A chylothorax (or chyle leak) is a type of pleural effusion. It results from lymphatic fluid (chyle) accumulating in the pleural cavity due to either disruption or obstruction of the thoracic duct.
In patients on a normal diet, this effusion can be identified by its turbid, milky white appearance, since chyle contains high levels of triglycercides. It is important to distinguish chylothorax from pseudochylothorax (pleural effusions high in cholesterol), which has a similar appearance, but is caused by more chronic inflammatory processes, and has a different treatment. 
The condition is rare but serious, and appears in all mammals. It results from leakage of lymph fluid from the thoracic duct (or one of its tributaries). This can result from direct laceration (e.g., from surgery) or from nontraumatic causes. The most common nontraumatic cause is malignancy, especially lymphoma. Less common is left-heart failure, infections, and developmental abnormalities (Down syndrome, Noonan syndrome, etc.). 
Since the mechanism behind chylothorax is not well understood, treatment options are limited. Drainage of the fluid out of the pleural space is essential to obviate damage to organs, especially the inhibition of lung function by the counter pressure of the chyle. Another treatment option is pneumoperitoneal shunting (creating a communication channel between pleural space and peritoneal cavity). By this surgical technique loss of essential triglycerides that escape the thoracic duct can be prevented. Omitting fat (in particular FFA) from the diet is essential. Either surgical or chemical pleurodesis are options: the leaking of lymphatic fluids is stopped by irritating the lungs and chest wall, resulting in swelling and closure of the pleural space. The medication octreotide has been shown to be beneficial and in some cases will stop the chylothorax after a few weeks.
In animals, the most effective form of treatment until recently has been surgical ligation of the thoracic duct combined with partial pericardectomy. There is at least one case report (in a cat) of clinical response to treatment with rutin.