Molecular and Clinical Study of Feline Infectious Peritonitis Virus in Iran Showing a Paraphyletic Tree: Emphasizing the “Internal Mutation” Hypothesis

Background: Feline infectious peritonitis (FIP) is a severe and often fatal disease affecting feline species. Despite the high prevalence of feline coronavirus (FCoV) infections, the manifestation of FIP occurs in only a small percentage (1%-5%) of cases. The intricate aspects of FIP differential diagnosis persist, and a comprehensive understanding of the molecular mechanisms driving FIP pathogenesis remains elusive. Objectives: This study aims to thoroughly investigate the characteristics of Iranian feline infectious peritonitis viruses (FIPV), encompassing sequence analysis and detailed examination of laboratory and clinical findings. The primary objective is to unravel the hypothesized genesis of the FIP virus, with a specific focus on the membrane (M) gene level. Methods: Our methodology involved examining abdominal or thoracic fluids from 17 cats suspected of having FIP, utilizing biochemical tests, such as total serum protein, albumin to globulin (A/G) ratio, and the Rivalta test. A molecular approach utilizing reverse transcription-polymerase chain reaction (RT-PCR) based on the M gene was employed. Sequence analysis of five crucial residues in the M genes and subsequent construction of a phylogenetic tree using the five sequenced viruses further enriched our investigation. Results: The study confirmed FIP in 6 of 17 cats through the Rivalta test, guiding subsequent evaluations. Significant gender disparities in FIP occurrence were observed among young cats (9-30 months old), with males exhibiting a two-fold higher incidence than females. Affected cats within the 9-30 months age range consistently exhibited an albumin to globulin (A/G) ratio below 0.66 and total serum protein exceeding 0.43 g/dL. Cavity fluid cytology indicated non-degenerated macrophages and neutrophils against a basophilic background due to a high protein percentage, confirming FIP diagnosis. Importantly, sequence analysis of five M protein amino acid hotspots revealed negligible differences in nucleotide sequences between feline enteric coronavirus (FECoV) and FIPV, aligning with their biotypic patterns. Conclusion: The phylogenetic tree generated in this study displayed a paraphilic pattern, emphasizing the “internal mutation” hypothesis, suggesting that viral mutations occur within the cat’s body and no significant differences are observed in FECoV and FIPV-generating viruses. These results provide valuable insights into the discourse surrounding FIP pathogenesis, potentially guiding future diagnostic and therapeutic approaches.