Infectious spleen and kidney necrosis virus-associated large-scale mortality in farmed giant gourami, Osphronemus goramy, in India

Megalocytivirus cause diseases that have serious economic impacts on aquaculture, mainly in East and South-East Asia. Five primary genotypes are known: infectious spleen and kidney necrosis virus (ISKNV), red sea bream iridovirus (RSIV), turbot reddish body iridovirus (TRBIV), threespine stickleback iridovirus (TSIV) and scale drop disease virus (SDDV). ISKNV-mediated infectious spleen and kidney necrosis disease (ISKND) is a major viral disease in both freshwater and marine fish species. In this study, we report the isolation of ISKNV from diseased giant gourami, Osphronemus goramy, in India. Transmission electron microscopy of ultrathin sections of kidney and spleen revealed the presence of numerous polygonal naked viral particles having an outer nucleocapsid layer within the cytoplasm of enlarged cells (115-125 nm). Molecular and phylogenetic analyses confirmed the presence of ISKNV and the major capsid protein (MCP) (1,362 bp) gene in the infected fish had a high similarity to the other ISKNV-I isolates. Moreover, ISKNV was propagated in the Astronotus ocellatus fin (AOF) cell line and further confirmed genotypically. A high mortality rate (60%) was observed in gourami fish injected with ISKNV-positive tissue homogenate through challenge studies. Considering the lethal nature of ISKNV, the present study spotlights the implementation of stringent biosecurity practices for the proper control of the disease in the country.

Viruses Infecting the European Catfish (Silurus glanis)

In aquaculture, disease management and pathogen control are key for a successful fish farming industry. In past years, European catfish farming has been flourishing. However, devastating fish pathogens including limiting fish viruses are considered a big threat to further expanding of the industry. Even though mainly the ranavirus (Iridoviridea) and circovirus (Circoviridea) infections are considered well- described in European catfish, more other agents including herpes-, rhabdo or papillomaviruses are also observed in the tissues of catfish with or without any symptoms. The etiological role of these viruses has been unclear until now. Hence, there is a requisite for more detailed information about the latter and the development of preventive and therapeutic approaches to complete them. In this review, we summarize recent knowledge about viruses that affect the European catfish and describe their origin, distribution, molecular characterisation, and phylogenetic classification. We also highlight the knowledge gaps, which need more in-depth investigations in the future.

Description of a Natural Infection with Decapod Iridescent Virus 1 in Farmed Giant Freshwater Prawn, Macrobrachium rosenbergii

Macrobrachium rosenbergii is a valuable freshwater prawn in Asian aquaculture. In recent years, a new symptom that was generally called "white head" has caused high mortality in M. rosenbergii farms in China. Samples of M. rosenbergii, M. nipponense, Procambarus clarkii, M. superbum, Penaeus vannamei, and Cladocera from a farm suffering from white head in Jiangsu Province were collected and analyzed in this study. Pathogen detection showed that all samples were positive for Decapod iridescent virus 1 (DIV1). Histopathological examination revealed dark eosinophilic inclusions and pyknosis in hematopoietic tissue, hepatopancreas, and gills of M. rosenbergii and M. nipponense. Blue signals of in situ digoxigenin-labeled loop-mediated isothermal amplification appeared in hematopoietic tissue, hemocytes, hepatopancreatic sinus, and antennal gland. Transmission electron microscopy of ultrathin sections showed a large number of DIV1 particles with a mean diameter about 157.9 nm. The virogenic stromata and budding virions were observed in hematopoietic cells. Quantitative detection with TaqMan probe based real-time PCR of different tissues in naturally infected M. rosenbergii showed that hematopoietic tissue contained the highest DIV1 load with a relative abundance of 25.4 ± 16.9%. Hepatopancreas and muscle contained the lowest DIV1 loads with relative abundances of 2.44 ± 1.24% and 2.44 ± 2.16%, respectively. The above results verified that DIV1 is the pathogen causing white head in M. rosenbergii. M. nipponense and Pr. clarkii are also species susceptible to DIV1.

Mortality from scale drop disease in farmed Lates calcarifer in Southeast Asia

In Southeast Asia, a new disease called scale drop disease (SDD) caused by a novel Megalocytivirus (SDDV) has emerged in farmed Asian sea bass (Lates calcarifer) in Singapore, Malaysia and Indonesia. We received samples from an Eastern Thai province that also showed gross signs of SDD (loss of scales). Clinical samples of 0.2-1.1 kg L. calcarifer collected between 2016 and 2018 were examined for evidence of SDDV infection. Histopathology was similar to that in the first report of SDDV from Singapore including necrosis, inflammation and nuclear pyknosis and karyorrhexis in the multiple organs. Intracytoplasmic inclusion bodies were also observed in the muscle tissue. In a density-gradient fraction from muscle extracts, TEM revealed enveloped, hexagonal megalocytiviral-like particles (~100-180 nm). By PCR using primers derived from the Singaporean SDDV genome sequence, four different genes were amplified and sequenced from the Thai isolate revealing 98.7%-99.9% identity between the two isolates. Since viral inclusions were rarely observed, clinical signs and histopathology could not be used to easily distinguish between SDD caused by bacteria or SDDV. We therefore recommend that PCR screening be used to monitor broodstock, fry and grow-out fish to estimate the current impact of SDDV in Southeast Asia and to prevent its spread.

