Scale Drop Disease Virus Associated Yellowfin Seabream (Acanthopagrus latus) Ascites Diseases, Zhuhai, Guangdong, Southern China: The First Description

Scale drop disease virus (SDDV), an emerging piscine iridovirus prevalent in farmed Asian seabass Lates calcarifer in Southeast Asia, was firstly scientifically descripted in Singapore in 2015. Here, an SDDV isolate ZH-06/20 was isolated by inoculating filtered ascites from diseased juvenile yellowfin seabream into MFF-1 cell. Advanced cytopathic effects were observed 6 days post-inoculation. A transmission electron microscopy examination confirmed that numerous virion particles, about 140 nm in diameter, were observed in infected MFF-1 cell. ZH-06/20 was further purified and both whole genome and virion proteome were determined. The results showed that ZH-06/20 was composed of 131,122 bp with 135 putative viral proteins and 113 of them were further detected by virion proteome. Western blot analysis showed that no (or weak) cross-reaction was observed among several major viral proteins between ZH-06/20 and ISKNV-like megalocytivirus. An artificial challenge showed that ZH-06/20 could cause 100% death to juvenile yellowfin seabream. A typical sign was characterized by severe ascites, but not scale drop, which was considerably different from SDD syndrome in Asian seabass. Collectively, SDDV was confirmed, for the first time, as the causative agent of ascites diseases in farmed yellowfin seabream. Our study offers useful information to better understanding SDDV-associated diseases in farmed fish.

The End of a 60-year Riddle: Identification and Genomic Characterization of an Iridovirus, the Causative Agent of White Fat Cell Disease in Zooplankton

The planktonic freshwater crustacean of the genus Daphnia are a model system for biomedical research and, in particular, invertebrate-parasite interactions. Up until now, no virus has been characterized for this system. Here we report the discovery of an iridovirus as the causative agent of White Fat Cell Disease (WFCD) in Daphnia WFCD is a highly virulent disease of Daphnia that can easily be cultured under laboratory conditions. Although it has been studied from sites across Eurasia for more than 60 years, its causative agent had not been described, nor had an iridovirus been connected to WFCD before now. Here we find that an iridovirus-the Daphnia iridescent virus 1 (DIV-1)-is the causative agent of WFCD. DIV-1 has a genome sequence of about 288 kbp, with 39% G+C content and encodes 367 predicted open reading frames. DIV-1 clusters together with other invertebrate iridoviruses but has by far the largest genome among all sequenced iridoviruses. Comparative genomics reveal that DIV-1 has apparently recently lost a substantial number of unique genes but has also gained genes by horizontal gene transfer from its crustacean host. DIV-1 represents the first invertebrate iridovirus that encodes proteins to purportedly cap RNA, and it contains unique genes for a DnaJ-like protein, a membrane glycoprotein and protein of the immunoglobulin superfamily, which may mediate host-pathogen interactions and pathogenicity. Our findings end a 60-year search for the causative agent of WFCD and add to our knowledge of iridovirus genomics and invertebrate-virus interactions.

Identification and characterization of a novel lymphocystis disease virus isolate from cultured grouper in China

Grouper Epinephelus spp. is one of the most important mariculture fish species in China and South-East Asian countries. The emerging viral diseases, evoked by iridovirus which belongs to genus Megalocytivirus and Ranavirus, have been well characterized in recent years. To date, few data on lymphocystis disease in grouper which caused by lymphocystis disease virus (LCDV) were described. Here, a novel LCDV isolate was identified and characterized. Based on the sequence of LCDV major capsid protein (MCP) and DNA polymerase gene, we found that the causative agents from different species of diseased groupers were the same one and herein were uniformly defined as grouper LCDV (GLCDV). Furthermore, H&E staining revealed that the nodules on the skin were composed of giant cells that contained inclusion bodies in the cytoplasm. Numerous virus particles with >210 nm in diameter and with hexagonal profiles were observed in the cytoplasm. In addition, phylogenetic analysis based on four iridovirus core genes, MCP, DNA polymerase, myristoylated membrane protein (MMP) and ribonucleotide reductase (RNR), consistently showed that GLCDV was mostly related to LCDV-C, followed by LCDV-1. Taken together, our data firstly provided the molecular evidence that GLCDV was a novel emerging iridovirus pathogen in grouper culture.

