Viruses of amphibia

Presence of ascites in bullfrog breeders reared and kept on a frog farm

ABSTRACT Ascites is a pathology characterized by the extravasation of fluid from blood vessels and its accumulation in the abdominal cavity, caused by several associated factors. In this paper, we report the occurrence of this syndrome in breeding male and female Bullfrogs fed a commercial fish feed with 40% crude protein. Liver malformation was already observed due to lack of protein deposition in adult animals from the same spawning. The culture of ascitic fluid showed absence of bacterial growth. Kidney histopathology showed, in both sexes, a large number of tubular structures with strongly eosinophilic hyaline material suggesting colloid growth in both the cortical region and spinal cord. The analysis also revealed granulomas in various stages of development, many showing central necrotic material. The kidneys, glomerular, were mostly hypoplastic, with enlarged Bowman space, and many were hyalinized or hemorrhagic; the parenchyma showed dystrophic calcification, and many tubules containing fibrinoid material. The liver displayed a large amount of melanomacrophages in the parenchyma and foci of mono-lymphocytic hepatitis and marked cytoplasmic rarefaction, as well as several hepatocytes with pyknotic nuclei and necrotic cells and dissociation of the hepatic trabeculae. Ascites may be caused by the lesions observed in these organs.

Frog Virus 3 Replication: Analysis of Structural and Nonstructural Polypeptides in Infected BHK Cells by Acidic and Basic Two-Dimensional Gel Electrophoresis

Analysis of frog virus 3-infected BHK cells by two-dimensional, acidic and basic gel electrophoresis showed that at least 90 infected cell-specific polypeptides could be detected. These polypeptides represent between 70 and 85% of the coding capacity of the viral genome. The polypeptides were sequentially induced in at least three phases. The virus gradually suppressed host cell polypeptide synthesis during infection, although the synthesis of a few cell polypeptides may be "switched off" early in infection.

Phosphonoacetic Acid inhibition of frog virus 3 replication

Phosphonoacetic acid at concentrations above 200 mug/ml inhibited the replication of frog virus 3 in BHK cells. The inhibition of viral DNA replication observed in these cells was reversible and correlated with the inhibition of the virus-induced DNA polymerase activity in an in vitro assay. The synthesis of frog virus 3-induced late or gamma polypeptides was also inhibited by phosphonoacetic acid, although the early (alpha and beta) polypeptides were unaffected.

Frog virus 3 replication: induction and intracellular distribution of polypeptides in infected cells

The synthesis of the polypeptides induced in frog virus 3-infected cells was analyzed by high-resolution sodium dodecyl sulfate-polyacrylamide gel electrophoresis of radiolabeled cell extracts. Purified frog virus 3 contained 22 polypeptides, with molecular weights in the range 9 x 10(3) to 114 x 10(3). All of the structural and an additional seven nonstructural polypeptides were detected in infected cell lysates. The following three classes of induced polypeptides (under temporal control) were observed in BHK cells: at 2 h, four alpha polypeptides; at 4 h, 13 beta polypeptides; and at 6 h, the remaining 12 gamma polypeptides. The total molecular weight of the infected cell-specific polypeptides (ICPs) was approximately 1.5 x 10(6), which accounts for about 30% of the coding capacity of the viral genome. At least 10 of the induced polypeptides were phosphorylated, but none was glycosylated or sulfated. No evidence for posttranslation cleavage of polypeptides in pulse-chase and inhibition experiments was obtained. The synthesis of gamma polypeptides was not detected in the presence of the viral DNA replication inhibitors cytosine arabinoside and hydroxyurea, but halogenated nucleotides apparently had no effect. These results suggest that alpha and beta polypeptides are "early" events and that detectable gamma polypeptide synthesis is dependent on the production of progeny viral DNA. The regulation of frog virus 3-induced polypeptide synthesis in infected BHK cells was examined by using inhibitors of protein and RNA synthesis and amino acid analogs. These experiments confirmed the existence of three sequentially synthesized, coordinately regulated classes of polypeptides, designated alpha, beta, and gamma. The requirements for the synthesis of each class were as follows: (i) alpha polypeptides did not require previous cell protein synthesis; (ii) beta polypeptides required a prescribed period of alpha polypeptide synthesis and new mRNA synthesis; and (iii) gamma polypeptides required prior synthesis of functional beta polypeptides and new mRNA synthesis. alpha polypeptide synthesis was controlled by beta and gamma polypeptides, and alpha and beta polypeptides were involved in the suppression of host cell polypeptide synthesis. Indirect evidence was obtained for the temporal regulation of frog virus 3 transcription. The intracellular distribution of virus-induced polypeptides in cells infected with frog virus 3 was investigated by using standard cell fractionation techniques. Most of the 29 induced polypeptides were bound to structures within the nucleus, and only two ICPs were not associated with purified nuclei. When isolated nuclei were incubated in an infected cell cytoplasm preparation, all of the nuclear ICPs were incorporated in vitro. All of the ICPs were associated with ribosomal and rough endoplasmic reticulum fractions of infected cells, and a number of ICPs were found on smooth intracellular membranes. Most of the ICPs were also associated with purified plasma membranes of infected cells, and one polypeptide (ICP 58) was highly enriched in the plasma membrane compared with whole cell extracts or purified frog virus 3.