Iridovirus-like viruses in erythrocytes of lacertids from Portugal

Icosahedral nucleo-cytoplasmic large DNA viruses (NCLDV)-like viruses, which forminclusions in the erythrocyte cytoplasm of reptiles, were previously presented as candidates for a new genus of the Iridoviridae family. The present work describes the distribution of infected lizard hosts and ultrastructural characteristics of the viral inclusions of NCLDV-like viruses from Portugal and adjacent locations in Spain. Giemsa-stained blood smears of 235 Lacerta schreiberi from Portugal and Spain, 571 Lacerta monticola from the mountain Serra da Estrela (Portugal), 794 Podarcis hispanica from several localities in Portugal and Spain, and 25 Lacerta dugesii from Madeira Island, were studied. Infection in L. schreiberi was only found in mountain populations, up to 30% in Serra da Estrela and 9-11% elsewhere. It was absent in lizards from lowlands. Prevalence of infection among L. monticola in Serra da Estrela was 10%; infected lizards were found during March to July and October but not in August and September. Infection in P. hispanica was below 3.3%. Only one infected specimen of L. dugesii was identified by light microscopy. Ultrastructural examination of infected samples revealed that the inclusions are virus assembly sites of icosahedral cytoplasmic iridovirus-like virions. Virions from different host species have different ultrastructural features and probably represent different related viruses.

Megalocytivirus infection in orbiculate batfish Platax orbicularis

Megalocytiviruses cause systemic disease in both marine and freshwater fishes, negatively impacting ornamental and food fish aquaculture. In this report, we characterize a megalocytivirus infection in a captive marine ornamental fish, the orbiculate batfish Platax orbicularis. Histologic examination revealed cytomegalic cells characterized by strongly basophilic granular intracytoplasmic inclusions within various organs. Transmission electron microscopy revealed icosahedral virus particles within the cytoplasm of cytomegalic cells consistent with an iridovirus infection. Analysis of the major capsid protein gene sequence confirmed that the orbiculate batfish virus is a member of the family Iridoviridae and is identical to the only other megalocytivirus reported from a marine ornamental fish, the Banggai cardinalfish Pterapogon kauderni iridovirus.

Establishment, characterization of a new cell line from heart of half smooth tongue sole (Cynoglossus semilaevis)

A new cell line was established from the heart of a cultured marine fish, half smooth tongue sole (Cynoglossus semilaevis), designated as CSH (Cynoglossus semilaevis heart cell line). The CSH cells grow over 400 days in minimum essential medium (MEM) supplemented with 10% fetal bovine serum (FBS) and 2 ng/ml basic fibroblast growth factor (bFGF). The suitable temperature for the cell growth was 24-30°C with the optimum growth at 24°C and a reduced growth at 12 and 30°C. FBS and bFGF concentration were the two important components for CSH cells proliferation. Twenty percent FBS in the medium was found to be the optimum concentration and bFGF promoted the growth of CSH cells. The double time of the cells at 24°C was determined to 73.39 h. Chromosome analysis revealed that 44% of the cells maintained a normal diploid chromosome number (2n=42) in the CSH cells at Passage 58. The fluorescent signals were observed in CSH after the cells were transfected with green fluorescent protein (GFP) reporter plasmids. CSH cells showed the cytopathic effect (CPE) after infection with lymphosystis disease virus (LCDV). Moreover, the LCDV particles can be observed in the cytoplasm of virus-infected cells by electron microscopy, and a segment of MCP gene for major capsid protein of LCDV was found by PCR amplification DNA of virus-infected cells.

Characterization of virus distribution in Rock Bream (Oplegnathus fasciatus; Temminck and Schlegel) infected with megalocytivirus

The distribution of virus-infected cells in the organs of Rock Bream naturally infected with megalocytivirus is reported. Examination of sections of liver, spleen and kidney stained by haematoxylin and eosin (HE) and periodic acid Schiff (PAS) revealed the presence of swollen and degenerate cells having morphology consistent with leucocytes. Many of these cells were shown to contain viral DNA by in-situ hybridization (ISH). Cells containing viral DNA were also found in the connective tissue of other organs in which there was no prominent infiltrate of degenerate leucocytes. Viral DNA was also found in the cytoplasm of leucocytes in blood smears. Transmission electron microscopy (TEM) confirmed the presence of viral particles in the cells within tissue and free within blood. The tissue distribution of virus in this infection is suggested to reflect the infiltration of virus-infected leucocytes.

Differential adsorption of occluded and nonoccluded insect-pathogenic viruses to soil-forming minerals

Soil represents the principal environmental reservoir of many insect-pathogenic viruses. We compared the adsorption and infectivity of one occluded and two nonoccluded viruses, Helicoverpa armigera single nucleopolyhedrovirus (HaSNPV) (Baculoviridae), Cricket paralysis virus (CrPV) (Dicistroviridae), and Invertebrate iridescent virus 6 (IIV-6) (Iridoviridae), respectively, in mixtures with a selection of soil-forming minerals. The relative infective titers of HaSNPV and CrPV were unchanged or slightly reduced in the presence of different minerals compared to their titers in the absence of the mineral. In contrast, the infective titer of IIV-6 varied according to the mineral being tested. In adsorption studies, over 98% of HaSNPV occlusion bodies were adsorbed by all the minerals, and a particularly high affinity was observed with ferric oxide, attapulgite, and kaolinite. In contrast, the adsorption of CrPV and IIV-6 differed markedly with mineral type, with low affinity to bentonites and high affinity to ferric oxide and kaolinite. We conclude that interactions between soil-forming minerals and insect viruses appear to be most important in nucleopolyhedroviruses, followed by invertebrate iridescent viruses, and least important in CrPV, which may reflect the ecology of these pathogens. Moreover, soils with a high content of iron oxides or kaolinite would likely represent highly effective reservoirs for insect-pathogenic viruses.

