Pyothorax in Cats

What is pyothorax?

Pyothorax (pleural empyema) is the presence of pus in the pleural space of the chest cavity, which is the thin, fluid-filled space that lies between the lungs and the chest wall.

Usually, a small amount of fluid is located in the pleural space (5 ml or less), which allows for the smooth movement of the structures in the thorax during respiration. Small blood vessels in the pleural lining produce this fluid, blood and lymph nodes take away any excess to keep it in balance. Pyothorax is a septic form of pleural effusion.

The average age of cats who develop pyothorax is 4-6 years; however, it can occur in cats of any age. Multiple cat households have a greater incidence as well as entire male cats who roam outside, presumably because of the increased risk of bite wounds.


There are several possible ways cats can develop pyothorax. In many cases, the initial cause may remain undetermined.

  • Penetrating bite wounds to the thorax
  • Penetrating trauma to the thorax
  • Inhalation of a foreign object such as a grass awn
  • Bacteria ascending from the mouth into the pleural space
  • Extension from other sites such as lungs (pneumonia) or bronchi
  • Diffusion of bacterial infection via the blood from a distant source
  • Parasitic migration
  • Perioperative aspiration (aspiration of gastric contents during anesthesia)
  • Oropharyngeal aspiration from an upper respiratory infection
  • Tumours
  • Ruptured abscess
  • Lung lobe torsion

Any pathogenic organism can cause pyothorax including fungal, viral, bacterial or protozoal; however, a bacterial infection is by far the most common pathogen. The collection of pus is due to dead white blood cells and dead bacteria. Common bacteria involved in pyothorax include Pasteurella spp, Nocardia spp, Streptococcus spp, Clostridium spp, Proteus spp, Bacteroides spp and Mycoplasma spp. Less common causes include toxoplasma (protozoa), cryptococcosis and Blastomyces (fungal infections). Most cases of pyothorax are polymicrobial, which means there is more than one organism.

Clinical signs

As fluid builds up in the pleural cavity, the lungs become compressed, making it harder for your cat to breathe. Symptoms may progress slowly, over some time and can include:


The veterinarian will perform a complete physical examination of your cat and obtain a medical history from you. During the exam, the veterinarian will listen to the heart and lungs with a stethoscope which may reveal muffled heart and lung sounds.

It is crucial to stabilise the cat if they are having breathing difficulty before diagnostics.

Diagnostic workup:

  • Baseline tests: Biochemical profile, complete blood count, and urinalysis. CBC may reveal elevated white blood cell count and anemia. A biochemical profile is usually normal, although hypoalbuminemia may occur. Other findings may include protein in the urine, elevated liver enzymes, elevated urea, creatinine, electrolyte imbalances and increased bile acids.
  • Radiographs: Xrays of the chest to look for the presence of unilateral or bilateral pleural effusion, foreign objects or tumours.
  • CT scan: An advanced non-invasive medical imaging technique that uses a combination of x-ray imaging and a digital computer to produce detailed 3-dimensional images to look for penetrating injury, foreign object or lung lobe torsion.
  • Thoracocentesis (pleural tap): A procedure in which a needle is inserted into the pleural cavity to remove excess fluid.
  • Cytology: Microscopic analysis of the thoracic fluid removed from the pleural cavity during thoracocentesis. Fluid samples should be sent for aerobic and anaerobic bacteria culture and sensitivity.
  • Abdominal ultrasound: To look for other organ involvement, abscess, structural abnormalities or foreign body.


The goal of treatment is to relieve symptoms and treat the underlying cause. Antibiotic therapy will be necessary for up to six weeks. Most cats will require hospitalisation during treatment.

  • Respiratory support such as oxygen therapy, bronchodilators or nebulisers may be necessary. Cats are often very sick and need to be treated with great care. The cat will be placed in an oxygen tent to stabilise him.
  • Thoracic drainage tubes to remove fluids from the pleural cavity. Local anesthesia and IV sedation are administered to keep your cat comfortable during this procedure. Tubes will be flushed several times a day with 0.9% saline or lactated Ringers solution warmed to body temperature to wash the chest cavity (pleural lavage). Tubes will need to remain in place for between 4-6 days.
  • While the tubes are in place, the cat will wear an Elizabethan collar to prevent him from pulling them out.
  • Cytologic examination of the fluids removed which will be checked daily to look for signs of resolution.
  • Administration of broad-spectrum IV antibiotics until cytology results return, after which, your veterinarian will tailor antibiotics to the type of bacteria present.
  • Pain medication.
  • Supportive care as required which may include IV fluids to correct dehydration, increase blood volume and electrolyte imbalance.
  • Nutritional support if the cat is not eating.
  • Surgery may be necessary to remove foreign objects and masses, treat abscesses or lung lobe torsion and debride damaged tissue.
  • Exploratory surgery for cases unresponsive to antibiotics.

Home care

Long-term antibiotic therapy will be necessary, administer medications as prescribed by your veterinarian.

The veterinarian will perform follow-up radiographs and antibiotics will continue for at least a month after radiographs show all evidence of fluid/pus build-up has resolved.

Confine the cat in a cage or small room during recovery.


If the owner is willing to follow the veterinarian’s instructions the prognosis fair, this will include medicating for several weeks and regular follow-up appointments and chest radiographs.

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  • Julia Wilson, 'Cat World' Founder

    Julia Wilson is the founder of Cat-World, and has researched and written over 1,000 articles about cats. She is a cat expert with over 20 years of experience writing about a wide range of cat topics, with a special interest in cat health, welfare and preventative care. Julia lives in Sydney with her family, four cats and two dogs. Full author bio