Pleural Effusion in Cats

pleural effusion in cats

Pleural effusion is an abnormal build up of fluid up in the pleural cavity, the thin fluid-filled space that lies between the lungs and the chest wall.

Two membranes line the thorax and lungs, and the space between naturally has a small amount of fluid which helps to lubricate the lungs as he breathes in and out. Small blood vessels in the pleural lining produce this fluid, blood and lymph nodes take away excess fluid to keep it all in balance.

Pleural effusion is a symptom, with an underlying disorder causing this fluid to build up. The buildup of excessive fluid leads to difficulty breathing, due to the inability of the lungs to fully expand.

Most fluids can be classified as transudates or exudates. Transudate (protein poor/clear watery fluid) effusion or exudate (protein rich/thick fluid) effusion. The type of fluid can help your veterinarian determine the underlying cause.

Cats of any age of sex can be affected by pleural effusion. It can occur on just one side (unilateral) or both (bilateral).

What causes pleural effusion in cats?

There are a lot of causes of pleural effusion in cats, transudate or exudate. How the fluid came to be in the pleural space is tied in with this.

Transudate (hydrothorax):

Caused by either an increase in intravascular hydrostatic pressure or a decrease in intravascular oncotic pressure in the vessels of the pleural membranes, causing fluid to leak out of them. The protein particles are too large to cross the vessel, resulting in only the fluid exiting (hence low protein). Common causes include:

  • Congestive heart failure
  • Liver failure
  • Hypoalbuminemia can result in decreased oncotic pressure, this is usually due to nephrotic syndrome
  • Fluid overload (in cats most commonly during fluid therapy)
  • Lung lobe torsion
  • Pulmonary embolism
  • Diaphragmatic hernia

Exudate effusion is usually due to inflammation or increased vascular permeability, they can be one of several types, including:


  • Cancer (most commonly lymphoma)
  • Fungal infection
  • Viral infection such as feline infectious peritonitis (wet form)
  • Lung lobe torsion
  • Pulmonary embolism
  • Diaphragmatic hernia
  • Pancreatitis
  • Drug reaction


  • Pneumonia
  • Penetrating trauma
  • Foreign body inhalation
  • Ruptured esophagus
  • Abscess
  • Bacterial infection

Chyle (chylothorax):

Chyle is a milky white fatty fluid which flows within the lymphatic vessels.

  • Neoplasia
  • Pulmonary hypertension
  • Heart disease
  • Heartworm disease
  • Lung lobe torsion
  • Obstruction of the thoracic duct
  • Tear or leak in the thoracic duct

Blood (hemothorax):

  • Trauma
  • Torn blood vessel
  • Coagulopathy (most commonly due to ingestion of rat poison)

This list is by no means complete, there are many other possible causes of pleural effusion in cats.

What are the symptoms of pleural effusion in cats?

As the excess fluid accumulates in the pleural cavity the lungs become compressed, making it much harder for your cat to breathe. This is the most common symptom of pleural effusion in cats.

Other symptoms include:

  • Difficulty breathing (dyspnea)
  • Coughing
  • Increased respiration rate
  • Lethargy
  • Blue-tinged gums, lips, and tongue due to cyanosis
  • Loss of appetite
  • Open-mouthed breathing
  • Unusual sitting position, hunched over, unable to lie down or stay comfortable

How is pleural effusion diagnosed?

Your veterinarian will perform a complete physical examination of your cat and obtain a medical history from you. Tests he will wish to perform can include:

  • Focused Assessment Sonography for Trauma (FAST procedure): This is a chest ultrasound to evaluate the emergency patient for the presence of fluid within the pleural cavity. This non-invasive and quick test can help your veterinarian to quickly evaluate your cat. More information can be found on the benefits of the FAST procedure here.
  • Xrays: Not all veterinary practices have an ultrasound machine to perform a quick assessment, in which case an xray may be used to evaluate the chest.
  • Thoracentesis: If the FAST ultrasound does reveal pleural effusion, a thoracentesis can be carried out. This procedure removes excess fluid from the pleural space using a needle. This not only relieves pressure on your cat’s lungs, but also provides your veterinarian with pleural fluid samples.

Once the cat has been stabilised and his breathing managed, further tests may need to be carried out to determine what caused the pleural effusion.

  • Ultrasound or xray of the heart, lungs (for obstructions, structural abnormalities and tumours).
  • CT scan may be recommended to look for penetrating injury, lung lobe torsion or foreign object.
  • Abdominal ultrasound may also be performed to look for a possible cause of pleural effusion such as pancreatitis or abdominal neoplasia.
  • Pleural fluid analysis to determine the cause of the pleural effusion. Different tests will be ordered depending on the type of fluid removed. Tests may include cell count, packed cell volume, culture, and smear, glucose, fluid pH, and cytology.
  • Biochemical profile, complete blood count, and urinalysis to check organ function, protein in the urine, red blood cell count (anemia) and look for signs of possible infection.
  • FIV and FeLV antigen tests.
  • Echocardiogram (ultrasound of the heart) to evaluate the heart.
  • Electrocardiogram to look at the rhythm of the heart.

How is pleural effusion treated?

Medical treatment is urgent in any cat experiencing breathing difficulty. Cats are exceptional at hiding symptoms of sickness and may be quite far along by the time pet owners notice there is a problem.

Treatment is twofold. Remove the excess fluid from the pleural cavity to enable your cat to breathe properly and address the underlying cause.

  • Your cat will need to be stabilised before treatment commences. This is typically with oxygen therapy to relieve dyspnea. Some cats resist wearing a face mask so your veterinarian may set up an oxygen tent for him.
  • Fluid is removed by thoracentesis which allows the lungs to expand. A needle or drainage tube will be inserted through the skin and into the pleural cavity to remove the fluid. A chest drain may remain in place for several days to help drain away excess fluid.

Treatment for the underlying condition varies depending on the cause. But may include:

  • Antibiotics to treat bacterial infections.
  • Furosemide to promote diuresis in cats with chronic heart failure.
  • Surgery for neoplasia, trauma, foreign body inhalation, obstruction,  hernia, lung lobe torsion.
  • Supportive care and anti-virals for FIP.
  • Nutritional support, pain medications, supportive care, antibiotics, and antiemetics for pancreatitis.
  • Medications to treat heartworm or fungal infections.
  • Induce vomiting, activated charcoal, stomach pump, vitamin K and possibly blood transfusion for rat poisoning.

What is the prognosis for cats with pleural effusion?

Prognosis depends on the underlying cause of pleural effusion but it is usually guarded. Particularly in cases of neoplasia.