A systemic iridoviral disease in mullet, Mugil cephalus L., and tiger grouper, Epinephelus fuscoguttatus Forsskal: a first report and study

A systemic iridoviral disease associated with high mortality was initially recognized in cultured mullet, Mugil cephalus L., and tiger grouper, Epinephelus fuscoguttatus Forsskal, by histopathology and transmission electron microscopy. Polymerase chain reaction was performed on tissues and viral isolates, using four published primer sets developed for the Red Sea bream iridovirus (RSIV). An indirect fluorescent antibody test was also performed on virus-infected ATCC gruntfin (GF) and seabass, Lates calcarifer Bloch, (SB) cells using a monoclonal antibody, RSIV M10. Our results suggested that the mullet and tiger grouper iridovirus bears genetic and antigenic similarities to RSIV.

Complementary approaches to structure determination of icosahedral viruses

Few biological macromolecular complexes exhibit the combination of massive size and hierarchical, symmetrical architecture embodied in icosahedral viruses. X-ray crystallography, electron cryomicroscopy and small-angle X-ray scattering provide complementary approaches to studying these remarkable structures. Through a combined approach, progress has been made towards providing detailed structures of highly complex and very large viruses, and towards imaging the dynamic structural changes performed by viruses at key stages in their life cycles.

Infection and propagation of lymphocystis virus isolated from the cultured flounder Paralichthys olivaceus in grass carp cell lines

The causative agent of lymphocystis disease that frequently occurs in cultured flounder Paralichthys olivaceus in China is lymphocystis virus (LV). In this study, 13 fish cell lines were tested for their susceptibility to LV. Of these, 2 cell lines derived from the freshwater grass carp Ctenopharyngodon idellus proved susceptible to the LV, and 1 cell line, GCO (grass carp ovary), was therefore used to replicate and propagate the virus. An obvious cytopathic effect (CPE) was first observed in cell monolayers at 1 d post-inoculation, and at 3 d this had extended to about 75% of the cell monolayer. However, no further CPE extension was observed after 4 d. Cytopathic characteristics induced by the LV were detected by Giemsa staining and fluorescence microscopic observation with Hoechst 33258 staining. The propagated virus particles were also observed by electron microscopy. Ultrastructure analysis revealed several distinct cellular changes, such as chromatin compaction and margination, vesicle formation, cell-surface convolution, nuclear fragmentation and the occurrence of characteristic 'blebs' and cell fusion. This study provides a detailed report of LV infection and propagation in a freshwater fish cell line, and presents direct electron microscopy evidence for propagation of the virus in infected cells. A possible process by which the CPEs are controlled is suggested.

Structural analyses ofPhycodnaviridaeandIridoviridae

The Phycodnaviridae, Iridoviridae and related viruses, with diameters of 1500-2000 A, are formed from large trigonal arrays of hexagonally close-packed capsomers forming the faces of icosahedra [Yan et al. (2000), Nature Struct. Biol. 7, 101-103; Nandhagopal et al. (2002), Proc. Natl Acad. Sci. USA, 99, 14758-14763]. Caspar and Klug predicted that such structures could be assembled from hexameric capsomers [Caspar & Klug (1962), Cold Spring Harbor. Symp. Quant. Biol. 27, 1-24], as was subsequently found in numerous icosahedral viruses. During the course of evolution, some viruses, including the virus families mentioned above, replaced hexameric capsomers with pseudo-hexameric trimers by gene duplication. In large dsDNA icosahedral viruses, the capsomers are organized into 'pentasymmetrons' and 'trisymmetrons'. The interactions between the trimeric capsomers can be divided into three groups, one between similarly oriented trimers and two between oppositely oriented trimers (trimers related by an approximately sixfold rotation). The interactions within a trisymmetron belong to the first class, whereas those between trisymmetrons and within the pentasymmetron are of the other two types. Knowledge of these distances permits a more accurate fitting of the atomic structure of the capsomer into the cryo-electron microscopy (cryoEM) reconstruction of the whole virus. The adoption of pseudo-hexagonal capsomers places these viruses into a subset of the Caspar and Klug surface lattices.

A comparison of techniques for detecting Invertebrate iridescent virus 6

The aim of this study was to compare the sensitivity and precision of various methods for the detection and quantification of Invertebrate iridescent virus 6 (IIV-6) (Iridoviridae) isolated from a the stem-boring moth Chilo suppressalis, and to apply these techniques to the detection of covert infections in the wax moth, Galleria mellonella. The relationship between the virus concentration and absorbance at 260 nm was linear over the range of 1.6 x 10(9)-5.6 x 10(10) particles/ml. TCID(50) assays using 12 different cell lines indicated that two Drosophila lines, DL2 and DR1, had the highest susceptibility whereas cell lines from Aedes albopictus and Plutella xylostella were four orders of magnitude less sensitive. TCID(50) values for IIV-6 in Spodoptera frugiperda Sf9 cells gave the particle-infectivity ratios of 15-64 virus particles/IU. An insect bioassay involved injecting doses of 1-100 IIV-6 particles into the third instar G. mellonella larvae. The prevalence of patent infection was 20-26% at a dose of 1 particle per larva rising to 86-92% at 10 particles and 100% at doses of 50 or 100 particles. Of the insects that survived to adulthood, between 5.8 and 75% caused patent infections when injected into G. mellonella larvae, indicating that they were covertly infected. A PCR technique resulted in 95% detection at 1000 virus particles per insect. Of the insects that proved positive for covert infection by insect bioassay, 41% also proved positive by PCR analysis. It is concluded that the G. mellonella bioassay is highly reliable for detection of doses of 10 particles or more and for determining the relative activity of IIV-6 preparations at doses as low as 1 particle per insect. PCR had a slightly lower sensitivity followed by the insect cell culture assay.

The familiar and the unexpected in structures of icosahedral viruses

Viruses were the first large macromolecular assemblages to be visualized at high resolution. New virus structures continue to challenge our understanding of specificity in protein-protein "recognition". The evolution of virus structures has been even more opportunistic than previously imagined.

