Hydrothorax in Cats: What You Need to Know

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Hydrothorax means that the thoracic cavity contains an increased amount of fluid, which surrounds and limits the space for the thoracic organs, especially the lungs, with acute or gradually increasing shortness of breath as a result.

Liquid leakage to the thoracic cavity can occur in connection with, among other things, heart failure, thinning of the blood (for example in the event of a protein deficiency) or bleeding.

Depending on the type of fluid that has accumulated in the chest cavity, the condition can also be called hemothorax (blood in the chest cavity), chylothorax (the fluid is lymph) or pyothorax (where in the chest cavity).

If the liquid is clear and mostly consists of water, it is called transudated, while proteinaceous liquid is called exudated.


The causes of the different variants of hydrothorax are several. Hemothorax is most often seen in connection with trauma when bleeding has occurred in the thoracic organs or blood vessels in the thoracic cavity.

Pyothorax is associated with a purulent inflammation, for example as a result of external stab wounds. Pyothorax can develop slowly and it is not always possible to find the original cause. Chylothorax can occur in several different diseases, but the cause is often difficult to determine, so-called idiopathic chylothorax.

Exudate or transudate exits can occur when the pressure in the blood vessels is higher than outside so that fluid is forced out through the vessel wall, which can occur in, among other things, right-sided heart failure or rotation of a lung lobe.

A special type of modified transudate occurs in the wet form of FIP, feline infectious peritonitis.

Withdrawal of transudate can also be seen when the blood is thinner than normal, such as in protein deficiency due to disease of the intestines, kidneys or liver. Increased production of fluid of any kind can also be a sign of tumor disease in the chest cavity.


A cat with hydrothorax usually has one or more of the following symptoms:

  • Cough
  • Breathing difficulties: the cat breathes quickly and shallowly, breathes with abdominal pressure, takes special body positions to get air easier, breathes with open mouth
  • Bluish mucous membranes
  • Impaired condition
  • Reduced appetite
  • Lethargy

Depending on what caused the condition, other symptoms are also seen.

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