In clinical practice we tend to become overwhelmed at the thought of the unknown. How many differentials are there truly for disease x, y, or z?

In my experience, veterinarians tend to become overwhelmed quickly with the thought of pleural effusion (abnormal fluid within the chest cavity).

In health, our small animal patients have a very small amount of fluid within body cavities. We cannot see this radiographically, and most novice users of the ultrasound machine would also likely miss this effusion. The main goal of this fluid; lubricate the surfaces of the organs and body walls like motor oil for your car engine. This allows the organs to glide over each other without friction, avoiding inflammation. That is in health.

In states of disease we see effusion develop which needs to be identified and characterized for both diagnosis and targeted treatment.

While this is a simplistic approach, or thought process, below I have listed common differentials and categories for pleural effusion. My hope is that this would be a great start, a great reference, for a clinician to have to better assess and characterize their patient’s effusion.

Feline Pleural Effusion (in order of statistical likelihood)
– Cardiomyopathy (heart rate changes? hypothermia?)
– Neoplasia (older patient?)
– Feline infectious peritonitis
– Pyothorax
– Chylothorax
– Other diseases

Canine Pleural Effusion
– Neoplasia
– Infection / Pyothorax
– Third spacing
– Chylothorax
– Anticoagulant rodenticide
NOTE: Cardiac disease is VERY unlikely to cause pleural effusion in dogs

Knowledge is power! (and safe travels!)
Garret Pachtinger, VMD, DACVECC
Board Certified Emergency and Critical Care Specialist
Co-Founder, VETgirl