Antibody dependent enhancement of frog virus 3 infection

Background

Viruses included in the family Iridoviridae are large, icosahedral, dsDNA viruses that are subdivided into 5 genera. Frog virus 3 (FV3) is the type species of the genus Ranavirus and the best studied iridovirus at the molecular level. Typically, antibodies directed against a virus act to neutralize the virus and limit infection. Antibody dependent enhancement occurs when viral antibodies enhance infectivity of the virus rather than neutralize it.

Results

Here we show that anti-FV3 serum present at the time of FV3 infection enhances infectivity of the virus in two non-immune teleost cell lines. We found that antibody dependent enhancement of FV3 was dependent on the Fc portion of anti-FV3 antibodies but not related to complement. Furthermore, the presence of anti-FV3 serum during an FV3 infection in a non-immune mammalian cell line resulted in neutralization of the virus. Our results suggest that a cell surface receptor specific to teleost cell lines is responsible for the enhancement.

Conclusions

This report represents the first evidence of antibody dependent enhancement in iridoviruses. The data suggests that anti-FV3 serum can either neutralize or enhance viral infection and that enhancement is related to a novel antibody dependent enhancement pathway found in teleosts that is Fc dependent.

Expression of frog virus 3 genes is impaired in mammalian cell lines

Frog virus 3 (FV3) is a large DNA virus that is the prototypic member of the family Iridoviridae. To examine levels of FV3 gene expression we generated a polyclonal antibody against the FV3 protein 75L. Following a FV3 infection in fathead minnow (FHM) cells 75L was found in vesicles throughout the cytoplasm as early as 3 hours post-infection. While 75L expressed strongly in FHM cells, our findings revealed no 75L expression in mammalian cells lines despite evidence of a FV3 infection. One explanation for the lack of gene expression in mammalian cell lines may be inefficient codon usage. As a result, 75L was codon optimized and transfection of the codon optimized construct resulted in detectable expression in mammalian cells. Therefore, although FV3 can infect and replicate in mammalian cell lines, the virus may not express its full complement of genes due to inefficient codon usage in mammalian species.

Isolation of lymphocystis disease virus from sole, Solea senegalensis Kaup, and blackspot sea bream, Pagellus bogaraveo (Brunnich)

Two viruses were isolated from cultured sole, Solea senegalensis, and wild blackspot sea bream, Pagellus bogaraveo, and preliminarily characterized as lymphocystis disease viruses (LCDVs). Viral isolates were characterized by morphological, biochemical and biophysical properties. In addition, the susceptibility of four fish cell lines was also tested. LCDV isolates developed cytopathic effects on the SAF-1 cell line at 5 and 6 days post-infection and reached titres of 10(6) TCID50 mL(-1). The antigenic and structural protein analysis of the two new LCDV isolates showed identical profiles to that obtained for LCDV strain Leetown NFH (ATCC VR-342), used as a reference viral strain, and for an LCDV isolate collected from gilt-head sea bream, Sparus aurata, cultured in southern Spain. Molecular confirmation was performed by polymerase chain reaction. Specific primers for LCDV produced a 270-bp DNA fragment, the expected size for LCDV.