Case report: concurrent herpesviral and presumptive iridoviral infection associated with disease in cultured shortnose sturgeon, Acipenser brevirostrum (L.), from the Atlantic coast of Canada

Approximately 8 weeks after a chlorine insult associated with the city water supply, shortnose sturgeon, Acipenser brevirostrum (L.), from one group presented with small (3-4 mm) irregular foci of cutaneous pallor that involved the dorsocranial integument with progressive ulceration of the nascent lesions. Various bacterial organisms were isolated from the cutaneous lesions, but not from the internal viscera. Histologically, the nuclei of the intralesional and perilesional epidermal cells often exhibited margination of the chromatin that resulted in a homogenous, pale, amphophilic, tinctorial quality of the nucleoplasm consistent with a herpesvirus infection. In addition, rare lamellar epithelial cells were prominently enlarged due to an abundant, dense, basophilic cytoplasm characteristic of an iridovirus infection. Inoculation of cutaneous lesion and kidney, spleen, liver sample pools from affected shortnose sturgeon onto white sturgeon spleen (WSS-2) cell line induced cytopathic effect characterized by syncytia formation. Ultrastructural analysis of infected WSS-2 cells revealed viral particles with a characteristic herpesvirus morphology. Intranuclear hexagonal capsids had a diameter of 95-108 nm, and enveloped particles present in the cytoplasm of infected cells had a diameter of 176-196 nm. This is the first report of a herpesvirus and a possible iridovirus-like infection in shortnose sturgeon.

[Virosphere and giruses]

Novel findings and concepts in the field of virology particularly regarding virosphere and giruses--a group of large nuclear-cytoplasmic deoxyriboviruses are briefly summarized. In the context of novel understanding the major taxonomic features and virus pathogenicity including African swine plague are interpreted.

Development and characterization of a new marine fish cell line from turbot (Scophthalmus maximus)

A new marine fish cell line, TK, derived from turbot (Scophthalmus maximus) kidney, was established by the method of trypsin digestion and subcultured for more than 50 passages over a period of 300 days. The TK cells were maintained in Minimum Essential Medium Eagle (MEM) supplemented with HEPES, antibiotics, fetal bovine serum (FBS), 2-Mercaptoethanol (2-Me), and basic fibroblast growth factor (bFGF). The suitable growth temperature for TK cells was 24°C, and microscopically, TK cells were composed of fibroblast-like cells. Chromosome analysis revealed that the TK cell line has a normal diploid karyotype with 2n=44. Two fish viruses LCDV-C (lymphocystis disease virus from China) and TRBIV (turbot reddish body iridovirus) were used to determine the virus susceptibility of TK cell line. The TK cell line was found to be susceptible to TRBIV, and the infection was confirmed by cytopathic effect (CPE) and transmission electron microscopy, which detected the viral particles in the cytoplasm of virus-infected cells. Finally, significant green fluorescent signals were observed when the TK cells were transfected with pEGFP-N3 vector, indicating its potential utility for fish virus study and genetic manipulation.

Identification of an iridovirus in Acetes erythraeus (Sergestidae) and the development of in situ hybridization and PCR method for its detection

An iridovirus (tentatively named SIV, sergestid iridovirus) that causes high mortality in the sergestid shrimp, Acetes erythraeus, was found in Madagascar in 2004. Severely affected shrimp exhibit a blue-green opalescence. Histological examination revealed massive cytoplasmic inclusions in the cuticular epithelial cells, connective tissues, ovary and testes. The electron microscopic examination showed paracrystalline arrays of virions at a size of 140nm, suggesting infection with an iridovirus. A pair of PCR primers were selected from the conserved region of the major capsid protein (MCP)-coding sequence among insect iridoviruses and used to amplify a 1.0kb fragment from the infected A. erythraeus. This fragment was cloned, sequenced and found to be highly similar (upto 80% similarity in translated amino acids with an E value of 1e-124) to the MCP of invertebrate iridoviruses. This clone was then labeled with digoxigenin-11-dUTP and hybridized to tissue sections of infected A. erythraeus, which reacted positively to the probe. The reacting tissues included epithelial cells, connective tissues, and the germinal cells; the same cells as those with inclusions. A PCR method was also developed from the MCP coding sequence for detecting SIV.

Inhibition of reporter gene and Iridovirus-tiger frog virus in fish cell by RNA interference

We describe the specific silencing of reporter gene lacZ in FHM cells (muscle cells of fathead minnow, a fish cell line) by either expressing small hairpin RNAs (shRNAs) from plasmids or transfecting small interfering RNAs (siRNAs) transcribed in vitro. Two types of dsRNAs could inhibit reporter gene expression, and siRNAs were more effective, while both of them worked very well in HeLa cells. siRNAs were tested for silencing expression of the major capsid protein (MCP) encoded by tiger frog virus (TFV), an iridovirus causing severe disease in fish. siRNAs targeting mcp gene effectively inhibited TFV replication in fish cells as demonstrated by reduced mcp RNA level, postponed emergence of cytopathogenic effect, as well as reduced TFV titer and particles in cells. The results suggest that the siRNA method suppressed TFV efficiently in fish cells, providing a potential approach to the therapy of aquaculture viral diseases.

