Suspected Paramalignant Pleural Effusion as the Primary Presentation of Disseminated Cholangiocarcinoma in a Cat

A 12-year-old neutered male domestic shorthair cat presented with recurrent pleural effusion following repeated thoracentesis. Initial analysis of the pleural effusion did not reveal neoplastic cells. Thoracic imaging demonstrated pleural effusion without identifiable nodular lesions. However, abdominal computed tomography and histopathological examination of an ultrasound-guided percutaneous core liver biopsy identified a large hepatic mass, confirmed to be cholangiocarcinoma, with suspected metastases to the spleen. After exclusion of other potential causes, the effusion was suspected to be a paramalignant pleural effusion. Despite palliative management, the disease progressed, and the cat was euthanized 98 days after definitive diagnosis. Postmortem examination revealed widespread metastatic cholangiocarcinoma involving the lungs, spleen, and intercostal muscles. No gross mass lesions were identified in the pulmonary parenchyma. Histological results demonstrated neoplastic cell infiltration within both the pulmonary lymphatic and blood vessels, a finding consistent with lymphovascular spread, which can contribute to the development of pleural effusion. This case show that while the characteristics of neoplasia-associated pleural fluid may vary according to disease stage, cholangiocarcinoma-associated pleural effusion may occur in the absence of cytologic evidence of malignancy, pulmonary nodules on thoracic imaging, or typical hepatobiliary signs.