Presumed monoclonal gammopathy and parasitized immune cells in peritoneal fluid of a dog with canine leishmaniasis.

Canine leishmaniasis (CanL) is a systemic parasitic disease characterized by a wide spectrum of clinical and clinicopathological manifestations, often associated with chronic immune stimulation and marked alterations in serum protein profiles. This report describes an uncommon combination of clinicopathological findings in a dog naturally infected with Leishmania infantum, including a presumed monoclonal gammopathy and the presence of parasitized immune cells in peritoneal fluid. Cytological examination of the ascitic fluid revealed a highly cellular inflammatory cell profile, composed predominantly of neutrophils, macrophages and lymphocytes, with amastigote forms present both intra- and extracellularly. Serum protein electrophoresis demonstrated marked hypoalbuminaemia and a well-defined peak in the gamma-globulin fraction, consistent with a presumed monoclonal gammopathy. Although monoclonal patterns are typically associated with plasma cell neoplasms, similar findings may occur in infectious diseases under sustained immune stimulation. This case highlights the immunopathological complexity of advanced CanL and emphasizes the diagnostic value of combining effusion cytology and serum protein profiling in the recognition of atypical and severe disease presentations.