Feline infectious peritonitis (FIP) is a disease caused by a genetic mutation in the feline enteric coronavirus, resulting in the transformation from an avirulent to a virulent form (3,4,6,7,9,10,12-15). FIP primarily affects young cats aged between 6 months and 2 years, with male cats being more susceptible and having a higher mortality rate (4,6,10,11,14,15). FIP is traditionally classified into two clinical forms: wet (effusive) and dry (non-effusive) (5,9,10,12,13). The wet form is characterized by inflammation with effusions in the abdominal or pleural cavity and/or pericardium, whereas the dry form is characterized by granulomatous changes in various organs, including the eyes and central nervous system. Mixed forms, combining wet and dry types, are occasionally observed (1,9,10,17). The clinical signs are generally nonspecific and include recurrent fever, anorexia, chronic weight loss, lethargy, jaundice, and lymph node enlargement (3,6,9,11,12,14,15). The wet form may present with abdominal distension, respiratory distress, and muffled heart sounds, whereas the dry form may present with various neurological symptoms and ocular abnormalities (9,10,12,13). Common laboratory abnormalities associated with FIP include neutrophilia, lymphopenia, anemia, thrombocytopenia, hyperbilirubinemia, hyperglobulinemia, and hypoalbuminemia, often with a low albumin-to-globulin (A:G) ratio of less than 0.6 (6,9,11-15,17). However, there are no specific clinical signs or laboratory findings for a definitive diagnosis of FIP (6,10,15). Therefore, the diagnosis of FIP is challenging and usually relies on a combination of clinical suspicion based on signalment, history, and clinical signs, along with routine clinicopathological results (1,6,11,12,14). Immunohistochemistry of histopathological tissue lesions or effusions containing infected macrophages is considered the gold standard for a definitive diagnosis (1,3,5,6,9,11-14). Reaching a definitive ante-mortem diagnosis is particularly difficult in cats without body cavity effusion (3-6). However, recent reports in cats have shown ante-mortem diagnosis using reverse transcriptase polymerase chain reaction (RT-PCR) testing of abdominal or pleural effusion, cerebrospinal fluid, and affected organs such as the mesenteric lymph nodes, liver, and spleen (3-7,14,16). In this report, we describe a rare case of atypical mixed-type FIP diagnosed ante-mortem using real-time RT-PCR of pericardial effusion with ultrasonography in a cat.

