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Pyothorax in Dogs and Cats
Revised: January 28, 2023
Published: June 05, 2005

First, an Anatomy Lesson

Pyothorax happens in an area of the chest called the pleural space. Thorax is another way of saying chest cavity. We know our chest cavities contain our heart and lungs and, if we think about it a little, there are some other important organs as well: the esophagus transverses the entire chest cavity to get to the stomach; there are some large blood vessels coming off the heart; the trachea (windpipe) connects to the lungs, etc. But all these important organs aren't just packed in there, they're organized.

A structure called the mediastinum separates the chest cavity into right and left (sort of the way your nasal septum divides your nasal cavity into right and left). Inside the mediastinum, the heart, large vessels, esophagus, and any other central organs all rest so that they don't go flopping around whenever you roll over or turn in a circle. The lungs fill in the right and left sides of the chest cavity but keep in mind they are not attached to the chest wall directly. They mostly fill the space but not completely.

The space around the lungs is the pleural space.

The canine pleural cavity and its contents. The pleural space is shown in yellow. Illustration by VIN.

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)

The Story of Pyothorax

Normally, the pleural space is small as most of the chest cavity is taken up by the expanding lungs. A small amount of fluid is there 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 in addition to air-filled lungs. The patient cannot breathe and begins to use abdominal muscles 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, meaning the abdominal muscles are recruited to assist respiration.

How Does Pus Get into the Pleural Space?

Cat bite wound. Photo by MarVistaVet

In cats, the problem usually starts as a bite wound. The bite may penetrate the chest cavity outright and deposit bacteria into the pleural space where abscessation occurs, or the bite may abscess externally but eventually rupture inward. In either case, the result is a bacterial infection in the pleural space and the formation of pus.

A foxtail that has been removed surgically. Photo by Dr. Jean Battig

In dogs, the problem usually starts as a piece of plant material, such as a foxtail, which is inhaled and works its way into the pleural space and deposits bacteria as it tracks along. The infection generates inflammation and pus. This situation tends to be more complicated than in cats as it involves an actual foreign body that must be removed to resolve the pyothorax (see later). 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.

Other causes of pyothorax include rupture of the esophagus, bacterial pneumonia leading to rupture of a lung abscess, or lung parasites.

A normal chest cavity. Photo by MarVistaVet

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


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

Pyothorax is one of those conditions where the prognosis can be reasonably good, assuming the patient is not too far gone when brought to the veterinarian, as long as aggressive treatment is pursued. If you try 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). This generally demands 24-hour hospitalization.
  • Fluid is flushed into the tubes and the pus is drained out several times daily.
  • Intravenous antibiotics geared towards both anaerobic and aerobic growing bacteria are required (or antibiotics are selected based on culturing the pus).
  • Ideally some kind of advanced imaging is taken to see if surgery is required, such as an abscessed lung lobe harboring an inhaled foxtail or a physical structure on the internal chest wall that harbors the infection such as a broken-off twig, etc. If a foreign body or abscessed internal structure is inside the chest, it has to be removed, which adds tremendous expense but may be the only way to resolve the pyothorax.

Less likely to be adequate would be:

  • Periodically tapping the chest with a needle to withdraw the pus. This will probably not provide enough drainage.

  • Oral antibiotics without beginning with intravenous antibiotics in the initial stages.

  • 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 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.

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. This problem is more likely if the anaerobic organisms such as Actinomyces or Nocardia have been cultured. In a review of several pyothorax studies, recurrence rates ranged from 0 - 14 percent in cats and 11.6 percent in dogs. Positive outcome/recovery was achieved in 58-100 percent of the patients depending on the study.

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 of success. Surviving patients will need several weeks of medication at home. Some patients will require surgery.

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