Abstract
ObjectivesFeline infectious peritonitis (FIP) emerges when feline coronaviruses (FCoVs) mutate within their host to a highly virulent biotype and the immune response is not able to control the infection. FCoV spike (S) gene mutations are considered to contribute to the change in virulence by enabling FCoV infection of and replication in macrophages. This study investigated the presence of FCoV with and without S gene mutations in cats with FIP using two different real-time RT-PCRs on different samples obtained under clinical conditions.MethodsFine-needle aspirates (FNAs) and incisional biopsies (IBs) of popliteal and mesenteric lymph nodes, liver, spleen, omentum and kidneys (each n = 20), EDTA blood (n = 13), buffy coat smears (n = 13), serum (n = 11), effusion (n = 14), cerebrospinal fluid (n = 16), aqueous humour (n = 20) and peritoneal lavage (n = 6) were obtained from 20 cats with FIP diagnosed by immunohistochemistry. Samples were examined by RT-PCR targeting the FCoV 7b gene, detecting all FCoV, and S gene mutation RT-PCR targeting mutations in nucleotides 23531 and 23537. The prevalence of FCoV detected in each sample type was calculated.ResultsIn 20/20 cats, FCoV with S gene mutations was present in at least one sample, but there was variation in which sample was positive. FCoV with mutations in the S gene were most frequently found in effusion (64%, 95% confidence interval [CI] 39–89), followed by spleen, omentum and kidney IBs (50%, 95% CI 28–72), mesenteric lymph node IBs and FNAs (45%, 95% CI 23–67), and FNAs of spleen and liver and liver IBs (40%, 95% CI 19–62).Conclusions and relevanceIn these 20 cats with FIP, FCoVs with S gene mutations were found in every cat in at least one tissue or fluid sample. This highlights the association between mutated S gene and systemic FCoV spread. Examining a combination of different samples increased the probability of finding FCoV with the mutated S gene.
Highlights
Feline infectious peritonitis (FIP) is one of the most important infectious diseases in cats, but its pathophysiology is still not fully understood
Serum samples and buffy coat smears showed no positive results for S gene mutation RT-PCR in any cats
All samples positive in S gene mutation RT-PCRs had the mutation in nucleotide 23531; in none of the examined samples was a mutation in nucleotide 23537 present
Summary
Feline infectious peritonitis (FIP) is one of the most important infectious diseases in cats, but its pathophysiology is still not fully understood. Following mutation, increased virulence of FCoV is the result of a change in viral cell tropism from enterocytes to macrophages and efficient replication within these cells.[11,12] As the FCoV spike (S) protein plays a key role in viral cell entry,[13] studies have investigated the mutations in the S gene as possible contributing reasons for the change in virulence.[14,15,16] One study identified mutations in close proximity in the S gene’s nucleotides 23531 and 23537, causing two different amino acid substitutions in the S protein.[5] In contrast to other S gene mutations,[14] mutations in nucleotides 23531 and 23537 were identified in 96% of FCoVs isolated from cats with FIP in that study. These mutations were not identified in faecal samples of clinically healthy control cats in that study; no organ samples from these control cats were analysed.[5]
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