Lymphocystivirus is one of five genera

Genus

In biology, a genus is a low-level taxonomic rank used in the biological classification of living and fossil organisms, which is an example of definition by genus and differentia...

 of virus

Virus

A virus is a small infectious agent that can replicate only inside the living cells of organisms. Viruses infect all types of organisms, from animals and plants to bacteria and archaea...

es within the viral family Iridoviridae

Iridoviridae

Iridoviridae are a family of viruses with double stranded DNA genomes. The name is derived from Iris the Greek goddess of the rainbow. This name was chosen because of the "rainbow like" iridescence observed in heavily infected insects and pelleted samples of invertebrate iridoviruses.-Taxonomy:The...

, and one of three genera within this family which infect teleost

Teleostei

Teleostei is one of three infraclasses in class Actinopterygii, the ray-finned fishes. This diverse group, which arose in the Triassic period, includes 20,000 extant species in about 40 orders; most living fishes are members of this group...

 fish

Fish

Fish are a paraphyletic group of organisms that consist of all gill-bearing aquatic vertebrate animals that lack limbs with digits. Included in this definition are the living hagfish, lampreys, and cartilaginous and bony fish, as well as various extinct related groups...

es, along with Megalocytivirus

Megalocytivirus

Megalocytivirus is one of five genera of viruses within the family Iridoviridae and one of three genera within this family which infect teleost fishes, along with Lymphocystivirus and Ranavirus. The megalocytiviruses are an emerging group of closely related dsDNA viruses which cause systemic...

and Ranavirus

Ranavirus

Ranavirus is one of five genera of viruses within the family Iridoviridae, one of the five families of nucleocytoplasmic large DNA viruses . Ranavirus is the only genus within Iridoviridae that includes viruses that are infectious to amphibians and reptiles, and one of only three genera within...

. Lymphocystiviruses infect more than 140 freshwater

Freshwater

Fresh water is naturally occurring water on the Earth's surface in ice sheets, ice caps, glaciers, bogs, ponds, lakes, rivers and streams, and underground as groundwater in aquifers and underground streams. Fresh water is generally characterized by having low concentrations of dissolved salts and...

 and marine

Seawater

Seawater is water from a sea or ocean. On average, seawater in the world's oceans has a salinity of about 3.5% . This means that every kilogram of seawater has approximately of dissolved salts . The average density of seawater at the ocean surface is 1.025 g/ml...

 species, spanning at least 42 host families worldwide, causing the chronic, self-limiting clinical disease, lymphocystis

Lymphocystis

Lymphocystis is a common viral disease of freshwater and saltwater fish.Aquarists often come across this virus when their fish are stressed such as when put into a new environment and the virus is able to grow....

.
While lymphocystis does not cause mass mortality events like megalocytiviruses and ranaviruses, fish with lymphocystis exhibit grossly visible papilloma-like skin lesions which substantially reduce their commercial value. No vaccines are currently available for lymphocystis viruses.

Taxonomy

The genus Lymphocystivirus has at least two viral species: Lymphocystis disease virus 1 (LCDV-1) and Lymphocystis disease virus 2 (LCD-2). LCDV-1 infects European flounder

European flounder

The European flounder is a flatfish of European coastal waters from the White Sea in North to the Mediterranean and the Black Sea in South. Introduced into the USA and Canada accidentally through transport in ballast water. It is a well-known food fish.-Description:The European flounder is oval in...

 Platichthys flesus and European plaice (Pleuronectes platessa). LCDV-2 infects the common dab (Limanda limanda). A third species Lymphocystis disease virus C has also been proposed based on the relative lack of sequence similarity of an isolate from the olive flounder (Paralichthys olivaceous) to LCDV-1 and LCDV-2.

LCDV Genome

Lymphocystiviruses are Group I viruses with a dsDNA genome

Genome

In modern molecular biology and genetics, the genome is the entirety of an organism's hereditary information. It is encoded either in DNA or, for many types of virus, in RNA. The genome includes both the genes and the non-coding sequences of the DNA/RNA....

. The LCDV-1 genome is approximately 102.7 kilobase pairs (kbp) in length, with 195 potential open reading frames (ORF), and codes for two DNA-dependent RNA polymerase

RNA polymerase

RNA polymerase is an enzyme that produces RNA. In cells, RNAP is needed for constructing RNA chains from DNA genes as templates, a process called transcription. RNA polymerase enzymes are essential to life and are found in all organisms and many viruses...

 subunits, a DNA methyltransferase

DNA methyltransferase

In biochemistry, the DNA methyltransferase family of enzymescatalyze the transfer of a methyl group to DNA. DNA methylation serves a wide variety of biological functions...

, a DNA polymerase

DNA polymerase

A DNA polymerase is an enzyme that helps catalyze in the polymerization of deoxyribonucleotides into a DNA strand. DNA polymerases are best known for their feedback role in DNA replication, in which the polymerase "reads" an intact DNA strand as a template and uses it to synthesize the new strand....

