Feline Pyothorax: Dispelled Notions

Ten retrospective case series of cats with pyothorax were reviewed and yielded new information on this important medical problem. Pyothorax was found to be a disease of young cats and had no breed or gender predisposition. Previously, the most common route of infection was believed to be a bite wound; however, these studies revealed that the most common pathogens were polymicrobial and were similar to the normal feline oropharyngeal flora. Fewer than 20% of cases were caused by pathogens other than normal oral flora. Kittens were most likely to have infections caused by unusual organisms. Aspiration of oral flora appears to be the most common and significant route of infection. Aspiration of normal oral flora and subsequent colonization of the lower respiratory tract and direct extension of the infection from the bronchi and lungs is the presumed pathogenesis. Pyothorax was found to be 3.8 times more likely in multicat households, although behavior studies do not support intercat aggression as the cause. A more plausible explanation is the greater risk for upper respiratory infections in multicat households. Clinical signs were related to systemic illness and were often nonspecific. Bradycardia was more common in cats with hypothermia, and cats with bradycardia had a worse prognosis. Useful diagnostic tests include complete blood count, serum biochemistry panel, diagnostic imaging, thoracocentesis, and analysis of pleural effusion. Neutrophilic leukocytosis with a left shift was the most common findings. Hypoalbuminemia, hypo- or hyperglycemia, hyponatremia, hypochloremia, and mild elevations of aspartate transaminase and bilirubin levels were most commonly found on serum chemistry panels. Thoracic radiography was more sensitive than ultrasonography in detecting small amounts of pleural fluid. Malodorous fluid from thoracocentesis was most commonly associated with anaerobic infection. Septic infections were turbid to opaque and flocculent. Results of fluidanalysis were consistent with those of an exudate, and cytologic examination, including Gram’s and acid-fast staining, were helpful in rapid identification of pathogens. Both aerobic and anaerobic cultures were recommended and provided useful information.

Feline pyothorax has had a poor prognosis until recently, and in many cases treatment was not attempted. Large retro- or prospective studies on treatment trials are not available; however, Barrs and Beatty report on their success in treating cats with pyothorax. Most nonsurviving cats die or are euthanized within 48 hours after presentation. However, patients that live beyond the first 48 hours can usually be successfully treated with aggressive medical management. Stabilizing the patient with oxygen, minimizing stress, and performing therapeutic thoracocentesis are considered critical. Closed-tube thoracostomy, twice-daily aspiration, and lavage and antimicrobial therapy resulted in resolution of most cases (95%). Lavage facilitates removal of exudates, prevents obstruction of the thoracostomy tube, and provides hydraulic debridement of the pleura (including breakdown of adhesions and dilution of bacteria and inflammatory mediators). Initial antimicrobial therapy is empirical and based on the results of cytologic evaluation with Gram’s stain and acid-fast stain. Initially, antibiotics against obligate and facultative anaerobes should be selected. These include penicillin combined with metronidazole or ticarcillin-clavulanate monotherapy; intravenous administration is necessary initially. Surgical intervention may be needed when a pulmonary or mediastinal abscess or extensive loculated effusion is seen on postdrainage radiographs. In addition, persistence of loculated or generalized effusion 3 to 7 days after thoracostomy drainage, development of pneumothorax, and drain obstruction are also indications for surgery. Surgery can be curative in many cases where medical management fails. Prevention of pyothorax may be possible by routine antimicrobial therapy in cats with viral upper respiratory tract infections and after dental procedures.

COMMENTARY: These 2 articles are good examples of the value of evidence-based medicine. For years, pyothorax has been widely believed to be the result of direct inoculation of the pleural cavity by bite wounds. However, close examination of the literature revealed that of the 128 cases described in the literature, only 20 cats had evidence of bite wounds. Fifty of the 128 cats were tested for FIV, a disease transmitted by bite wounds, yet only 3 of 50 cats were positive. This is compelling evidence that there is little association between FIV infection and pyothorax. These papers present strong evidence that the most likely source of infection was oropharyngeal bacteria. This led the authors to recommend using antimicrobial prophylaxis in cats with upper respiratory tract infection and after general anesthetic dental procedures. Over the years, successful treatment has increased from 60% to nearly 95%, most likely because of the availability of intensive care procedures. Currently, antimicrobial resistance is not a problem in management of feline pyothorax, but veterinarians will need to be vigilant about this development, particularly in light of the recommendation for prophylactic antibiotic therapy for viral upper respiratory infection.Feline pyothorax—New insights into an old problem: Part 1: Aetiopathogenesis and diagnostic investigation. Barrs VR, Beatty JA. VET J 179:163-170, 2009.Feline pyothorax—New insights into an old problem: Part 2: Treatment recommendations and prophylaxis. Barrs VR, Beatty JA. VET J 179:171-178, 2009.