. 

The treatment time and recovery for a feline diagnosed with a fungal lung infection is often lengthy. To ensure all colonized fungal spores can been eliminated, medications will be administered for several weeks to a month. During the recovery time, the feline will be restricted from physical activity and confined to the home. Pet owners will be advised to provide an air filter or purifier for the feline, as the clean air will aid in the cat’s recovery.