Comparative studies of piscine and amphibian iridoviruses

A total of 30 iridoviruses collected from Australia, South-East Asia, North America, South America and Europe were characterised. With the exception of the South-East Asian iridoviruses all viruses were found to belong to the genus Ranavirus. All viruses, except those originating from South-East Asia, cross-reacted with antisera against epizootic haematopoietic necrosis virus (EHNV). Viruses or virus-infected cells were examined using electron microscopy, SDS PAGE, restriction endonuclease (RE) digestion, DNA hybridisation, and DNA sequencing. Data from RE digestion of genomic DNA, and from the sequencing of PCR products indicated that the viruses generally grouped according to their geographic and taxonomic (i.e. amphibian or fish) origin. The one exception to this was the viruses from the United Kingdom that grouped with the North American ranaviruses. The differences between specified genomic regions were small. To assess the validity of the differences in sequence homology, similar studies were performed with different isolates from two viruses (EHNV and Guatopo virus (GV), collected from different animals at different locations and time). The sequence data showed complete homology for the isolates for any one virus over the 200 and 586 bp regions examined. Collectively, the data showed that the coding region for the major coat protein (MCP) is stable for any one species (e.g. EHNV).

Reconstruction principles of icosahedral virus structure determination using electron cryomicroscopy

Electron cryomicroscopy is a useful tool for studying the three-dimensional structure of icosahedral viruses. This review is intended to provide beginners with an understanding of icosahedral virus structure determination focusing on the data processing aspects. We begin with an overview of the entire structure determination process and a brief summary of the sample preparation and imaging aspects. Next, we provide detailed descriptions of each data processing step leading to three-dimensional reconstruction, including application of image corrections, resolution assessment, and structure visualization. To aid in understanding this reconstruction process we provide a variety of illustrative examples. Last, we summarize future prospects for icosahedral virus structural studies.

A new iridovirus isolated from soft-shelled turtle

A virus was isolated from soft-shelled turtle (Trionyx sinensis) with 'red neck disease' on a farm in Shenzhen, China, the virus multiplied and caused a cytopathogenic effect (CPE) at 15-30 degrees C in CO, FHM, CK and BF-2 cells. The optimum conditions for replication was in CO cells at 25-30 degrees C. The virus was sensitive to chloroform treatment, acid (pH 3) or alkaline (pH 10) conditions and heating at 56 degrees C for 30 min. Treatment with 5-iodo-2-deoxyuridine (IUDR) inhibited viral replication, indicating the presence of a DNA genome. Electron microscopy of infected CO culture fluid revealed spherical particles measuring 120-160 nm in diameter. Observation of ultra-thin sections showed numerous hexagonal viral particles in the cytoplasm and nucleus of cells typical of an iridovirus. This virus was moderately virulent for turtles in infection tests. We suggest that this virus is named soft-shelled turtle iridovirus (STIV).

Comparison of European systemic piscine and amphibian iridoviruses with epizootic haematopoietic necrosis virus and frog virus 3

Iridovirus-like agents isolated from systemic infected fish (Silurus glanis, SFIR; Ictalurus melas, CFIR I, CFIR II, CFIR III) and from frogs (Rana esculenta, REIR) in Europe, Epizootic Haematopoietic Necrosis Virus (EHNV) isolated in Australia from redfin perch (Perca fluviatilis), and Frog Virus 3 (FV 3) isolated from frogs (Rana pipiens) in the USA were investigated by electron microscopy, polypeptide composition, immunofluorescence, restriction endonuclease digestion, Southern-blot hybridization and polymerase chain reaction (PCR) amplification. All virus isolates proved to be similar in morphology and in size and reacted with EHNV polyclonal antiserum in the immunofluorescence. Whilst DNA restriction profiles of the European piscine isolates cleaved by BamH I were similar, they differed clearly from those of EHNV, REIR and FV 3. Southern-blot analysis of viral BamH I digested DNA using an EHNV DNA probe revealed cross-hybridization with DNA of the investigated iridoviruses. Using a set of primers designed for an open reading frame of the EHNV genome, PCR products of about 250 bp were obtained with the DNA of systemic piscine and amphibian iridoviruses. The data suggest that the systemic piscine and amphibian iridoviruses should be regarded as members of the the genus Ranavirus within the family Iridoviridae.

Viral hemorrhagic septicemia virus, Ichthyophonus hoferi, and other causes of morbidity in Pacific herring Clupea pallasi spawning in Prince William Sound, Alaska, USA

Pacific herring Clupea pallasi populations in Prince William Sound, Alaska, USA, declined from an estimated 9.8 x 10(7) kg in 1992 to 1.5 x 10(7) kg in 1994. To determine the role of disease in population decline, 233 Pacific herring from Prince William Sound were subjected to complete necropsy during April 1994. The North American strain of viral hemorrhagic septicemia virus (VHSV) was isolated from 11 of 233 fish (4.7%). VHSV was significantly related to myocardial mineralization, hepatocellular necrosis, submucosal gastritis, and meningoencephalitis. Ichthyophonus hoferi infected 62 of 212 (29%) fish. I. hoferi infections were associated with severe, disseminated, granulomatous inflammation and with increased levels of plasma creatine phosphokinase (CPK) and aspartate aminotransferase (AST). I. hoferi prevalence in 1994 was more than double that of most previous years (1989 to 1993). Plasma chemistry values significantly greater (p < 0.01) in males than females included albumin, total protein, cholesterol, chloride, glucose, and potassium; only alkaline phosphatase was significantly greater in females. Hypoalbuminemia was relatively common in postspawning females; other risk factors included VHSV and moderate or severe focal skin reddening. Pacific herring had more than 10 species of parasites, but they were not associated with significant lesions. Two of the parasites have not previously been described: a renal intraductal myxosporean (11% prevalence) and an intestinal coccidian (91% prevalence). Transmission electron microscopy of a solitary mesenteric lesion revealed viral particles consistent with lymphocystis virus. No fish had viral erythrocytic necrosis (VEN). Prevalence of external gross lesions and major parasites was not related to fish age, and fish that were year-lings at the time of the 1989 'Exxon Valdez' oil spill (1988 year class) had no evidence of increased disease prevalence.