A 1-year-old castrated male Korean Shorthair cat presented with dyspnea, anorexia, lethargy, and seizures. On physical examination, the patient showed salivation and right forelimb hemiparesis, as well as a rapid respiratory rate of 60 breaths per minute. No abnormalities were detected during auscultation, and the body temperature was within the normal range (38.4°C). Complete blood count revealed no abnormalities. Serum biochemistry findings revealed elevated levels of bilirubin (3.5 mg/dL; reference range, 0.1-0.4 mg/dL), aspartate aminotransferase (AST) (105 U/L; reference range, 18-51 U/L), globulin (6.3 g/dL; reference range, 2.7-5.2 g/dL), glucose (231 mg/dL; reference range, 71-148 mg/dL), ammonia (89 μg/dL; reference range, 23-78 μg/dL), and total protein (9.0 g/dL; reference range, 5.7-7.8 g/dL). The A:G ratio was 0.4. Levels of both N-terminal pro-B-type natriuretic peptide (NT-proBNP) (1,392 pmol/L; reference range, 0-100 pmol/L) and feline serum amyloid A (fSAA) (93.7 μg/mL; reference range, 0-5 μg/mL) were significantly elevated. Kit tests for feline leukemia virus and feline immunodeficiency virus yielded negative results. Thoracic radiography revealed mild cardiomegaly (vertebral heart score 8.4 v, upper reference value 8.0 v), diffuse increased interstitial pulmonary infiltration, and a triangular soft tissue opacity in the pleura of the right caudal lung field (Fig. 1). No abnormalities were observed on the abdominal radiographs. Based on the patient’s history, laboratory, and radiographic findings, feline cardiomyopathy with pulmonary edema and pleural effusion, and feline infectious diseases, such as FIP, were considered in the differential diagnosis. During hospitalization, the patient received oxygen and diuretic treatment, resulting in a decrease in the respiratory rate (40 breaths/min). Approximately 6 h later, repeat thoracic radiography showed no significant changes compared with the previous radiograph, including heart size and overall pulmonary infiltration. To further investigate the cause of mild cardiomegaly, soft tissue opacity in the right caudal pleural space, and pulmonary infiltration, echocardiography, and lung and abdominal ultrasonography were performed. Echocardiography revealed normal wall thickness and lumen size of the heart; however, a mild amount of echogenic pericardial effusion was identified (Fig. 2). Lung ultrasonography revealed a triangular echogenic soft tissue mass with no blood signal in the right caudal pleural space, suggestive of granulomatous inflammation (Fig. 2). Abdominal ultrasonography showed the presence of the medullary rim sign in both kidneys, with faint effusion in the perirenal space, and no other significant abnormalities (Fig. 3). To analyze the pericardial effusion, ultrasound-guided pericardiocentesis was performed, yielding approximately 5 mL of viscous, cloudy, and straw-yellow pericardial fluid consistent with a purulent exudate (Fig. 4). The total nucleated cell count (TNCC) in the pericardial fluid was 30 × 10⁹/L, bilirubin level was 3 mg/dL, total protein level was 9.1 g/dL, and albumin level was 4.8 g/dL. Cytological examination of the pericardial effusion revealed marked neutrophils and macrophages, along with a small number of lymphocytes, monocytes, and red blood cells (Fig. 4); however, no bacteria were observed. Based on the ultrasonographic findings and analysis of pericardial effusion, FIP was strongly suspected, whereas heart disease was ruled out. To confirm the diagnosis, a PCR test for pericardial effusion was performed. IDEXX FIP Virus real-time RT-PCR confirmed the presence of the FIP virus biotype in the samples. However, the patient deteriorated and showed neurological symptoms and stupor. The patient was euthanized at the owner’s request. This case represents a rare occurrence of an atypical mixed form of FIP diagnosed ante-mortem through pericardial effusion analysis in a cat, which has seldom been reported.

Figure 1. Right lateral (A) and ventrodorsal (B) thoracic radiographs in this case show mild cardiomegaly (vertebral heart score 8.4 v, upper reference value 8.0 v), diffuse increased interstitial pulmonary infiltration, and a triangular soft tissue opacity in the pleura of the right caudal lung field (dotted circle).

Figure 2. The echocardiographic images (A, B) reveals normal wall thickness and lumen size of the heart, but a mild amount of echogenic pericardial effusion was identified (C). Lung ultrasonography reveals a triangular, echogenic soft tissue mass with no blood signal in the right caudal pleural space, suggestive of a granulomatous lesion (D). The asterisks indicate the normal hepatic parenchyma (C, D). FF, free fluid; LA, left atrium; LV, left ventricle; RA, right atrium; RV, right ventricle.

Figure 3. Abdominal ultrasonography shows the medullary rim sign in both kidneys (A,B) with faint effusion in the perirenal space (C), with no other significant abdominal abnormalities. FF, free fluid; LK, left kidney; RK, right kidney.

Figure 4. Approximately 5 mL of pericardial fluid was obtained through ultrasound-guided pericardiocentesis, showing viscous, cloudy, and straw-yellow colored fluid consistent with purulent exudate (A). Cytology of the pericardial effusion revealed marked neutrophils and macrophages, along with a few lymphocytes, monocytes, and red blood cells, but no bacteria were observed (B).

