Canis ISSN: 2398-2942

Pleural: effusion

Contributor(s): Yvonne McGrotty, Elizabeth Rozanski


  • Accumulation of fluid within the pleural cavity.
  • Cause: fluid may be blood, chyle, exudate, transudate or modified transudate.
  • Signs: respiratory distress related to reduced lung volume (tachypnea, dyspnea).
  • Diagnosis: ultrasonography, radiography and thoracocentesis.
  • Treatment: thoracocentesis and treatment of underlying cause where possible.
  • Prognosis: guarded overall, uncommonly cured but may be successfully managed.




  • Accumulation of fluid within the pleural space by a variety of mechanisms → reduced lung volume → respiratory compromise.


  • Trauma, anticoagulant rodenticide Anticoagulant rodenticide poisoning or neoplasia bleeding into pleural cavity.
  • PCV of effusion is similar to that of blood and has similar white cell number.
  • Usually doesn't clot due to rapid loss of fibrin (in contrast to inadvertant aspiration from blood vessel or heart).


  • Thoracic duct rupture is currently considered a RARE cause of chylothorax Chylothorax.
  • True chylous effusion is usually caused by abnormal or obstructed lymph channels:
    • Lymphangectasia.
    • Idiopathic.
    • Inflammation or obstruction of flow, eg neoplasia, right-sided heart failure.


  • Introduction of bacteria into pleural space, eg bite wound, foreign body, esophageal perforation Esophagus: perforation , extension from pneumonic lung Lung: bacterial pneumonia.
  • Migration of WBCs → pleural cavity, ie pyothorax Pyothorax or inflammatory conditions affecting pleura, eg neoplasia, immune-mediated disease or chylothorax Chylothorax.


  • Due to imbalance of absorption and filtration of fluid across pleura.
  • May be due to:
    • Increased hydrostatic pressure.
    • Negative intra-pleural pressure, eg severe upper airway obstruction.
    • Reduction in resorption of fluid due to reduced lymphatic permeability or obstruction, eg immune-mediated disease, inflammatory or infectious conditions, eg uremia Uremia , pancreatitis Pancreatitis: acute , trauma, neoplasia, bacterial, viral and fungal infections.

Modified transudate

  • Long-standing transudates which have increased protein and cells secondary to pleural inflammation.


  • Acute to chronic.


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Further Reading


Refereed papers

  • Recent references from PubMed and VetMedResource.
  • Smeak D D, Stephen J, Birchard S J, McLoughlin M A et al (2001) Treatment of chronic pleural effusion with pleuroperitoneal shunts in dogs 14 cases (1985-1999). JAVMA 219 (11), 1590-1597 PubMed.
  • Clikenbeard K D (1992) Diagnostic cytology - carcinomas in pleural fluid. JAAHA 14, 187-195 AGRIS FAO.
  • Stowater J L & Lamb C L (1989) Ultrasonography of noncardiac thoracic disease in small animals. JAVMA 195 (4), 514-520 PubMed.
  • Forrester S D, Troy G C & Fossum T (1988) Pleural effusions - pathophysiology and diagnostic considerations. Comp Cont Ed Pract Vet 10 (2), 121-136 VetMedResource.
  • Myer W (1978) Radiography review - pleural effusion. J Am Vet Rad Soc 19 (3), 75-79 Wiley Online Library.

Other sources of information

  • Murphy K & Papasouliotis K (2011) Pleural effusions in dogs and cats 1. Diagnostic investigation. In Practice 33, 462-469.