Since 1993, an epizootic viral disease has occurred in net-cage cultured red sea bream, Pagrus major (Temminck & Schlegel), in Peng-hu Island located on the south-western coast of Taiwan. The diseased fish exhibited abnormal swimming and were lethargic, but few visible external signs were observed. The cumulative mortality because of the disease sometimes reached 50-90% over 2 months. Histopathogical studies of the affected fish showed enlarged basophilic cells in the gill, kidney, heart, liver and spleen. These necrotic cells were Feulgen-positive and stained blue using Giemsa. Transmission electron microscopy revealed icosahedral virions in the cytoplasm of the necrotic cells. The viral particles consisted of a central nucleocapsid (75-80 nm) and envelope, and were 120-150 nm in diameter. These results suggest that the virus belongs to the Iridoviridae. Using polymerase chain reaction (PCR), approximately 570 bp fragments were produced from the viral DNA using as a template 1-F and 1-R primers derived from red seabream iridovirus (RSIV) from red sea bream in Japan. Similar results were also obtained using nested-PCR with different primer sets (1-F, 2-R and 2-F, 1-R). Although the size and some features of epizootics of this virus differed from RSIV in Japan, it shows close genetic affinities with the latter and it is suggested that RSIV has been introduced to Taiwan.
Viruses belonging to the genus Megalocytivirus in the family Iridoviridae are one of the major agents causing mass mortalities in marine and freshwater fish in Asian countries. Outbreaks of iridovirus disease have been reported among various fish species in Taiwan. However, the genotypes of these iridoviruses have not yet been determined. In this study, seven megalocytivirus isolates from four fish species: king grouper, Epinephelus lanceolatus (Bloch), barramundi perch, Lates calcarifer (Bloch), silver sea bream, Rhabdosargus sarba (Forsskal), and common ponyfish, Leiognathus equulus (Forsskal), cultured in three different regions of Taiwan were collected. The full open reading frame encoding the viral major capsid protein gene was amplified using PCR. The PCR products of approximately 1581 bp were cloned and the nucleotide sequences were phylogenetically analysed. Results showed that all seven PCR products contained a unique open reading frame with 1362 nucleotides and encoded a structural protein with 453 amino acids. Even though the nucleotide sequences were not identical, these seven megalocytiviruses were classified into one cluster and showed very high homology with red sea bream iridovirus (RSIV) with more than 97% identity. Thus, the seven iridovirus strains isolated from cultured marine fish in Taiwan were closer to the RSIV genotype than the infectious spleen and kidney necrosis virus genotype.
Megalocytiviruses are classified into three genotypes, infectious spleen and kidney necrosis virus (ISKNV), red seabream virus (RSIV), and turbo reddish body iridovirus (TRBIV), based on the major capsid protein and ATPase genes. However, only a few complete genome sequences have been obtained. This paper reports the complete genome sequence and phylogenetic analysis of an RSIV-Ku strain megalocytivirus. The genome sequence comprises 111,154 bp, has 132 putative open reading frames, and is homologous mostly to ISKNV, except for the sequence in the region 58981-66830, which is more closely related to that of the RSIV genotype. The results imply that RSIV-Ku is actually a natural recombinant virus.
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