Major viral diseases affecting fish aquaculture in Spain

The number of viruses isolated from fish has grown in the last few years as a reflection of the increasing interest in fish diseases, particularly those occurring in aquaculture facilities. Of all the described viruses, only a few are considered to be of serious concern and economic importance; they are described in this review, drawing special attention to the four families of viruses (Birnaviridae, Rhabdoviridae, Iridoviridae and Reoviridae) that have been reported in Spanish aquaculture. Infectious pancreatic necrosis virus, a member of the first family, is the most spread virus with a prevalence of 39%. Viral diseases are untreatable and because effective and safe vaccines for fish are not yet commercially available, a great care needs to be exercised when moving fish or eggs from one site or country to another. Some fish health control regulations have been legislated in Europe and USA.

Intraerythrocytic inclusions associated with iridoviral infection in a fer de lance (Bothrops moojeni) snake

Intraerythrocytic inclusions associated with infection by an iridovirus were observed in a fer de lance (Bothrops moojeni) snake that was being evaluated for the presence of renal carcinoma. The erythrocytes contained two types of inclusions, one viral and one crystalline, usually concomitantly. The snake was markedly anemic and exhibited a marked regenerative response. Ultrastructural analysis identified the virus to be an iridovirus consistent with snake erythrocyte virus and the crystalline structures to be of a different nature than hemoglobin.

Respiratory and pharyngo-esophageal iridovirus infection in a gopher tortoise (Gopherus polyphemus)

A free-living adult male gopher tortoise (Gopherus polyphemus) was found on Sanibel Island, Florida (USA), on 18 February 1992 with signs of upper respiratory disease. On necropsy after euthanasia on 27 February 1992, severe, extensive necrotizing ulcerative tracheitis, multifocal necrotizing pneumonia, and multifocal necrotizing ulcerative pharyngitis and esophagitis were observed. Large ovoid to round intracytoplasmic basophilic inclusions, which appeared to displace the nucleus to the cell periphery, occurred within degenerate and necrotic epithelial cells of the above tissues. On transmission electron microscopy of formalin-fixed trachea and lung, intracytoplasmic viral particles were observed within necrotic cells in the tracheal lumen and epithelial cells of the lung. Most infected cells also had a roughly spherical granular cytoplasmic inclusion that contained clusters of viral particles. Viral particles had an electron dense spherical to icosahedral core surrounded by a less electron dense icosahedral capsid. Mature extracellular virions were surrounded by an envelope and were 150 to 220 nm in diameter. Virions and cytoplasmic inclusions were morphologically similar to those of the Family Iridoviridae.

Lizard erythrocytic virus in east African chameleons

Giemsa-stained peripheral blood films from 9 of 50 flap-necked chameleons, Chamaeleo dilepis, and 1 of 18 Fischer's chameleons, Bradypodion fischeri, collected in Tanzania had intraerythrocytic inclusions. In C. dilepis, acidophilic inclusions were associated with the albuminoid vacuoles reported in typical pirhemocytonosis of saurians. Under transmission electron microscopy, the acidophilic inclusions were aggregations of partially or completely formed viral particles consistent with those of the family Iridoviridae. Enveloped viral particles were 140 to 180 nm in diameter, with a mean of 159 nm (SD, 12). Albuminoid vacuoles were not seen in the B. fischeri infection, in which erythrocytes contained multiple acidophilic inclusions of variable shape. Viral particles in B. fischeri were 156.3 to 200.0 nm in diameter; the mean was 180 nm (SD, 18). This represents the first confirmation of the viral identity of pirhemocyton in a lizard. We recommend that the epithet pirhemocyton no longer be used as a generic name under the International Code of Zoological Nomenclature, and that future reports of the etiologic agent refer to it as Lizard Erythrocytic Virus.

Infectivity and pathogenesis of iridescent virus type 22 in various insect hosts

This paper reports the results of a series of laboratory experiments to determine the infectivity and pathogenesis of iridescent virus type 22 (IV 22) for six species of mosquitoes, phlebotomine sand flies and triatomid bugs. Following inoculation, IV 22 replicated in all of the species tested, without producing noticeable mortality within a 14 day observation period. Examination of the infected insects by immunofluorescence demonstrated large amounts of viral antigen in many different organs. Electron microscopy done on infected mosquitoes (Aedes aegypti) showed large numbers of virus particles within cells of the fat body, muscle tracheal and midgut epithelium. Virus replication in the mosquitoes was confirmed to host cell cytoplasm and was similar to that described in the natural blackfly (Simulium) host. Transovarial transmission of IV 22 could not be demonstrated in A. aegypti, and only a small percentage of mosquito larvae could be infected orally. Results of these experiments are compatible with observations of other iridescent viruses; IV 22 is highly infectious for a wide range of insects when introduced into their hemolymph, but it is not very infectious per os. These characteristics would appear to limit its value as a potential biocontrol agent for Diptera.

Ultrastructure of lymphocystis disease virus (LDV) as compared to frog virus 3 (FV3) and chilo iridescent virus (CIV): effects of enzymatic digestions and detergent degradations

Ultrastructure of fish lymphocystis disease virus (LDV), the largest of all known icosahedral viruses, has been studied under electron microscopy using enzymatic digestions and detergent degradations. LDV structure appeared roughly the same as those of frog virus 3 (FV3) and chilo iridescent virus (CIV), two other well known viruses of the family Iridoviridae, although the great flexibility of its capsid as observed on negatively stained and shadow cast particles, and its three electron dense layers visualized in ultrathin sections, differed from observations made with the two other viruses. Specific degradation of the virions with enzymes or detergents revealed that the composition of the three iridoviruses was very much alike. In fact, their capsid was composed of two layers as observed in negative staining: an external one, which was removed following digestion with proteinase K, and an internal one which could be digested with phospholipase A2. Thus, the outermost layer is probably made of surface protein units, more or less tightly bound to each other, while the internal one would be a lipoprotein membrane. Consequently, these three iridoviruses appeared structurally related.

