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A CLEARLY defined optimal strategy for the management of pyothorax in dogs, a recurrent and potentially fatal disease, remains elusive. Not only is it an uncommon diagnosis; it is also likely to have a multifactorial aetiology that is often difficult to determine with certainty in an individual dog without an invasive procedure and, even then, the aetiology often remains a mystery. So while there is an easily accessible veterinary literature on pyothorax dating back to the 1960s, the majority, if not all, of these publications are case reports, case series and retrospective reviews, which are compromised to different degrees by varying case presentation, diagnostic techniques and treatment strategies.
Pyothorax is defined by the presence of a septic neutrophilic exudate within the pleural cavity, and diagnosis is based on cytological examination and/or culture of pleural fluid or histopathology and/or culture of the pleura. It commonly presents as dyspnoea (with or without cough), lethargy and pyrexia. Mixed bacterial infections are common. In a study of bacteria isolated from pleural fluid from 51 dogs, obligate anaerobes were isolated from 60 per cent of samples, and a mixture of obligate anaerobes and facultative anaerobes from 44 per cent ( Walker and others 2000 ). The most common anaerobes isolated were Peptostreptococcus anaerobius , Bacteroides species and Fusobacterium species, and aerobes isolated were Actinomyces species, Pasteurella species, Escherichia coli and Streptococcus canis . Therefore, empirical antibiotics chosen for canine pyothorax should have activity against anaerobic and facultative bacteria, be distributed into the pleural space and have low toxicity. Walker and others (2000) , recommend the beta lactams ceftizoxime, clindamycin and metronidazole. Cytological and histopathological descriptions of pleural samples often report 'filamentous bacteria'. In dogs these could represent Gram-positive Actinomyces species and Nocardia species, or Gram-negative Bacteroides species.
The route of entry of bacteria into the pleural space often remains undefined. Reported modes of entry include: inhalation with a foreign body ( Frendin 1997 , Robertson and others 1983 , Demetriou and others 2002 , Rooney and Monnet 2002 ); escape from a pulmonary abscess ( Demetriou and others 2002 ); oesophageal tear associated with bone foreign body or parasites ( Hamir 1986 , Trinterud and others 2014 ); penetrating wound or thoracic trauma ( Piek and Robben 2000 , Doyle...