| Feline infectious peritonitis(FIP)is a feline lethal disease caused by feline infectious peritonitis virus(FIPV).To elucidate the pathological changes of FIP,the genetic evolutionary pattern of the whole virus gene and the prevalence of feline coronavirus(FCoV)in the northern region,this study conducted a systematic pathological study of nine cats that died of FIP,genetic evolutionary analysis of the whole gene sequence of two FIPV strains,as well as a molecular epidemiological investigation.FIP pathology: this study was conducted by clinical history investigation,necropsy,histopathology and immunohistochemistry(IHC)to investigate the systemic pathology of nine confirmed FIP cases collected between 2016-2023.The results showed that all nine cases of sick cats presented with progressive mental depression,diarrhea and marked increase in abdominal girth before birth,and marked fluctuation in the abdominal cavity on palpation.Necropsy showed that the abdominal cavity of all nine cats was filled with plasma or plasma fibrinous exudate,and small gray-white nodules were scattered or diffusely attached to the peritoneal plasma surface.Histopathology showed foci of inflammatory cell infiltrates of variable size attached to the peritoneal surface,which consisted of lymphocytes,macrophages,plasma cells,and large amounts of fibrin.Necrotizing enterocolitis,interstitial pneumonia,interstitial pancreatitis,interstitial necrotizing hepatitis,mild acute inflammatory splenomegaly,and interstitial nephritis.IHC results showed strong positive signals for FIPV in the cytoplasm of macrophages in the intestine and various tissues and organs,as well as in the cytoplasm of pancreatic alveolar epithelial cells.The results indicated that all nine cases were mixed FIP and that the intestine and pancreas were important target organs for viral infection.FIPV whole genome sequence and genetic evolution analysis: the whole genome sequences of two FIPV strains were amplified by RT-PCR.Genetic evolution showed that the study virus IMAU No.11685 had the highest homology of 93.7% with the 2016 Daqing strain DQ(gene bank.KY292377.1)and No.12 had the highest homology of93.1% with the 2018 Wuhan strain QS(gene bank.MW030108),which were located within the same evolutionary branch and had the closest kinship,probably originating from a common ancestor.The S gene sequences of the two strains had lower homology of44.4%～89.2% with mutations at several loci,indicating rich genetic diversity.3c,7ab,and N gene sequences had higher homology with the reference strain,87.4%～96.8%,89.7%～96%,and 87.5%～94%,respectively,with mutations,deletions,and insertions at several loci,deletions and insertions.Molecular epidemiological investigation of FCoV: 543 cat anal swabs were randomly collected from Changchun,Beijing,Huhhot and Baotou cities during 2022-2023;swab genomes were extracted,nested PCR was applied with FCoV-specific primers,and three positive PCR products were randomly selected from each region for sequencing and genetic evolutionary analysis.The results showed that the total positive rate of FCoV was33.7%(183/543),with the highest positive rate of 70.7% in Changchun City area.infection with FCoV was highly significantly correlated with collection site(P<0.001)and breed(P<0.001);significantly correlated with age(P=0.002);and more significantly correlated with feeding status(P=0.046).There was no correlation with sex(P=0.429)and whether they were neutered(P=0.376).183 positive samples belonged to FCoV Ⅰ.In terms of affinity,they had the highest nucleotide and amino acid homology with the domestic 2020 Chengdu isolate(registration number: MW316843.1),with 93.5%～95.3% and95.7%～98.3% respectively and located in the same evolutionary branch.The S amino acid of Baotou 29 and 40 strains was mutated from asparagine(N)to aspartic acid(D)at position 224.This study elucidated the systemic pathological changes of natural cases of mixed FIP,analyzed the genetic evolution pattern of the whole genome sequence of two strains of the virus and investigated the epidemiological characteristics of FCoV in four regions of Changchun,Beijing,Huhhot and Baotou,which enriched the research data of FIP in China and laid the foundation for the diagnosis and in-depth study of FIP. |