, a guanosine triphosphate phosphohydrolase (GTPase

GTPase

GTPases are a large family of hydrolase enzymes that can bind and hydrolyze guanosine triphosphate . The GTP binding and hydrolysis takes place in the highly conserved G domain common to all GTPases.-Functions:...

), a helicase

Helicase

Helicases are a class of enzymes vital to all living organisms. They are motor proteins that move directionally along a nucleic acid phosphodiester backbone, separating two annealed nucleic acid strands using energy derived from ATP hydrolysis.-Function:Many cellular processes Helicases are a...

, protein kinase

Protein kinase

A protein kinase is a kinase enzyme that modifies other proteins by chemically adding phosphate groups to them . Phosphorylation usually results in a functional change of the target protein by changing enzyme activity, cellular location, or association with other proteins...

s, a ribonucleoside diphosphate reductase, and zinc-finger proteins, among others. The LCDV-2 genome is similar to that of LCDV-1 but is slightly smaller, approximately 98 kilobase pairs (kbp) in length.

Structure and Replication

Megalocytiviruses are large icosahedral DNA viruses measuring from 198-227 nm in diameter (in some cases as large as 380) nm in diameter with a large single linear dsDNA genome

Genome

In modern molecular biology and genetics, the genome is the entirety of an organism's hereditary information. It is encoded either in DNA or, for many types of virus, in RNA. The genome includes both the genes and the non-coding sequences of the DNA/RNA....

. The main structural component of the protein

Protein

Proteins are biochemical compounds consisting of one or more polypeptides typically folded into a globular or fibrous form, facilitating a biological function. A polypeptide is a single linear polymer chain of amino acids bonded together by peptide bonds between the carboxyl and amino groups of...

 capsid

Capsid

A capsid is the protein shell of a virus. It consists of several oligomeric structural subunits made of protein called protomers. The observable 3-dimensional morphological subunits, which may or may not correspond to individual proteins, are called capsomeres. The capsid encloses the genetic...

 is the major capsid protein (MCP)

Lymphocystiviruses attach to the host cell and enter by receptor-mediated endocytosis

Receptor-mediated endocytosis

Receptor-mediated endocytosis , also called clathrin-dependent endocytosis, is a process by which cells internalize molecules by the inward budding of plasma membrane vesicles containing proteins with receptor sites specific to the molecules being internalized.-Process:After the binding of a...

 similar to other iridoviruses. Viral particles are uncoated and move to the nucleus

Cell nucleus

In cell biology, the nucleus is a membrane-enclosed organelle found in eukaryotic cells. It contains most of the cell's genetic material, organized as multiple long linear DNA molecules in complex with a large variety of proteins, such as histones, to form chromosomes. The genes within these...

 of the cell, where DNA replication

DNA replication

DNA replication is a biological process that occurs in all living organisms and copies their DNA; it is the basis for biological inheritance. The process starts with one double-stranded DNA molecule and produces two identical copies of the molecule...

 begins via a virally encoded DNA polymerase

DNA polymerase

A DNA polymerase is an enzyme that helps catalyze in the polymerization of deoxyribonucleotides into a DNA strand. DNA polymerases are best known for their feedback role in DNA replication, in which the polymerase "reads" an intact DNA strand as a template and uses it to synthesize the new strand....

. Viral DNA then moves to the cytoplasm for the second stage of DNA replication, which results in the formation of DNA concatemers. The concatameric viral DNA is subsequently packaged via a headful mechanism into virions. The lymphocystis viral genome is circularly permuted with terminally redundant DNA

Terminally redundant DNA

Terminally redundant DNA is DNA that contains repeated sequences at each end called terminal repeats. These ends are used to join the ends of the linear DNA to form a cyclic DNA....

.

Pathogenesis

Lymphocystis disease is a chronic disease that rarely causes mortality. Infection causes transformation and hypertrophy (approximately 1000x) of cells in the dermis, forming grossly visible lymphocystis nodules, as well as transformation and hypertrophy in cells of the connective tissues of various internal organs. Fibroblasts and osteoblasts are specifically targeted by the virus. Lymphocystis viruses are not easily grown in cell culture

Cell culture

Cell culture is the complex process by which cells are grown under controlled conditions. In practice, the term "cell culture" has come to refer to the culturing of cells derived from singlecellular eukaryotes, especially animal cells. However, there are also cultures of plants, fungi and microbes,...

, placing limitations on in vitro

In vitro

In vitro refers to studies in experimental biology that are conducted using components of an organism that have been isolated from their usual biological context in order to permit a more detailed or more convenient analysis than can be done with whole organisms. Colloquially, these experiments...

molecular pathogenesis experiments.

Diagnostic Pathology

As Lymphocystis viruses are not easily grown in cell culture, diagnosis

Diagnosis

Diagnosis is the identification of the nature and cause of anything. Diagnosis is used in many different disciplines with variations in the use of logics, analytics, and experience to determine the cause and effect relationships...

 is based on clinical signs, gross pathology

Gross pathology

Gross pathology refers to macroscopic manifestations of disease in organs, tissues, and body cavities. The term is commonly used by anatomical pathologists to refer to diagnostically useful findings made during the gross examination portion of surgical specimen processing or an autopsy.It's vital...