A possible iridovirus in erythrocytes of Bufo marinus in Costa Rica

Icosahedral viral particles were found in the cytoplasm of erythrocytes and splenic reticular cells of a marine toad (Bufo marinus) collected from Costa Rica. Capsids had a maximum diameter of 312 nm and a spherical core with biphasic electron density. Viruses in erythrocytes were associated with cytoplasmic assembly areas and vacuoles in cytoplasm. Nuclei had finely granular material of decreased electron density located centrally, but contained no viral particles. A group of unenveloped viral particles was seen extracellularly in a splenic vessel. The virus was consistent with an iridovirus. In a blood smear stained with Giemsa round basophilic bodies with average diameters of 1.70 microns and morphologically similar to Pirhemocyton sp. were seen in the cytoplasm of erythrocytes and occasionally in the cytoplasm of monocytes or extracellularly. Erythrocytes containing these bodies had vacuoles and irregular pale-staining areas in the cytoplasm and pale-staining areas in the nucleus. These changes corresponded to the viral particles, assembly areas, vacuoles and nuclear changes at the ultrastructural level.

Semipermissive replication of Tipula iridescent virus in Aedes albopictus C6/36 cells

Comparative studies were carried out using two different insect cell lines, Aedes albopictus and Estigmene acrea, for Tipula iridescent virus (TIV) propagation. Light microscope autoradiography showed viral DNA present in viroplasmic centers (VCs) and an inhibition of nuclear DNA synthesis. These VCs appeared to be morphologically similar in both cell lines when examined by light and electron microscopy. Radiolabeled cDNA was synthesized from RNA samples obtained from infected cells at different times after infection and hybridized to TIV DNA digested with various restriction endonucleases. The results indicated that the pattern of transcription and the kinetics of TIV infection were qualitatively similar in both cell lines. The major TIV DNA components, L (greater than 174 kbp) and S1 (10.8 kbp) that are found in virions in approximately equivalent amounts, were made in both infected cell lines. However, the infected cell lines produced S1 DNA at higher levels relative to L than in virions. The cDNA hybridization studies also revealed that the S1 DNA has sequences that are transcribed and are TIV specific. While VC morphology, levels of L and S1 DNA synthesis, transcription, and capsid protein synthesis were similar in both cell lines, time course electron microscope studies revealed that progeny virions were detected only in the VCs of E. acrea cells and not in the VCs of A. albopictus cells, even by 96 hr p.i. These data suggest that the A. albopictus C6/36 cell line is semipermissive for TIV replication.

Ultrastructural, biochemical and biophysical properties of an erythrocytic virus of frogs from Ontario, Canada

Frog erythrocytic virus (FEV), one of the largest icosahedral viruses, is enveloped, measures up to 450 nm in diameter, and contains double stranded DNA. The virus is found in the cytoplasm of erythrocytes of Rana catesbeiana, Rana septentrionalis, and Rana clamitans from Algonquin Park, Ontario (Canada). Acidophilic inclusions in infected erythrocytes stained with Giemsa's stain correspond to viroplasms from which FEV buds and forms aggregates of virus particles as seen in the electron microscope. Frog erythrocytic virus appears to acquire its envelope from lamellar membranes which surround the virus particles. The virus is structurally sensitive to cesium chloride, potassium tartrate and glycerol. It is also sensitive at pH 1 to 5 and a temperature of 56 C for 15 min. The virus contains at least 16 proteins which range in relative molecular mass from 19.5 to 91.0 kilodaltons (kDa), with two major proteins of 31.0 and 43.0 kDa. The viral DNA has a buoyant density of 1.690 +/- 0.005 g/ml, guanine plus cytosine ratio of 25 to 36%, and a melting temperature of 82 to 86 C. Data from this study indicate that FEV should be included in the family Iridoviridae.

[Infection of gypsy moth Porthetria dispar cultured cells with Aedes caspius caspius mosquito iridovirus]

The results of inoculation of gipsy moth cell culture with mosquito iridovirus are presented. As a result of the virus infection the cell monolayer became loosened, and in damaged cells, numerous cytoplasmic processes appeared. Virus virions were found in the cytoplasm of the involved cells.

Fish viruses: isolation of an icosahedral cytoplasmic deoxyribovirus from sheatfish (Silurus glanis)

An icosahedral cytoplasmic deoxyvirus has been isolated from moribund sheatfish (Silurus glanis) fry of a commercial warm water recirculation aquaculture unit with cumulative mortalities of up to 100%. The agent replicated in BF-2 and in FHM cells at 20-30 degrees C producing cytoplasmatic inclusion bodies followed by lysis of the cells. The DNA containing virus proved to be labile to chloroform. Infected BF-2 cells revealed hexagonal particles in the cytoplasm measuring about 125-135 nm in diameter. The virus consisted of a central electron-dense core and a electron-translucent zone. The isolate shares characteristics with the Iridoviridae.

