Chylothorax is a build-up of a milky fluid known as chyle in the pleural cavity. Chyle is a fatty lymphatic fluid which drains from the intestine. Normally, chyle is channelled through the tiny ducts from one lymph node to another and collects in the thoracic duct near the heart. The thoracic duct empties into the vena cava, the large blood-carrying vein that enters the heart. 
The pleurae are two membranes which surround the lungs. The parietal pleura is the outer membrane, which is attached to the chest wall, the inner one is known as the visceral pleura and is attached to the lung. In between these two membranes is the pleural cavity. The pleural cavity contains a small amount of pleural fluid which lubricates the pleural surfaces.
Some diseases can cause a build-up of fluid which collects in the pleural space resulting in compression of the lungs. This build-up of fluid is known as pleural effusion. Different types of fluid can accumulate in the pleural space. Blood (hemothorax), pus (pyothorax), serous fluid (hydrothorax) and chyle (chylothorax).
- Weight loss
- Shortness of breath due to pleural effusion
- Blue-tinged mucous membranes
- Loss of appetite
Your veterinarian will perform a complete physical examination of your cat and obtain a medical history. He will also wish to find out the cause of chylothorax if possible.
- X-ray – Fluid in the pleural space will be visible
- A needle aspirate some of the fluid, chyle is milky white in appearance
- Ultrasound – This to evaluate the heart and lungs
- Remove fluid and repeat x-rays to look for any masses or evidence of heart disease
It is also important that the veterinarian determine the underlying cause, which can include additional diagnostics depending on the index of suspicion.
Treatment of the underlying condition as well as medical therapy.
- Thoracentesis – Remove the fluid from the pleural cavity to relieve pressure.
- Rutin – A drug from the benzopyrone family that is readily available at health food stores which can increase removal of lymph from the tissues.  The exact mechanism is unknown at this stage but it is proving to be of benefit to cats with chylothorax.
- Surgery of the thoracic duct (thoracic duct ligation) with pericardectomy (removal of a portion or all of the pericardium).
 Cat Health Encyclopedia – Edited by Lowell Ackermann.
 The Feline Patient – Gary D. Norsworthy, Mitchell A. Crystal, Sharon K. Fooshee, Larry P. Tilley.