Felis ISSN 2398-2950

Pleural effusion

Contributor(s): Philip K Nicholls, Elizabeth Rozanski, Penny Watson, Kim Willoughby


  • Serious condition requiring radical and often long-term therapy to achieve control.
  • Pleural effusion: accumulation of fluid in the pleural space.
  • Four standard effusion types recognized (in addition to blood):
    • True transudates: associated with hypoproteinemia.
    • Modified transudates: found with right congestive heart failure, obstruction to lymphatic drainage by tissue adhesions in pleural space, lung lobe torsion, neoplasms and abdominal contents herniating through diaphragmatic rupture.
    • Exudates: associated with active inflammatory processes. May be septic (pyothorax) or non-septic.
    • Chyle: lymphatic fluid leaking from thoracic duct (or other lymphatics in chest), into pleural space (differentiate true chylous effusion from pseudochylous effusion, containing accumulated lectins from cells in exudates). Associated with congestive heart failure, neoplasia, trauma, lymphangiectasia, congenital lesions, venous thrombosis or ideopathic.
      Pseudochylous effusions are very rare.
    • Blood: neoplasia, trauma, coaguloopathy, lung lobe torsion.
  • Signs: respiratory distress, irrespective of nature of effusion. Dyspnea is usually inspiratory.
  • Diagnosis: signs, radiography, ultrasonography, echocardiography, cytology and bacterial culture of thoracic fluid.
  • Treatment: treat underlying condition, thoracocentesis, placement of chest drain, surgery to break down adhesions and open pockets of effusion.
  • Prognosis: depends on nature and cause of effusion, generally very guarded.



Transudates and modified transudates


  • Bacterial invasion by aerobes and anaerobes, eg StreptococcusStaphylococcusProteus and Pasturella spp and Escherichia coli, following puncture of chest wall, eg by a bite wound, migrating foreign body   →   inflammation Pyothorax.
  • Feline infectious peritonitis Feline infectious peritonitis.
  • Chest trauma   →   lung contusion or lung lobe torsion.
  • Extension of mediastinal diseases.
  • Neoplasia.


  • Severe intrapulmonary hemorrhage.
  • Trauma.
  • Blood vessel erosion (neoplasia), rupture of blood vessels in pleural adhesions.
  • Coagulopathies:


  • Chylothorax
  • Congestive heart failure Heart: congestive heart failure.
  • Neoplasia Lymphoma, often mediastinal lymphoma.
  • May not be identified.
  • Thoracic duct erosion by inflammatory processes and neoplasia.
  • Heart worm disease.
  • Coughing and vomiting.


  • Fluid accumulation in pleural space   →   lung lobes unable to expand   →   dyspnea.
  • True transudates associated with hypoproteinemia. Plasma albumin <2 g/dl may   →   plasma leakage, levels <1.5 g/dl usually required for significant development of pleural effusion. Protein-losing glomerulonephropathy, amyloidosis, protein-losing enteropathies and reduced albumin production by liver are most common causes of hypoproteinemia. Reduction in plasma oncotic pressure   →   effusion.
  • True transudates are rarely recognized as they soon become modified, ie have increased protein content and added cellular elements. Modified transudates are yellow to pink in color and more opaque than true transudates.
  • Modified transudates are found with right-sided congestive heart failure, obstruction to lymphatic drainage by tissue adhesions in pleural space, neoplasms and abdominal contents herniating through the diaphragm.
  • Exudates are associated with active inflammatory processes in pleural space, infections with bacteria penetrating wounds of chest wall, microerophilic agents such as Nocardia spp and Actinomyces spp, neoplasia, thoracic trauma and mediastinal disease.
  • Hemothorax is found with coagulopathies, trauma and blood vessel erosion associated with neoplasms or destruction of pleural adhesions by inflammation or neoplastia.
  • Chyle leaks from lymphatics in chest into pleural space. The etiology is usually unknown, but heart failure, lymphoma, heart worm infection and lymphangiectasia are sometimes implicated.


  • Acute.
  • Insidious and progressive.


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Further Reading


Refereed papers

  • Recent references from PubMed and VetMedResource.
  • Ludwig L L (2000) Surgical emergencies of the respiratory system. Vet Clin North Am Small Anim Pract 30 (3), 531-553 PubMed.
  • Padrid P (2000) Canine and feline pleural disease. Vet Clin North Am Small Anim Pract 30 (6), 1295-1307 PubMed.
  • Reichle J K & Wisner E R (2000) Non-cardiac thoracic ultrasound in 75 feline and canine patients. Vet Radiol Ultrasound 41 (2), 154-162 PubMed.
  • Wright K N, Gompf R E, DeNovo R C Jr. (1999) Peritoneal effusion in cats - 65 cases (1981-1997). JAVMA 214 (3), 375-381 PubMed.
  • Tidwell A S (1998) Ultrasonography of the thorax (excluding the heart). Vet Clin North Am Small Anim Pract 28 (4), 993-1015 PubMed.
  • Frendin J & Obel N (1997) Catheter drainage of pleural fluid collections and pneumothorax. JSAP 38 (6), 237-242 PubMed.
  • Davies C & Forrester S D (1996) Pleural effusion in cats - 82 cases (1987 to 1995). JSAP 37 (5), 217-224 PubMed.