Establishment of a continuous embryonic cell line from Japanese flounder Paralichthys olivaceus for virus isolation

A continuous cell line, the flounder embryonic cell line (FEC), was established from gastrula-stage embryos of a marine cultured fish, the Japanese flounder Paralichthys olivaceus and cultured for more than 200 d with more than 60 passages. FEC cells were cultured in Dulbecco's Modified Eagle's Medium (DMEM) supplemented with antibiotics, fetal bovine serum (FBS), sea perch serum (SPS), and basic fibroblast growth factor (bFGF). The cells were small and round, and grew actively and stably in culture. The effect of temperature, FBS concentration and bFGF on FEC cell growth was examined. Cells grew well between 24 and 30 degrees C, but had a reduced growth rate below 18 degrees C. The growth rate of FEC cells in medium containing 15% FBS was higher than that in medium containing 7.5% FBS. Addition of bFGF to the medium also significantly increased the growth rate. Chromosome analysis revealed that FEC cells have a normal diploid karyotype with 2n = 48. High survival rate was obtained after cryopreservation of cell cultures. The susceptibility of the cell line to piscine viruses was examined. Two viruses tested were shown to induce CPE (cytopathic effect) on FEC cells. FEC cell culture infected with fish iridovirus was further elucidated by electron microscopy. Many virus particles were found in the cytoplasm of the virus-infected FEC cells. These results indicated that the FEC cell line could be potentially used to isolate and study fish viruses.

Histological, ultrastructural, and in situ hybridization study on enlarged cells in grouper Epinephelus hybrids infected by grouper iridovirus in Taiwan (TGIV)

Grouper iridovirus in Taiwan (TGIV) infection in the Epinephelus hybrid is a major problem in the grouper industry. ATPase gene sequences indicate that this virus is closely related to cell hypertrophy iridoviruses. Histologically, the appearance of basophilic or eosinophilic enlarged cells in internal organs is the most characteristic feature of this disease. These cells are acid-phosphatase positive and are able to phagocytose injected carbon particles. In our study, TGIV infection inhibited normal phagocytic ability in these cells in vivo after 4 d post-infection (p.i.) but not before 2 d p.i. Their staining properties and phagocytic ability suggested a monocyte origin of enlarged cells, which appeared in high numbers in the trunk kidney, head kidney, spleen and gill. After infection, the enlarged cells first appeared in the spleen, with an abundance peak at 64 h p.i. (Peak 1); at 120 h p.i., a second peak (Peak 2) occurred in the spleen, head kidney, trunk kidney and gill. Lower numbers of enlarged cells were observed in the liver, muscle, heart, eye, intestine, but no enlarged cells were found in the brain. A TGIV-specific DNA probe labeled most of the basophilic but not eosinophilic enlarged cells. Nuclei of infected cells were labeled during an early stage of the infection; at later stages, both nuclei and cytoplasms were labeled. Ultrastructurally, heterochromatins of the infected cells were marginated or aggregated to one side of the nuclei during the early stages of infection. Damage and rupture of the nuclear membrane started before formation of the viromatrix. Capsids were assembled in ring-shaped or disc-shaped structures. Bullet-shaped electron-dense material was present near the incomplete virus particles, and is speculated to be inserted into the capsids later.

The pathology of systemic iridoviral disease in fish

Iridoviruses have been associated with severe disease and economic loss in farmed food fish and ornamental fish, with mortality often reported to reach 50% or more. In the present study, three tropical marine food fish species and four tropical freshwater ornamental fish species with systemic iridovirus infections were examined histopathologically and ultrastructurally. Light microscopy consistently revealed pale to intensely basophilic hypertrophied virus-infected cells in spleen, kidney and intestine from all seven species. Ultrastructural examination showed changes in the vascular endothelium overlying hypertrophied virus-infected cells suggestive of pressure necrosis. Viral isolation was improved by the use of fibroblastic cell lines. This, together with the sub-endothelial location of infected cells in all infected species examined, suggests that systemic iridoviruses are mesotheliotropic.