Comparative genomic analyses of frog virus 3, type species of the genus Ranavirus (family Iridoviridae)

Frog virus 3 (FV3) is the type species member of the genus Ranavirus (family Iridoviridae). To better understand the molecular mechanisms involved in the replication of FV3, including transcription of its highly methylated DNA genome, we have determined the complete nucleotide sequence of the FV3 genome. The FV3 genome is 105903 bp long excluding the terminal redundancy. The G + C content of FV3 genome is 55% and it encodes 98 nonoverlapping potential open reading frames (ORFs) containing 50-1293 amino acids. Eighty-four ORFs have significant homology to known proteins of other iridoviruses, whereas twelve of these unique FV3 proteins do not share homology to any known protein. A microsatellite containing a stretch of 34 tandemly repeated CA dinucleotide in a noncoding region was detected. To date, no such sequence has been reported in any animal virus.

Sensitivity of Invertebrate iridescent virus 6 to organic solvents, detergents, enzymes and temperature treatment

The sensitivity of Invertebrate iridescent virus 6 (IIV-6) to a selection of organic solvents, detergents, enzymes and heat treatment was assayed in Spodoptera frugiperda (Sf9) cells and by injection of inoculum into larvae of Galleria mellonella. In several cases, the degree of sensitivity of the virus depended on the method of assay; cell culture assays indicated greater losses of activity than insect bioassay. IIV-6 was sensitive to chloroform but sensitivity to ether was only detected by cell culture assay. Sensitivity (defined as a reduction of at least 1 log activity) was detected following treatment by 1 and 0.1% SDS, 1% Triton-X100, 70% ethanol, 70% methanol, 1% sodium deoxycholate, pH 11.1 and 3.0. No sensitivity was detected to 1% Tween 80, 1 M MgCl2, 100 mM EDTA, lipase, phospholipase A2, proteinase K, or trypsin at the concentrations tested. Viral activity was reduced by approximately 4 logs following heating to 70 degrees C for 60 min or 80 degrees C for 30 min. The above observations highlight the need for studies on the role of the virus lipid component in the process of particle entry into cells, and may explain why vertebrate and invertebrate iridoviruses have been reported to differ in their sensitivity to organic solvents and enzymes.

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.

DNA-dependent RNA polymerase activity associated with subviral particles of polyhedral cytoplasmic dexoyribovirus

Polyhedral cytoplasmic deoxyribovirus virions contain a DNA-dependent RNA polymerase which catalyzes the incorporation of ribonucleotides into an acid-precipitable product. Treatment of virions with sodium deoxycholate and dithiothreitol resulted in the formation of subviral particles which could be separated from virions by rate zonal centrifugation in sucrose gradients. Subviral particles were RNA polymerase-positive and more active per unit mass of protein than virions. In vitro enzyme activity associated with subviral particles required addition of ribonucleotides, Mg(2+), and exogenous denatured DNA template. Optimal enzyme activity occurred over a broad pH (7.2 to 8.8) and Mg(2+) concentration (2 to 10 mumol) range. The specific activity of the RNA polymerase was maximal at 37 C. Addition of DNase or actinomycin D to the reaction mixture reduced the incorporation of [(3)H]UMP into an acid-precipitable product. The product of the reaction was sensitive to degradation by RNase but not to DNase or Pronase. These data suggest that the enzyme copies DNA into RNA.