Association of African swine fever virus with the cytoskeleton

The association of African swine fever virus (ASFV) with the cytoskeleton was investigated. Immunofluorescent studies of ASFV infected cells with anti-ASFV serum showed a temporal and spatial development of viral inclusions which moved from a peripheral to a perinuclear location and fused to give a single large perinuclear factory. The migration and fusion of viral inclusions was inhibited by colchicine suggesting a function for microtubules in assembly site organization not previously described. Accumulation of virions outside the inclusions and inhibition of viral release was also observed in colchicine treated cells. Viral antigens and structural elements were retained on the cytoskeleton fraction of Triton X-100 extracted cells. Reorganization of cytoskeletal elements around the assembly sites was demonstrated by transmission electronmicroscopy and by immunofluorescent studies using monoclonal antibodies against actin, tubulin and vimentin. Intermediate filaments accumulated around the viral factories, microtubules were greatly decreased in number and microfilaments were reorganized in association with the plasma membrane. Bundles of 15 nm tubules of unknown origin were also observed around the assembly sites. The distribution of viral proteins in soluble, cytoskeleton and detergent insoluble nuclear fractions was studied by pulse-chase experiments with [35S]methionine. SDS-PAGE analysis showed the presence in the cytoskeletal and nuclear fractions of 150, 72, 38, 28, 19 and 15 kDa virus structural proteins which increased after a 5 h chase. Our results indicate a close association of ASFV replication with the cytoskeleton similar to events described during FV3 replication but which differ from those occurring in poxvirus-infected cells.

Further characterization of an alkaline protease activity associated with iridescent virus type 6. Brief report

Iridescent virus type 6 infecting Galleria mellonella larvae contained an associated alkaline protease activity which appeared to be tightly bound to the virions and essentially localized on the outside of the viral particle. Under alkaline conditions the virus was degraded by proteolytic cleavage of viral envelope proteins. Proteolytic activity was not present in virus propagated in a permissive insect cell line or when purified from isolated larval fat-bodies instead from whole larvae. The results suggest that the protease associated with Iridescent virus type 6 is of larval origin.

Biochemical and structural studies of fish lymphocystis disease virions isolated from skin tumours of Pleuronectes

Fish lymphocystis disease viruses (FLDV) were isolated directly from lymphocystis disease lesions of various flatfish species and further purified. Subunits could be identified only after the purified virus was disrupted. In combination with different types of treatment, Nonidet-P40, dithiothreitol, proteases digestion and after ultrasonication and ultracentrifugation, the inner region of FLDV was studied. The purified virus was used for isolation of the virus nucleoid and for further study of the viral genome. Contour length measurements of 20 DNA molecules gave an average length of 40.44 +/- 3.2 micron. Lines of precipitation between isolated nucleoid material and FLDV-antibodies were shown by immunoelectrophoresis.

Analysis of the structure of fish lymphocystis disease virions from skin tumours of pleuronectes

Virions of fish lymphocystis disease (FLDV) from tumour-carrying-fishes (flounder, dab, plaice and gurnard) collected in the North Sea were isolated directly from the tumours and purified by sucrose and subsequent caesium chloride gradient centrifugation. They were studied by electron microscopy using embedding methods, negative staining and using metal shadowing methods. Tumours of dermal connective fish tissue showed particles with hexagonal outlines. Occasionally, an "empty structure" was observed. Subunits of the virus membrane were identified after 3 per cent phosphotungstic acid (PTA) or 0.2 per cent Ruthenium red (RR) staining. FLDV measures 199-227 nm in diameter.

Ultrastructural and biochemical evidence of the trimeric nature of frog virus 3 (FV3) six-coordinated capsomers

Image analysis of freeze-etch replicas of cylindrical aberrant forms of FV3 provided evidence for three morphological subunits protruding from the six-coordinated capsomers. Negatively stained capsomers displayed both triangular and hexagonal profiles which suggests that their innermost portion is pseudohexagonal. Images from underfocused micrographs of capsomers are indicative of a central channel. The trimeric nature of the capsomer has been established by electrophoresis in the presence of Triton X-100, which showed that the molecular weight of the nondissociated capsomer is about 140,000 whereas that of the polypeptide itself is 48,000. This trimeric association does not occur via disulfide bonds, and inside the capsomers there are no free amino groups accessible to the usual bifunctional reagents. Thus, the chemical nature of the interpolypeptide bonds inside the trimers is still unknown. We have previously estimated the triangulation number (T) of FV3 to be 147 or 133 (Darcy-Tripier et al., 1984). The present study, using optical diffraction of the facets of FV3, allowed a better determination of the angle of skewness and is in favor of T = 133 (h = 9, k = 4, 18 degrees).

The organization of frog virus 3 as revealed by freeze-etching

A variety of freeze-fracture techniques has been employed in this study with the aim of dissecting the frog virus 3 virion and obtaining further information about its architecture. The icosahedral capsid has a skew symmetry with a triangulation number of 133 or 147. The capsomers are closely packed with a center-to-center spacing of 72 A. The inner membrane contains transmembrane proteins which appear as intra-membranous particles on both fracture faces. Rod-like structures (about 100 A in diameter) are present in the virus interior suggesting that the DNA-protein complex is highly organized.

The role of DNA methylation in virus replication: inhibition of frog virus 3 replication by 5-azacytidine

Frog virus 3 (FV3) DNA is the most highly methylated DNA of any known DNA virus; about 20% of the cytosine residues in FV3 DNA are methylated (D. Willis and A. Granoff, 1980, Virology 107, 250-257). To understand the role of DNA methylation in virus replication, we have examined the effect of 5-azacytidine, a drug that inhibits DNA methylation. 5-Azacytidine (10 microM) reduced the production of infectious FV3 by 100-fold or more and inhibited methylation of viral DNA by about 80%. Inhibition of DNA methylation did not affect viral gene expression since there was no detectable inhibition of virus-specific RNA or protein synthesis in 5-azacytidine-treated cells. In contrast, the size of the replicating DNA measured under completely denaturing conditions, was much smaller than that found during infection in the absence of drug. These results suggest that the undermethylated DNA was susceptible to endodeoxyribonuclease(s). Additionally, electron microscopic examination of FV3-infected, 5-azacytidine-treated cells revealed that preformed capsids remained empty or were only partially filled with viral DNA. Based on these data, it is suggested that methylation of DNA protects it from endonucleolytic cleavage and that the integrity of genomic DNA is required for its proper packaging into virions.