Establishment of cell lines from a tropical grouper, Epinephelus awoara (Temminck & Schlegel), and their susceptibility to grouper irido- and nodaviruses

Four tropical marine fish cell lines have been established from the eye, fin, heart and swim bladder of grouper, Epinephelus awoara (Temminck & Schlegel). Optimum media and temperature conditions for maximum growth were standardized. The eye and swim bladder cells were mostly epithelial, but the fin and heart cells were mostly fibroblastic. The viability of cells was 95% after 1 year of storage in liquid nitrogen (-196 degrees C). Besides these four cell lines, previously established grouper brain, kidney and liver cell lines were also used for a viral susceptibility study which showed that all the cell lines were sensitive to grouper iridovirus, whereas only brain, fin and liver cell lines were susceptible to the yellow grouper nervous necrosis virus (a nodavirus). Electron microscopy studies of the grouper irido- and nodaviruses in ultrathin sections of infected cells showed an abundance of viral particles in the cytoplasm of the virus-infected cells indicating the effective replication of these two viruses. It is suggested that these cell lines can be used for the isolation of putative fish specific viruses and provide a valuable tool to study the mechanisms of host-pathogen interactions. Furthermore, these cell lines upon transfection, using pEGFP-C1 and pEGFP-aMT2.5 (ayu metallothionein promoter), produced significant fluorescent signals indicating their utility for exogenous studies.

Iridovirus infections in farm-reared tropical ornamental fish

A systemic viral infection in both gourami Trichogaster spp. and swordtail Xiphophorus hellerii and an outbreak of lymphocystis in scalare Pterophyllum scalarae and gourami are reported to have occurred in fish reared in ornamental fish farms in Israel. The systemic infection developed in endothelial cells that became hypertrophic and their contents were modified. The presence of such cells in light-microscopically examined stained smears and sections provides an initial indication for this systemic viral infection. Infection in gourami caused hemorrhagic dropsy. Transmission electron microscopic (TEM) images of iridovirus-like particles recovered from gouramies showed them to be 138 to 201 nm from vertex to vertex (v-v); those from swordtails were 170 to 188 nm v-v. TEM images of lymphocystis virions from scalare were 312 to 342 nm v-v and from gourami 292 to 341 nm v-v. Lymphocystis cells from the gourami were joined by a solid hyaline plate, which was lacking in the infection in scalare where the intercellular spaces between the lymphocystis cells consisted of loose connective tissue.

Electron microscopic observations of a marine fish iridovirus isolated from brown-spotted grouper, Epinephelus tauvina

The morphogenesis and the ultrastructure of a marine fish iridovirus isolated from diseased grouper, Epinephelus tauvina were studied by electron microscopy. The virus was grown on a marine fish cell line (GP) at 25 degrees C. After appearance of advanced cytopathic effect (CPE), various morphogenetic stages of virus amplification, maturation and assembly were detected in the cytoplasm of virus-infected cells. The matured nucleocapsids were probably formed by insertion of electron-dense core material into a partly forming empty capsid just before completely sealed. The nucleocapsids were located at the assembly sites as pseudocrystalline arrays or scattered individually. In the late phase of infection, the nucleocapsids were enveloped and released by budding from the plasma membrane. The budding virus particles could directly enter neighbouring cells by endocytosis to start the next round infection. Ultrastructure of the grouper iridovirus was studied using the methods of enzymatic digestions and detergent degradations. The purified iridovirus particles showed a three-layered membrane including an external lipoprotein envelope, an inner periodic protein capsid and a lipid-containing membrane. The regular array of surface capsid subunits was observed after degradation with detergent.

Isolation and characterization of an iridovirus from Hermann's tortoises (Testudo hermanni)

A virus was isolated from tissues of 2 diseased Hermann's tortoises (Testudo hermanni) and preliminarily characterized as an iridovirus. This conclusion was based on the presence of inclusion bodies in the cytoplasm of infected cells, sensitivity to chloroform, inhibition of virus replication by 5-iodo-2'-desoxyuridine and the size and icosahedral morphology of viral particles. The virus was able to replicate in several reptilian, avian and mammalian cell lines at 28 degrees C, but not at 37 degrees C. Restriction enzyme analysis showed resistance of the ral DNA to digestion with HpaII due to methylation of the internal cytosine at CCGG sequences. Part of the genomic region encoding the major capsid protein was amplified by PCR and subjected to sequence analysis. Comparative analysis of the obtained nucleotide sequence revealed that the isolate is closely related to frog virus 3, the type species of the genus Ranavirus.

