Pyothorax is accumulation of exudates in the pleural cavity, also known as empyema. It is characterized by accumulation of purulent fluid due to bacterial or fungal contamination. Clinical signs of pyothorax are not specific and commonly include dyspnea, tachypnea, anorexia, pyrexia, and exercise intolerance. Duration of the clinical signs varied from 1–300 days, with a median of 33 days in cats and 40 days in dogs. Disease severity is influenced by the chronicity of the infectious. In order to facilitate establishment of treatment guidelines, the American Thoracic Society divides pleural infections into three stages: an exudative stage, fibropurulent stage, and organizational stage characterized by formation of a pleural peel (scar tissue). Although numerous retrospective studies, case reports, have been published on the subject, no data exist on the actual incidence of pyothorax in dogs and cats. Furthermore, the actual route of pleural infection often remains unknown with identification of an underlying cause reported in only 2–22% of dogs and 35–67% of cats. Diagnosis is based on clinical signs, thoracic X-ray, US and CT-scan, fluid sampling, cytological evaluation and bacterial/fungal culture and sensitivity. Definitive diagnosis is based on cytology examination of pleural effusion or subsequent aerobic and anaerobic bacteria/fungal culture results.

The isolated bacteria vary and are influenced by the primary source of contamination. The etiology of pyothorax includes; penetrating wound, primary bacterial pneumonia that may progress to parapneumonic effusion; migrating foreign body, esophageal perforation and lung abscess. In cats bite wound were considered as the most common cause of pyothorax with pasteurella contamination. A study evaluating the risk factors of 80 cats suffering from pyothorax, cats were 3.8 times more to live in a house load, but only 15% (20/128) had signs of bites. Another study of 27 cats with pyothorax in Australia has revealed that 78% of the cases had mixed oropharyngeal isolation including Pasteurella, Salmonella, and Mycoplasma species in kittens. Fifty-six percent (56%) of the cats suffered from pneumonia indicating the parapneumonic spread may be more common in cats as the source of the pleural contamination. Esophageal rupture may occur due to foreign body, neoplasia by Spirocerca lupi. The incidence of inhaled foreign bodies seems to vary depending on geographical location, climate, and vegetation, and the type of activity in which the dog is involved; hunting, working dogs or just dogs living in the country side were grass own are common. In a study evaluating 182 dogs with documented grass awn inhalation, only in 3 dogs grass awn was confirmed. Grass awns migration may be the source of Nocardia and Actinomyces spp. that are usually in organic matter.

Primary antimicrobial therapy is based on cytological evaluation and later on culture and sensitivity. Cytologic findings did not match microbiological findings in 13 of 44 (30%) dogs. Empirical treatment with penicillin-based antibiotics combined with fluoroquinolones and sulfadiazine (TMS) have shown that results of antimicrobial susceptibility testing suggested empirical antimicrobial selection was associated with a 35% risk of inefficacy. Several bacteria may produce sulfur granules in the septic pleural effusion. A study of 46 dogs with pyothorax a filamentous, branching organism was detected cytologically in 18 dogs, 11 of which had sulfur granules. Bacteria isolated from these 18 dogs included Prevotella spp., Clostridium, Bacteroides spp., Corynebacterium spp., Staphylococcus spp., Enterobacter spp., Fusobacterium spp., Nocardia and Actinomyces spp. The 2 latter are gram-positive, creating endospores that became reactive in optimal environment. They are not always identified in normal stain as they are partially acid fast, therefore, may need special staining (e.g., Zen Nielsen) to identify them in a smear. When these bacteria are highly suspected a combination of clindamycin and TMS should administrate. CT-scan and surgical intervention may be needed as pulmonary abscess is suspected as source of infection. Intrapleural antimicrobial treatment is not recommended in human empyema. Only one limited study in veterinary medicine has shown shorter thoracic tube placement in dogs treated intrapleurally with antibiotics (ampicillin and/or metronidazole) (4.8 vs. 6.8 days). However, the sample size was small and did not reach significant statistical difference.