This case represents a rarely reported atypical mixed-type FIP case diagnosed ante-mortem by real-time RT-PCR testing of the pericardial fluid. Previously reported cases of mixed-type FIP mostly involved pleural or abdominal effusion, and a few cases involved pericardial effusion alone (2,10,12,17). Additionally, in three cases of FIP with concurrent pericardial effusion, severe cardiomegaly with a globoid shape on thoracic radiography provided sufficient suspicion of pericardial effusion with or without the presence of pleural effusion (1,2,8). In comparison to previous cases, the present case had difficulty in identifying pericardial effusion due to mild cardiomegaly on radiography; however, pericardial effusion was incidentally detected through echocardiographic examination. Moreover, unlike previous cases diagnosed by postmortem histopathology, this case was diagnosed ante-mortem using real-time RT-PCR via ultrasound-guided pericardiocentesis.

The patient presented with severe dyspnea as the main clinical symptom and was admitted urgently. Immediately after admission, it was challenging to differentiate between feline cardiomyopathy with both cardiogenic pulmonary edema and pleural effusion, and FIP based on radiographic and NT-proBNP results. Particularly, after oxygen supply and diuretic treatment, improvement in the patient’s respiratory rate further complicated the differentiation. However, considering the lack of improvement in heart size and pulmonary infiltration on repeat radiographs, along with the patient’s signalment; neurological symptoms; elevated levels of bilirubin, globulin, and fSAA; and low A:G ratio, infectious or inflammatory causes other than cardiac diseases were suspected. Therefore, echocardiography, lung, and abdominal ultrasonography were performed, resulting in a very high suspicion of FIP based on echogenic pericardial effusion, granulomatous lesions in the pleura, and bilateral renal medullary rim sign, along with the rule-out of heart diseases.

Due to the invasive nature of procedures for histopathological examination for the definitive diagnosis of FIP and the poor status of the patient, most FIP cases often rely on patient signalment, history, clinical signs, and laboratory results (6,11,12,14,15). However, this case demonstrates the crucial role of ultrasound examination as a fundamental test in diagnosing FIP suspected cats. This technique not only helps distinguish between the dry and wet types based on the presence of body cavity effusion or lymph node enlargement but also plays a decisive role in diagnosing FIP. Therefore, ultrasonography should be considered an essential test to be performed alongside radiographic examination in cats suspected of having FIP. Echocardiography can offer crucial information for patient evaluation or diagnosis, especially in FIP cases where the clinical signs and radiographic findings are similar to those of feline cardiomyopathy.

Although faint fluid was visible in the perirenal space, there was no effusion in the other body cavities, except for mild pericardial effusion. Considering the possibility that dry FIP progresses to a mixed FIP type with body cavity effusion at the terminal stage of the disease and deteriorates quickly, it is believed that this FIP case also progressed from a dry form to a mixed one and deteriorated rapidly with grave prognosis (17).

To date, no case reports have diagnosed FIP ante-mortem via real-time RT-PCR using pericardial effusion samples. Previous reports confirming FIP using RT-PCR mostly focused on samples obtained from pleural or abdominal effusions and mesenteric lymph nodes (3-5). The specificity and sensitivity of FIP diagnosis using real-time RT-PCR have been reported with varying values, but for body cavity effusion, they were 100% and 85-89%, respectively, with high reliability (1,3,4,16). Therefore, the RT-PCR results in this patient can be considered highly reliable, and considering the patient’s signalment, clinical signs, laboratory tests, and diagnostic imaging, this case was confirmed as a mixed-type FIP. Given the current lack of effective treatment options for FIP and the need for new drug development that complies with legal regulations, a definitive ante-mortem diagnosis of FIP can play a crucial role in the development of commercially available antiviral drugs in the future (11,14).

This report describes a rare case of mixed-type FIP diagnosed ante-mortem using real-time RT-PCR for pericardial effusion in a cat. Ultrasonography can be helpful for the definitive diagnosis of mild effusive or mixed-type FIP. Furthermore, if ultrasound-guided sampling with a sufficient viral load is performed, ante-mortem diagnosis of FIP through RT-PCR becomes possible. The availability of ante-mortem diagnosis through RT-PCR testing will significantly contribute to advancing FIP treatment and improving outcomes in affected cats.

No funding was received for this study.

The authors have no conflicting interests.