is common, most infected cats do not develop FIP.
For each category of FIPV or FECV infection, cats harboring viruses with or without mutations at the S1/S2 site were counted.
Sequencing of the S1/S2 site of 30 S sequences from FECV fecal samples revealed an extremely well-conserved motif at the amino acid level (Figure 1, panel A).
Examination results of the entire portion of spike sequenced in this study indicate that the conserved R-R-S/A-R-R-S motif in FECV is present within a region of the spike gene that shows a high degree of variability, in contrast to other neighboring regions that are more highly conserved (online Technical Appendix Figure 1).
To establish a consistent cause for a virulence shift in FCoV, specifically the predominant serotype I FCoV, we sequenced the entire genome of several FECV and FIPV specimens and then concentrated on the most conspicuous region of consistent difference by collecting and sequencing additional FECV and FIPV samples.
FECV strain RM and FECV strain UCD (FECV UU2) were propagated in specific pathogen-free cats.
To identify key differences between FIPV and FECV, we analyzed their genomes and proteomes; for each nucleotide or amino acid position, we determined the rate at which FIPVs differed from all FECVs at that position.
The latter group consisted of 10 asymptomatic healthy cats (FECV; test specimens: feces) and 13 dead cats with FIP confirmed through pathology (FIPV; test specimens: organs, ascites).
When compared by phylogenetic analysis, the nucleotide sequences of FIPV and FECV M genes distributed into paraphyletic patterns rather than in monophyletic clusters (Figure, panel A).
Thus, as we observed earlier for the 3c gene (10), M gene sequences generally clustered according to the cattery from where they originated, irrespective of their pathotype (e.g., FECV 586 and FIPVs 584 and 585 from cattery A; FECV 620 and FIPVs 615 and 622 from cattery G; FECV 10 and FIPV 8 from cattery F).
Together with in vitro studies describing the FIPV strains affinity for macrophages in contrast to FECV strains (15), the hypothesis was extended to propose that the enteric coronavirus (FECV) undergoes a mutational shift in the gastrointestinal system, thus allowing infection of macrophages, systemic dissemination, and fatal disease manifestation (12,13).
An alternative circulating virulent-avirulent FCoV hypothesis of viral pathogenesis suggests that distinctive benign and pathogenic strains of FECV circulate in a population, and that the disease will develop only in those persons infected by the virulent strains.