Interaction of frog virus 3 with the cytomatrix. II. Structure and composition of the virus assembly site

We have described the structure of virus assembly sites in frog virus 3-infected tissue culture cells based on an examination of sectioned and whole cells by conventional and high voltage (1 000 kV) electron microscopy (HVEM), respectively. We have also attempted to identify the cellular and viral components within the assembly sites using immunofluorescence and a combination of DNase digestion and EM autoradiography. Immunofluorescence studies showed that the sites do not contain tubulin, vimentin, actin, or myosin, confirming EM and HVEM studies which showed the absence of these cytoskeletal filaments from these sites. Enzymatic digestion and autoradiographic studies identified viral DNA in clumps of electron-dense material which are suspended within the matrix of the assembly site. Examination of whole, virus-infected cells by HVEM revealed that the matrix of the assembly site appears as a compaction of the cytoplasmic matrix and appears to be continuous with it. The fine strands of cytomatrix, equated with the microtrabeculae of other cells, are covered with or contain granules measuring 6-8 nm; similar granules also populate the virus assembly sites. Disruption of purified FV 3 with chloroform yielded capsomeres measuring 6-8 nm. Based on all these observations, we postulate that the virus assembly sites are regions of the cytomatrix specialized for virus assembly and that the viral components are transported along the cytomatrix to the assembly sites.

Morphology and ultrastructure of Lymphocystis disease virus, a fish iridovirus, grown in tissue culture

The morphology and the ultrastructure of the Lymphocystis disease virus (LDV) strain Leetown , a fish iridovirus , was studied by electron microscopy. The virus was grown on bluegill fry (BF-2) cells at 21 degrees. LDV showed an icosahedral shape by ultrathin sections and negative staining, with a diameter of about 200 nm. The shell of the virion seemed to be composed of two unitary membranes with a total diameter of 16 nm. The outer membrane demonstrated swelling in negative staining, exhibiting a central core and the presence of globular subunits at its external surface organized in geometrical arrays of 60 and 90 degrees. Glutaraldehyde fixation preserved very effectively the icosahedral structure with the subunits remaining invisible. The internal structure of the virions was composed of osmiophilic threads or granules of 6 to 8 nm in diameter surrounded by an amorphous material of 10 to 20 nm in thickness. External filaments were observed at the surface of the particles in ultrathin sections, giving the appearance of a halo surrounding the shell. In negative staining these filaments were rarely observed; in one virus preparation, they appeared in bundles.

General morphology and capsid fine structure of African swine fever virus particles

The structure of African swine fever virus particles has been examined by electron microscopy. The analysis of virions prepared by negative staining, thin sectioning, and freeze-drying and shadowing showed that the virus particle was composed of several concentric structures with an overall icosahedral shape. The inner region of the virus particles was a nucleoid that was surrounded by a membrane covered by the capsid. The capsid had side-to-side dimensions of 172 to 191 nm and was built up by capsomers arranged in an hexagonal lattice. Computer-filtered electron micrographs of either negatively stained or freeze-dried and shadowed capsids revealed capsomers with a hexagonal outline and a hole in the center. The intercapsomer distance ranged from 7.4 to 8.1 nm. The triangulation number of the capsid was estimated to be 189 to 217, indicative of 1892 to 2172 capsomers. Extracellular African swine fever virus particles had an external membrane that resembled the cytoplasmic unit membrane.

Structural and thermodynamic investigation of the Chilo iridescent virus (Iridovirus type 6)

Structural features and thermodynamic parameters of the complete Chilo iridescent virus (Iridovirus type 6) and its constituents, isolated from the larvae of Galleria mellonella, were evaluated by means of UV spectroscopy and microcalorimetry. It can be demonstrated that the viral DNA is attached to the coat protein in a chromatin-like fashion, which is preserved after disruption of the virus by low temperature or partial digestion of the coat protein with proteinase K. At elevated temperature however the viral DNA is denaturated irreversibly. The coat protein appears to remain in its native state during the DNA transition and its own thermal denaturation profile shows its independence from the DNA denaturation.

Analysis of the genome of fish lymphocystis disease virus isolated directly from epidermal tumours of pleuronectes

Virions of fish lymphocystis disease virus (FLDV), a member of the iridovirus family, were isolated directly from lymphocystis disease lesions of individual flatfishes and purified by sucrose and subsequent cesium chloride gradient centrifugation to homogeneity as judged by electron microscopy. The isolated FLDV DNAs appear to be heterogeneous in size. Contour length measurements of 43 DNA molecules gave an average length of 49 +/- 23 microns, corresponding to 93 +/- 44 X 10(6) D. Molecular weight estimations of FLDV DNA by restriction enzyme analysis resulted in only 64.8 X 10(6) D indicating an excess length of the DNA of about 50%. FLDV DNA was sensitive to lambda 5'-exonuclease and to E. coli 3'-exonuclease III without preference of any one terminal DNA restriction fragment. Denaturation and reannealing experiments of FLDV DNA resulted in the formation of circular DNA molecules of 34.25 microns contour length (= 65.22 X 10(6) D). This result suggests that FLDV DNA contains directly repeated sequences at both ends and that it is terminally redundant. FLDV DNA is methylated in cytosine. FLDV DNA did not hybridize with frog virus DNA indicating that the two iridoviruses are not closely related to each other. Restriction enzyme analysis and Southern blot hybridizations revealed that FLDV isolates can be classified into two different strains: FLDV strain 1 occurs in flounders and plaice, whereas strain 2 is usually found in lesions of dabs.