Mar Vista Animal Medical Center

3850 Grand View Blvd.
Los Angeles, CA 90066




The thorax is another way of saying “chest cavity.” One’s thorax is bounded on the sides by the ribs and muscles between the ribs and by the diaphragm below. Inside the thorax are the heart and lungs as well as some rather large blood vessels and the esophagus (running down the throat and across the thorax to reach the stomach on the other side of the diaphragm.) The muscles between the ribs (called “intercostal” muscles) and the diaphragm, which is itself a muscle, enable the thorax to change its size so that air can enter and exit the lungs. The thorax is divided into right and left halves by a membrane called the “mediastinum” which anchors the heart, esophagus, and blood vessels in a central position so they do not flop around as one moves. The membranes that line the mediastinum, inside of the ribs and muscles, and the lungs are called the “pleurae” (singular would be “pleura”). The pleura on the rib side is called the “parietal pleura” and the pleura on the lung side is called the "visceral pleura." The space in between is the "pleural space" and this is the space that is of concern to the condition known as “pyothorax.”


It is easiest to show the pleural space on a human graphic 


The pleural space normally contains less than a teaspoon of fluid for lubrication. In disease, the pleural can fill with any of the following fluids:

  • Air (“Pneumothorax”)
  • Blood (“Hemothorax”)
  • Lymph (“Chylothorax”)
  • Clear fluid (“Hydrothorax”)
  • Pus (“Pyothorax”)



Normally, the pleural space is very small as most of the chest cavity is taken up by the expanding lungs. A small amount of fluid is present for lubrication purposes. If fluid actually fills the pleural space, however, it becomes difficult for the lung to expand as there simply is not enough room in the chest for lots of fluid and normal lung volume. The patient cannot breathe and begins to use abdominal muscle to expand the chest. Breaths become labored, rapid, and shallow. Because the fluid is so inflammatory, there is usually a fever. The pet is listless, will not eat, and has effortful rapid breathing (the abdominal muscles are recruited to assist respiration).



In cats, usually the problem starts as a bite wound. In dogs, usually the problem starts as a piece of plant material (such as a “foxtail”) which is inhaled and works its way into the pleural space. Bacteria are carried to the pleural space as the bite wound abscess ruptures inward (or the original bite may have penetrated the chest) or as the foxtail travels into the pleural space. Inflammation results from the infection. Extra fluid is produced and overwhelms the normal fluid drainage conduits so that fluid production and drainage are no longer in balance. Soon the entire pleural space is filled with pus. The patient has difficulty breathing and the toxic, inflammatory material in the fluid creates fever, listlessness, and death is expected without treatment.


In the dog, the definitive cause of the pyothorax is found in only 4 - 14% of cases
while in cats the cause is found in 40 - 67% of cases. 


A normal chest cavity

This cat has an effusion in its pleural cavity. In this case the fluid is chyle, not pus, but pyothorax might look the same.

Diagnosis is confirmed when the chest is tapped with a needle and pus is obtained from inside the chest cavity. The pus is generally cultured to get a better handle on what organisms are present and thus which antibiotics are likely to be the most helpful.


Pyothorax is one of those conditions where prognosis can be reasonably good (assuming the patient is not too far gone at the time of presentation) as long as aggressive treatment is pursued. If one tries to go with inexpensive alternatives to proper treatment, a poor outcome is likely. So what is needed to “do this disease right.”

  • In-dwelling chest tubes are placed (usually one on each side of the chest). The management of these tubes generally demands 24 hour hospitalization
  • Fluid is flushed into the tubes and the pus is drained out several times daily.
  • Intravenous antibiotic administration geared towards both anaerobic and aerobic growing bacterial is required (or antibiotics are selected based on culture of the pus).

Less likely to be adequate would be:

  • Periodically tapping the chest with a needle to withdraw the pus. This is will probably not provide enough drainage.
  • Oral antibiotics, at least not at first.
  • Leaving a chest tube unattended overnight. If the tube unclamps, the patient’s chest cavity becomes open to the outside atmosphere. When this occurs, the patient cannot breathe. In hospitals where that do not offer overnight care, the tube can be clamped and wrapped but an industrious animal can potentially chew through these safeguards and the owner should understand this risk if the patient is left alone overnight.
  • Antibiotics alone without some kind of chest drainage will not be effective.

In some cases, surgical exploration is recommended so that the chest cavity can be explored for foreign bodies (like foxtails, sticks, or other material broken off inside the chest cavity causing infection to persist). Surgery is generally reserved for patients for whom medical management (chest tubes and antibiotics) has failed.

After the fluid has stopped re-forming and the patient has a good appetite, the tubes are withdrawn and the patient can go home. Antibiotics are given for many weeks to ensure that this very deep infection is cleared. Recurrence is possible if the infection is not completely cleared and is felt to be more likely if the anaerobic organisms: Actinomyces or Nocardia have been cultured. In a review of several pyothorax studies, recurrence rates ranged from 0% to 14% in cats and 11.6% in dogs. Positive outcome/recovery was achieved in 58%-100% of the patients depending on the study.

The overall message is that pyothorax is a life-threatening condition and death can be expected without treatment. Proper treatment requires critical care, which may be expensive, but has a fair chance for success. Surviving patients will need several weeks of medication at home. Some patients will require surgery.

Page last updated: 1/29/2012
Page last reviewed 8/28/2014