Causes
Many possible causes: heart failure, FIP in cats, tumours, infections (pyothorax), trauma, clotting disorders.
Diagnosis
Chest X-ray or ultrasound. Analysis of the fluid drawn off points to the cause.
Treatment
EMERGENCY: drainage of the fluid (thoracocentesis) so the animal can breathe, oxygen, and treatment of the underlying cause.
Prevention
Difficult to prevent. Keep heart disease under control and treat infections promptly.

