Successful treatment of feline infectious peritonitis-associated myocarditis in a cat.

Case summary: A 4-year-old, indoor-only male castrated domestic shorthair cat was referred after identification of cardiomegaly and pleural effusion by the primary veterinarian. Echocardiography revealed generalised left ventricular hypertrophy with left atrial enlargement, pleural effusion and a small amount of pericardial effusion. Therefore, congestive heart failure was suspected, and the patient was treated with furosemide (2 mg/kg PO q12h). Subsequent investigations included pleural fluid analysis, plasma cardiac troponin I and serum alpha-1 acid glycoprotein (AGP) measurements. Plasma cardiac troponin concentration raised suspicion of myocarditis at 1.31 ng/ml (reference interval [RI] <0.05), while pleural fluid analysis revealed it to be a highly proteinaceous (84.3 g/l) exudate with mixed neutrophilic and macrophagic inflammation. A quantitative RT-PCR for feline coronavirus performed on the same fluid was positive in conjunction with markedly elevated serum AGP levels (2709 µg/ml; RI <500); therefore, a diagnosis of effusive feline infectious peritonitis (FIP) was made. The patient was treated with GS-441524 (10 mg/kg PO q12h) for 12 weeks, which resulted in resolution of the clinical signs, normalisation of AGP and fully reversed cardiac remodelling. Relevance and novel information: This case report summarises an unusual case of FIP as the putative cause of myocarditis in a cat. Although acute myocarditis has been well described in people as a cardiovascular complication of systemic coronavirus disease (COVID-19), this is the first suspected ante-mortem diagnosis in a cat with FIP. Furthermore, once the FIP was successfully treated, the cardiac abnormalities entirely resolved. This case also highlights the importance of pleural fluid analysis in cats with effusion, even when heart failure is suspected as the cause.