Phylogenomic characterization of two novel members of the genus Megalocytivirus from archived ornamental fish samples

Abstract

The genus Megalocytivirus is the most recently described member of the family Iridoviridae; as such, little is known about the genetic diversity of this genus of globally emerging viral fish pathogens. We sequenced the genomes of 2 megalocytiviruses (MCVs) isolated from epizootics involving South American cichlids (oscar Astronotus ocellatus and keyhole cichlid Cleithracara maronii) and three spot gourami Trichopodus trichopterus sourced through the ornamental fish trade during the early 1990s. Phylogenomic analyses revealed the South American cichlid iridovirus (SACIV) and three spot gourami iridovirus (TSGIV) possess 116 open reading frames each, and form a novel clade within the turbot reddish body iridovirus genotype (TRBIV Clade 2). Both genomes displayed a unique truncated paralog of the major capsid protein gene located immediately upstream of the full-length parent gene. Histopathological examination of archived oscar tissue sections that were PCR-positive for SACIV revealed numerous cytomegalic cells characterized by basophilic intracytoplasmic inclusions within various organs, particularly the anterior kidney, spleen, intestinal lamina propria and submucosa. TSGIV-infected grunt fin (GF) cells grown in vitro displayed cytopathic effects (e.g. cytomegaly, rounding, and refractility) as early as 96 h post-infection. Ultrastructural examination of infected GF cells revealed unenveloped viral particles possessing hexagonal nucleocapsids (120 to 144 nm in diameter) and electron-dense cores within the cytoplasm, consistent with the ultrastructural morphology of a MCV. Sequencing of SACIV and TSGIV provides the first complete TRBIV Clade 2 genome sequences and expands the known host and geographic range of the TRBIV genotype to include freshwater ornamental fishes traded in North America.

The impact of pooling samples on surveillance sensitivity for the megalocytivirus Infectious spleen and kidney necrosis virus

“…For example, ISKNV has been reported as the cause of significant mortality at marine farms holding barramundi ( Lates calcarifer ; Dong et al, ) whilst the geographic range is expanded with detections in ornamental fish in Brazil (de Lucca Maganha et al, ), Germany (Jung‐Schroers et al, ) and the USA (Subramaniam et al, ). TRBIV was recently reported in a marine food fish (starry flounder, Platichthys stellatus ) in Korea (Jin et al, ; Woong et al, ) and from archived clinical cases of freshwater ornamental fish from the late 1980s and early 1990s in the USA (Koda et al, ), Canada and Australia (Go et al, ). Further, Liu et al () reported that the original freshwater host of ISKNV, mandarin fish, is now frequently affected by RSIV in China.…”

“…Disease caused by MCVs have occurred most frequently in east and South‐East Asia (Subramaniam et al, ), and mortality is usually between 20% and 60% (Kurita & Nakajima, ). Recently, retrospective studies using novel molecular tools have traced the early emergence of MCVs in ornamental fish from the late 1980s and early 1990s in Australia, USA and Canada (Go et al, ; Koda et al, ). At present, the OIE Manual for Diagnostic Tests for Aquatic Animals does not identify a test suitable for targeted surveillance for MCVs (OIE, , version adopted 2012).…”

PCR Detection and Phylogenetic Analysis of Megalocytivirus Isolates in Farmed Giant Sea Perch Lates calcarifer in Southern Taiwan

“…According to previous studies, the distribution of traditional TRBIV megalocytiviruses that usually infect food flatfish species is restricted to areas around the Yellow Sea in East Asia [ 6 , 12 , 13 , 14 , 22 , 23 ]. Recently, however, these traditional TRBIV megalocytiviruses have been further classified as the TRBIV clade I genotype, which is different from the TRBIV clade II genotype so far discovered in freshwater ornamental fish [ 24 , 25 ] and marine-reared rock bream [ 8 ]. Go et al first reported and characterized the TRBIV clade II genotype based on the phylogenetic analyses of the MCP and ATPase gene sequences of iridoviruses from the archived materials of freshwater ornamental fish species (angelfish Pterophyllum scalare (KX354223), dwarf gourami Trichogaster lalius (KX354222) and oscar Astronotus ocellatus (KX354221)) in earlier outbreaks from the late 1980s through the early 1990s [ 24 ].…”

“…Since these archived ornamental fish were imported to Australia and North American from Southeast Asia, the TRBIV clade II megalocytiviruses may have spread around Southeast Asia for a long time starting from the late 1980s to the early 1990s [ 24 ]. Moreover, Koda et al have sequenced two other TRBIV clade II megalocytiviruses, South American cichlid iridovirus (SACIV) and three spot gourami iridovirus (TSGIV), isolated from archival ornamental fish samples to provide the first complete genome sequences of the TRBIV clade II genotype [ 25 ]. In the same study, Koda and his coauthors have also expanded the geographic range of the TRBIV clade II genotype to include freshwater ornamental fish, including South American cichlids (oscar Astronotus ocellatus and keyhole cichlid Cleithracara maronii ) and three spot gourami ( Trichopodus trichopterus ), traded in North America [ 25 ].…”

“…Those TRBIV isolates traditionally restricted in East Asia have been classified as the TRBIV clade I megalocytiviruses [ 24 ]. On the other hand, the newly found TRBIV clade II genotype has been so far discovered in freshwater ornamental fish [ 24 , 25 ] and marine-reared rock bream [ 8 ]. Therefore, much attention should be paid to further studies on the geographic distribution and the host range of TRBIV clade II megalocytiviruses.…”