GS-441524 Treatment in a Cat With Feline Infectious Peritonitis Virus and Pyogranulomatous Transverse Colon Lesion

Feline infectious peritonitis (FIP) is a fatal disease caused by feline coronavirus (FCoV), manifesting as effusive (wet) or non-effusive (dry) forms. Granulomatous lesions in the gastrointestinal tract, particularly the colon, are rare and pose diagnostic and therapeutic challenges. A 7-year-old castrated male domestic shorthair cat (4.1 kg) presented with anorexia and vomiting. Blood tests showed an albumin:globulin ratio of 0.5, a mild elevation in aspartate aminotransferase, mild leukocytosis, and a severe elevation of feline serum amyloid A. The abdominal ultrasound and computed tomography imaging showed a mass in the transverse colon and surrounding mesenteric lymph nodes. The mass was observed to be relatively well vascularized, with areas of low-density uneven necrosis. Histological examination revealed severe pyogranulomatous inflammation with macrophages, neutrophils, and lymphocytes predominantly present in both the affected colon and lymphoid tissues. Immunohistochemistry for feline infectious peritonitis (FIP) virus antigen of the lesion showed a strong positive result, confirming limited and localized lesion induction by FIP virus infection. The cat received GS-441524 for 12 weeks, resulting in clinical improvement, lesion resolution, and normalized lab results. RT-PCR and antibody tests were negative post-treatment. This case report describes a rare presentation of FIP virus infection, characterized by localized lesions confined to a specific segment of the colon. In this case, treatment with the nucleoside analog GS-441524 was very effective in improving the localized lesion and demonstrated excellent efficacy in clearing the FIP virus.