Fish rhabdovirus replication in non-piscine cell culture: new system for the study of rhabdovirus-cell interaction in which the virus and cell have different temperature optima

The replication of three rhabdoviruses associated with diseases of fish has been demonstrated in cells of continuously cultivated non-piscine cell lines. Spring viremia of carp (SVC) virus and the salmonid fish viruses, Egtved and infectious hematopoietic necrosis virus, all replicated in mammalian WI-38 (human diploid cell strain) and BHK/21 cells and in cells of one or more reptilian cell lines at the temperatures commonly used to propagate these viruses in fish cells. The infections were cytopathic: SVC virus plaque assays may be performed in several types of mammalian cell culture. "Autointerference" apparently mediated by abortive "T" particle formation was observed during serial nondiluted passages of SVC virus in BHK/21 and TH1 cells, but not in RTG-2 or WI-38 cells. Optimal temperatures for replication of SVC and Egtved viruses in BHK/21 cells were identical to those determined in poikilothermic vertebrate cell cultures. However, these viruses replicated relatively more efficiently at suboptimal temperatures in "cold-blooded" vertebrate cells than in the hamster cells. Studies of [(3)H]uridine incorporation into uninfected BHK/21 cells incubated at different temperatures revealed that [(3)H]uridine uptake is sharply reduced at temperatures below 24.5 C. Growth curve studies of SVC virus in BHK/21 cells incubated at 23 C revealed that a clear-cut large excess of virus-induced [(3)H]uridine incorporation could be demonstrated in the absence of actinomycin D. Actinomycin D treatment (1 mug/ml) led to efficient inhibition of control cell [(3)H]uridine uptake, but also markedly reduced the total counts per minute of virus-induced [(3)H]uridine uptake, without depressing the yield of released infectious virus. Actinomycin D added to SVC virus-infected BHK/21 cell cultures at concentrations as low as 0.01 mug/ml caused a significant decrease in the level of virus-induced [(3)H]uridine uptake, despite the fact that this concentration is insufficient to efficiently suppress "background" cellular [(3)H]uridine incorporation.

Failure of rifampin to inhibit frog polyhedral cytoplasmic deoxyribovirus multiplication

Resistance of frog virus multiplication to rifampin suggests that components peculiar to cytoplasmic deoxyribonucleic acid replicating viruses (e.g., poxvirus) are not equally sensitive to rifampin.

Lipids in Viruses

This chapter discusses lipids in viruses. Lipid forms an integral part of many viruses and exists either in the form of a continuous envelope or in lipoprotein complexes that surround a nucleoprotein core or helix. In general, the envelope can be described as a molecular container for the genetic material of the virus. Viruses are obligate intracellular parasites and are not known to carry genetic coding for enzymes involved in lipid synthesis. Hence, they generally contain the same classes of lipid as are found in the host cell or their membrane of assembly. Lipids make up 20–35% by weight of most viruses; however, there are exceptions such as vaccinia virus, which has only 5% lipid despite having a complex multimembrane envelope structure. Naked herpesvirus capsids closely resemble non-lipid-containing viruses such as adenovirus or polyoma virus, which are also assembled in the nucleus but show full infectivity without any envelope. Both naked and enveloped herpesvirus particles are found in infected cells; however, only enveloped particles are found in extracellular fluids.

Iguana virus, a herpes-like virus isolated from cultured cells of a lizard, Iguana iguana

An agent cytopathic for Terrapene and Iguana cell cultures was isolated from spontaneously degenerating cell cultures prepared from a green iguana (Iguana iguana). The agent, designated iguana virus, caused a cytopathic effect (CPE) of a giant cell type, with eosinophilic inclusions commonly observed within giant cell nuclei. Incubation temperature had a marked effect on CPE and on virus release from infected cells. Within the range of 23 to 36 C, low temperatures favored CPE characterized by cytolysis and small giant cell formation, and significant virus release was observed. At warmer temperatures, a purely syncytial type of CPE and total absence of released virus were noted. A unique type of hexagonal eosinophilic cytoplasmic inclusion was observed within syncytia of infected Terrapene cell cultures incubated at 36 C. In vivo studies revealed no evidence of pathogenicity of iguana virus for suckling mice, embryonated hen's eggs, or several species of reptiles and amphibians. Inoculation of iguana virus into young iguanas consistently caused infection that was "unmasked" only when cell cultures were prepared directly from the infected animal. Filtration studies revealed a virion size of >100 nm and <220 nm. Iguana virus is ether-sensitive and, as presumptively indicated by studies of inhibition by bromodeoxyuridine, possesses a deoxyribonucleic type of nucleic acid. The virus characteristics described, as well as electron microscopy observations described in a separate report, indicate that iguana virus is a member of the herpesvirus group.