Pathology, isolation, and preliminary molecular characterization of a novel iridovirus from tiger salamanders in Saskatchewan

All iridovirus was confirmed to be the cause of an epizootic in larval and adult tiger salamanders (Ambystoma tigrinum diaboli) from four separate ponds in southern Saskatchewan (Canada) during the summer of 1997. This organism also is suspected, based on electron microscopic findings, to be the cause of mortality of larval tiger salamanders in a pond over 200 km to the north during the same year. Salamanders developed a generalized viremia which resulted in various lesions including: necrotizing, vesicular and ulcerative dermatitis; gastrointestinal ulceration; and necrosis of hepatic, splenic, renal, lymphoid, and hematopoietic tissues. In cells associated with these lesions, large lightly basophilic cytoplasmic inclusions and vacuolated nuclei with marginated chromatin were consistently found. Virus was isolated from tissue homogenates of infected salamanders following inoculation of epithelioma papilloma cyprini (EPC) cells. The virus, provisionally designated Regina ranavirus (RRV), was initially identified as an iridovirus by electron microscopy. Subsequent molecular characterization, including partial sequence analysis of the major capsid protein (MCP) gene, confirmed this assignment and established that RRV was a ranavirus distinct from frog virus 3 (FV3) and other members of the genus Ranavirus. Intraperitoneal inoculation of 5 x 10(6.23) TCID50 of the field isolate caused mortality in inoculated salamanders at 13 days post infection. Field, clinical, and molecular studies jointly suggest that the etiological agent of recent salamander mortalities is a highly infectious novel ranavirus.

Replication and pathogenesis of white sturgeon iridovirus (WSIV) in experimentally infected white sturgeon Acipenser transmontanus juveniles and sturgeon cell lines

Characteristics of the in vitro propagation of the white sturgeon iridovirus (WSIV) were examined in 6 sturgeon cell lines. One new cell line originating from gonadal tissues (WSGO) produced up to 12-fold more WSIV [approximately 22 TCID50 (50% tissue culture infective dose) cell-1], than that of a previously established reference spleen cell line (WSS-2). Infected WSGO cell cultures were examined using phase microscopy, viral infectivity assay and transmission electron microscopy (TEM). At 15 degrees C, both mature virions and infectious virus were first detected after 7 d post-infection. Capsids acquired envelopes in the cytoplasm and virions remained primarily cell-associated during the 35 d replication cycle. Cellular changes including hyper-refractility and cytoplasmic swelling with dense cytoplasmic inclusions correlated to extensive proliferation of cytoplasmic vesicles and viral assembly sites. These cytological characteristics corresponded to changes in target cells of WSIV-infected juvenile white sturgeon following bath challenge. Microscopic changes in stained tissue sections of the host epithelium were detected 4 d post-challenge, approximately 8 d prior to the onset of clinical signs. Hypertrophied Malpighian cells surrounded by a prominent pericellular cisternum characterized epithelial lesions in the skin. Similar changes to epithelial cells of the barbels, olfactory organs and esophagus were also observed. Destruction of the sensory epithelium is suggested as a cause for cessation of feeding which occurs early in the infection of white sturgeon juveniles with WSIV.

Pathological and microbiological findings from incidents of unusual mortality of the common frog (Rana temporaria)

In 1992 we began an investigation into incidents of unusual and mass mortalities of the common frog (Rana temporaria) in Britain which were being reported unsolicited to us in increasing numbers by members of the public. Investigations conducted at ten sites of unusual mortality resulted in two main disease syndromes being found: one characterized by skin ulceration and one characterized by systemic haemorrhages. However, frogs also were found with lesions common to both of these syndromes and microscopic skin lesions common to both syndromes were seen. The bacterium Aeromonas hydrophila, which has been described previously as causing similar lesions, was isolated significantly more frequently from haemorrhagic frogs than from those with skin ulceration only. However, as many of the latter were euthanased, this may have been due to differences in post mortem bacterial invasion. An iridovirus-like particle has been identified on electron microscopical examination of skin lesions from frogs with each syndrome and iridovirus-like inclusions have been detected in the livers of frogs with systemic haemorrhages. Also, an adenovirus-like particle has been cultured from one haemorrhagic frog. A poxvirus-like particle described previously from diseased frogs has now been found also in control animals and has been identified as a melanosome. Both the prevalence of the iridovirus-like particle and its association with lesions indicate that it may be implicated in the aetiology of the disease syndromes observed. Specifically, we hypothesize that primary iridovirus infection, with or without secondary infection with opportunistic pathogens such as A. hydrophila, may cause natural outbreaks of 'red-leg', a disease considered previously to be due to bacterial infection only.