cds46, a highly variable carp edema virus gene

Abstract

Carp edema virus disease (CEVD) is a severe viral illness that causes substantial economic losses in wild and farmed common carp and koi. It is caused by carp edema virus (CEV), a member of the Poxviridae family, whose genetic diversity and genome evolution are poorly understood. Based on a genomic fragment of the 4a gene, two genogroups, genogroup I (gI) and genogroup II (gII), have been identified in samples of different origins. By analysing a series of recent samples, we highlight here a new genomic region of interest that varies by substitutions, indels and putative recombinations. In the Japanese reference sequence, this region encodes an ORF, cds46, whose function is unknown despite weak homologies with genes of some members of the Iridoviridae. Surprisingly, AlphaFold protein structure prediction analyses link cds46-encoded ORF with cellular endonucleases, providing insights into its possible origin. The ORF is absent in all gI haplotypes and in some gII haplotypes. Apart from the absence of cds46, gI haplotypes show an insertion of 121 bp with no homology to any viral sequence. When present, cds46 showed two groups of alleles differentiated by substitutions. The analysis of the cds46 locus showed that some samples from fish batches contained mixes of different haplotypes, irrespective of their origin (i.e. France, Japan or Israel). In a 2023 sample, we also found a virus carrying a gII-like atypical 4a allele first identified in France in 2015, indicating the limited but persistent spread of this virus in the country. The cds46 locus is a new target that may be useful for identifying and tracking CEV haplotypes.