Spontaneous cholecystopleural fistula leading to biliothorax and sepsis in a cat

Case summary A 13-year-old spayed female domestic shorthair cat presented with pleural effusion and suspected triaditis. Intake vitals and leukocytosis were consistent with a diagnosis of systemic inflammatory response syndrome. Biochemical analysis confirmed a pleural fluid-to-serum bile ratio consistent with a diagnosis of biliothorax. Abdominal ultrasound failed to identify a definitive gall bladder but noted a hypoechoic tubular structure ventral to the liver and contacting the diaphragm. Thoracic ultrasound identified a hyperechoic structure contacting the diaphragm at the same location. Thoracoabdominal CT scan identified a fluid-dense tubular structure extending from ventral to the liver, through a diaphragmatic defect and directly communicating with the pleural space, suspected to be an abnormal gall bladder. The cat was humanely euthanized, and post-mortem analysis confirmed a cholecystopleural fistula arising from the gall bladder with multifocal abscesses, mixed inflammatory hepatic infiltrates and small-cell gastrointestinal lymphoma. Culture of the abscess isolated Parabacteroides merdae, meeting the reported feline criteria for sepsis. Relevance and novel information To our knowledge, spontaneous cholecystopleural fistula formation leading to biliothorax and sepsis has not been previously reported in the cat. This case highlights a novel sequela of gall bladder disease in this species, and biliothorax should be a differential diagnosis for pleural effusion in cats with evidence of cholecystitis or triaditis.