, histopathology

Histopathology

Histopathology refers to the microscopic examination of tissue in order to study the manifestations of disease...

, serology

Serology

Serology is the scientific study of blood serum and other bodily fluids. In practice, the term usually refers to the diagnostic identification of antibodies in the serum...

, and/or polymerase chain reaction

Polymerase chain reaction

The polymerase chain reaction is a scientific technique in molecular biology to amplify a single or a few copies of a piece of DNA across several orders of magnitude, generating thousands to millions of copies of a particular DNA sequence....

 (PCR)-based molecular

Molecular biology

Molecular biology is the branch of biology that deals with the molecular basis of biological activity. This field overlaps with other areas of biology and chemistry, particularly genetics and biochemistry...

 assay

Assay

An assay is a procedure in molecular biology for testing or measuring the activity of a drug or biochemical in an organism or organic sample. A quantitative assay may also measure the amount of a substance in a sample. Bioassays and immunoassays are among the many varieties of specialized...

s.

Gross pathology

The pathology of lymphocystis consists of papilloma-like skin lesions composed of greatly hypertrophied infected host cells embedded in extracellular matrix, sometimes called lymphocystis tumor cells, which are grossly evident as white spots on the skin and fins of infected fish. These lesions proliferate as epithelial tumors in some cases.

Histopathology

In a recent comparison of lymphocystis histopathology of four unrelated marine species, lesion

Lesion

A lesion is any abnormality in the tissue of an organism , usually caused by disease or trauma. Lesion is derived from the Latin word laesio which means injury.- Types :...

s consistently associated with lymphocystis included hypertrophied

Hypertrophy

Hypertrophy is the increase in the volume of an organ or tissue due to the enlargement of its component cells. It should be distinguished from hyperplasia, in which the cells remain approximately the same size but increase in number...

 cells displaying irregular nuclei, basophilic

Basophilic

Basophilic is a technical term used by histologists. It describes the microscopic appearance of cells and tissues, as seen down the microscope, after a histological section has been stained with a basic dye. The most common such dye is haematoxylin....

 cytoplasm

Cytoplasm

The cytoplasm is a small gel-like substance residing between the cell membrane holding all the cell's internal sub-structures , except for the nucleus. All the contents of the cells of prokaryote organisms are contained within the cytoplasm...

ic inclusion bodies

Inclusion bodies

Inclusion bodies are nuclear or cytoplasmic aggregates of stainable substances, usually proteins. They typically represent sites of viral multiplication in a bacterium or a eukaryotic cell and usually consist of viral capsid proteins...

 that stained positively via Feulgen and Mann's reaction and Periodic acid-Shiff (PAS)-positive hyaline capsules. Hyaline capsules arise from the extracellular matrix that is produced by the infected cells, and are composed of sulphated and carboxylated glycoproteins (acid mucopolysaccharides). In contrast, the inclusion body shape, distribution of viral particles within the cytoplasm and overall appearance of lymphocystis nodules varied by species. The species examined in this study included the the white-spotted puffer (Arothron hispidus), the Japanese sea bass (Lateolabrax japonicus), olive flounder (Paralichthys olivaceus) and the "sting fish" or Schlegel's black rockfish (Sebastes schegeli)

Serology

Several serologic assays have been developed to identify LCDV infections, including flow cytometry

Flow cytometry

Flow cytometry is a technique for counting and examining microscopic particles, such as cells and chromosomes, by suspending them in a stream of fluid and passing them by an electronic detection apparatus. It allows simultaneous multiparametric analysis of the physical and/or chemical...

, immunoblot, and immunofluorescence

Immunofluorescence

Immunofluorescence is a technique used for light microscopy with a fluorescence microscope and is used primarily on biological samples. This technique uses the specificity of antibodies to their antigen to target fluorescent dyes to specific biomolecule targets within a cell, and therefore allows...

. However, PCR-based molecular assays are more practical for most applications.

Electron Microscopy

Transmission electron microscopy

Transmission electron microscopy

Transmission electron microscopy is a microscopy technique whereby a beam of electrons is transmitted through an ultra thin specimen, interacting with the specimen as it passes through...

 (TEM) of infected cells reveals cytoplasmic virus particles typically measuring from 198-227 nm in diameter (in some cases as large as 380 nm) and electron-dense substances in the perinuclear space.

Molecular Pathology

Published PCR primers and protocol are available to amplify a portion of the LCDV-1 MCP. When the PCR diagnostic assay is combined with slot blot, diagnostic sensitivity

Sensitivity and specificity

Sensitivity and specificity are statistical measures of the performance of a binary classification test, also known in statistics as classification function. Sensitivity measures the proportion of actual positives which are correctly identified as such Sensitivity and specificity are statistical...

is increased, facilitating the diagnosis of asymptomatic LCDV-1 infections.