Distribution of marine birnavirus in cultured marine fish species from Kagawa Prefecture, Japan

Detection and characterization of a novel marine birnavirus isolated from Asian seabass in Singapore

Background

Lates calcarifer, known as seabass in Asia and barramundi in Australia, is a widely farmed species internationally and in Southeast Asia and any disease outbreak will have a great economic impact on the aquaculture industry. Through disease investigation of Asian seabass from a coastal fish farm in 2015 in Singapore, a novel birnavirus named Lates calcarifer Birnavirus (LCBV) was detected and we sought to isolate and characterize the virus through molecular and biochemical methods.

Methods

In order to propagate the novel birnavirus LCBV, the virus was inoculated into the Bluegill Fry (BF-2) cell line and similar clinical signs of disease were reproduced in an experimental fish challenge study using the virus isolate. Virus morphology was visualized using transmission electron microscopy (TEM). Biochemical analysis using chloroform and 5-Bromo-2′-deoxyuridine (BUDR) sensitivity assays were employed to characterize the virus. Next-Generation Sequencing (NGS) was also used to obtain the virus genome for genetic and phylogenetic analyses.

Results

The LCBV-infected BF-2 cell line showed cytopathic effects such as rounding and granulation of cells, localized cell death and detachment of cells observed at 3 to 5 days’ post-infection. The propagated virus, when injected intra-peritoneally into naïve Asian seabass under experimental conditions, induced lesions similar to fish naturally infected with LCBV. Morphology of LCBV, visualized under TEM, revealed icosahedral particles around 50 nm in diameter. Chloroform and BUDR sensitivity assays confirmed the virus to be a non-enveloped RNA virus. Further genome analysis using NGS identified the virus to be a birnavirus with two genome segments. Phylogenetic analyses revealed that LCBV is more closely related to the Blosnavirus genus than to the Aquabirnavirus genus within the Birnaviridae family.

Conclusions

These findings revealed the presence of a novel birnavirus that could be linked to the disease observed in the Asian seabass from the coastal fish farms in Singapore. This calls for more studies on disease transmission and enhanced surveillance programs to be carried out to understand pathogenicity and epidemiology of this novel virus. The gene sequences data obtained from the study can also pave way to the development of PCR-based diagnostic test methods that will enable quick and specific identification of the virus in future disease investigations.

High prevalence of betanodavirus barfin flounder nervous necrosis virus as well as red-spotted grouper nervous necrosis virus genotype in shellfish

Using two serially executed PCRs, the discriminative multiplex two-step RT-PCR (DMT-2 RT-PCR) following the detection seminested two-step RT-PCR (DSN-2 RT-PCR), we found a high frequency presence of BFNNV genotype as well as RGNNV in various domestic and imported shellfish. This was definitely different from the previous reports of outbreaks and asymptomatic infection only by the RGNNV genotype in cultured finfish in Korea. Cultivation of NNV entrapped in shellfish was performed successfully by a blind passage. Thus, in an attempt to elucidate the epidemiology of betanodavirus, experiments conducted on 969 shellfish samples concluded that (i) distribution of NNV genotype, especially BFNNV, in shellfish is clearly different from that found in finfish of the world; (ii) unlike RGNNV, which showed a high rate in summer, BFNNV showed no seasonal variation and this result suggests BFNNVs in the marine environment remain fairly constant throughout the year; and (iii) the entrapped virus in shellfish was alive and culturable in vitro. These results are the first report of high level prevalence of in vitro culturable NNV in shellfish, for both BFNNV and RGNNV, which may present a potential risk in transmitting nodaviruses to host species in a marine environment.

Viruses of fish: an overview of significant pathogens

The growing global demand for seafood together with the limited capacity of the wild-capture sector to meet this demand has seen the aquaculture industry continue to grow around the world. A vast array of aquatic animal species is farmed in high density in freshwater, brackish and marine systems where they are exposed to new environments and potentially new diseases. On-farm stresses may compromise their ability to combat infection, and farming practices facilitate rapid transmission of disease. Viral pathogens, whether they have been established for decades or whether they are newly emerging as disease threats, are particularly challenging since there are few, if any, efficacious treatments, and the development of effective viral vaccines for delivery in aquatic systems remains elusive. Here, we review a few of the more significant viral pathogens of finfish, including aquabirnaviruses and infectious hematopoietic necrosis virus which have been known since the first half of the 20th century, and more recent viral pathogens, for example betanodaviruses, that have emerged as aquaculture has undergone a dramatic expansion in the past few decades.

Distribution of marine birnavirus in cultured olive flounder Paralichthys olivaceus in Korea

Surveys of marine birnavirus (MABV) were undertaken in cultured olive flounder Paralichthys olivaceus from the south and west coastal areas and Jeju in Korea during the period January 1999 to April 2007. MABV was detected in all seasons from the fry, juveniles and adult fish from the areas examined. Evident cytopathic effects of the virus including rounding and cell lysis were observed in chinook salmon embryo (CHSE-214) and rainbow trout gonad (RTG-2) cells, but not in fathead minnow (FHM) and epithelial papilloma of carp (EPC) cells. Nucleotide sequences of the VP2/NS junction region of the Korean isolates showed 97.8% ~ 100% similarity, and they belonged to the same genogroup. Cross neutralization tests with serotype-specific rabbit antisera against MABV strains exhibited a close antigenic relationships between strains, and were distinct from infectious pancreatic necrosis virus (IPNV) strains. Coinfection of MABV with bacteria (Streptococcus iniae, Vibrio spp.) and viruses (nervous necrosis virus, lymphocystis disease virus, viral hemorrhagic septicemia virus) was observed.

An approach for genogrouping of Japanese isolates of aquabirnaviruses in a new genogroup, VII, based on the VP2/NS junction region

Aquabirnaviruses, represented by Infectious pancreatic necrosis virus (IPNV), have been isolated from epizootics in salmonids and a variety of aquatic animals in the world; six genogroups of aquabirnaviruses have been identified. In comparisons of nucleotide sequences of the VP2/NS junction region, maximum nucleotide diversities of 30·8 % were observed among 93 worldwide aquabirnavirus isolates. A phylogenetic tree revealed the existence of a new genogroup, VII, for Japanese aquabirnavirus isolates from marine fish and molluscan shellfish. Nucleotide diversities between genogroups VII and I–VI were 18·7 % or greater. At the nucleotide level, Japanese IPNV isolates from epizootics in salmonids were nearly identical to a genogroup I strain from the USA or Canada. It is suggested that Japanese IPNV isolates belonging to genogroup I were originally introduced from North American sources, whereas Japanese aquabirnavirus isolates of genogroup VII were from marine aquatic animals indigenous to Japan.