Diagnostic algorithm for Feline Infectious Peritonitis: Integrating clinical manifestations, imaging findings, and immunopathological interpretation

Feline infectious peritonitis (FIP) is an immune-mediated disease caused by mutated strains of feline enteric coronavirus (FCoV). Differential diagnosis of the dry (granulomatous) and wet (effusive) forms of FIP, as well as their differentiation from other diseases with similar clinical presentations, including lymphoma, toxoplasmosis, bacterial peritonitis, and congestive heart failure, remains a major challenge in veterinary practice. The aim of this study was to identify differential diagnostic criteria for the dry (granulomatous) and wet (effusive) forms of FIP by integrating clinical signs, diagnostic imaging findings, and immunological interpretation of the observed changes. The study was conducted between 2023 and 2024 at the Department of Veterinary Epidemiology, Polissia National University, and at the Vet Centre Veterinary Clinic (Irpin, Kyiv Region, Ukraine). Ultrasonographic examinations were performed using a LOGIQ P6 PRO ultrasound system, whereas radiographic examinations were carried out using a Univet digital radiography unit. The diagnosis of FIP was confirmed using a comprehensive diagnostic approach that included hematological and biochemical analyses, polymerase chain reaction (PCR) testing, and serological assays. However, the primary focus of this study was the evaluation of imaging findings and their immunological interpretation. All procedures were performed with the written informed consent of the owners and in accordance with accepted ethical standards for animal research. The dry form of FIP was characterized by the presence of granulomatous lesions in parenchymal organs (liver, kidneys, and spleen) together with lymphadenopathy, in the absence of clinically significant effusions. In contrast, the wet form was characterized by anechoic abdominal effusion, lymphadenopathy, and radiographic evidence of pleural and/or peritoneal effusion. Notably, ultrasonographic findings typical of the dry form were also observed in a substantial proportion of cats with the wet form, suggesting a mixed immune response characterized by localized cell-mediated immune activation superimposed on a systemic humoral response. The study presents diagnostic flowcharts that facilitate the systematic differentiation between the dry and wet forms of FIP and their distinction from other diseases with similar clinical and imaging manifestations. In most cases, no single laboratory or imaging technique provides sufficient diagnostic accuracy for differentiating the clinical forms of FIP. The proposed diagnostic algorithm represents an effective tool for distinguishing between the dry and wet forms of feline infectious peritonitis and for differentiating FIP from other disorders presenting with similar clinical features.