Amarilloviruses of Aquatic Animals

The family <i>Flaviviridae</i> has been expanded to include the highly divergent flavi-like viruses into three new families, <i>Flaviviridae</i>, <i>Pestiviridae</i>, and <i>Hepaciviridae</i>, in the order <i>Amarillovirales</i>. Classical flavivirids are small, enveloped viruses with positive-sense ssRNA genomes lacking a 3' poly(A) tail and ~9.0-13.0 kb in length, with a single open reading frame (ORF) encoding structural proteins at the N-terminus and nonstructural proteins at the C-terminus. Members infect a wide range of mammals, birds, and insects, and many are host-specific and pathogenic. Although the <i>RNA-directed RNA polymerase (RdRP)</i> gene sequences of the flavi-like viruses group phylogenetically with those of classical flavivirids, flavi-like viruses often encode larger polyproteins and possess substantially longer genomes of up to ~40 kb, and some have a 3' poly(A) tail. Their host range extends across the whole animal kingdom and angiosperm plants. This review describes the reported flavi-like viruses of aquatic animals, providing a meaningful update on all three new families in <i>Amarillovirales</i> that have been discovered using metagenomics in fish, crustaceans, mollusks, and echinoderms. These amarilloviruses include pathogenic viruses of aquatic animals, such as <i>Cyclopterus lumpus</i> virus (CLuV) detected in moribund lumpfish, and infectious precocity virus (IPV) found in iron prawn syndrome (IPS)-affected farmed giant freshwater prawns.