Genus Lymphocystivirus

Type Species	Lymphocystis disease virus 1	(LCDV-1)

Distinguishing Features

Morphology

Particle size varies from 198-227  nm from Lymphocystis disease virus 1 (LCDV-1) and 200  nm for Lymphocystis disease virus 2 (LCDV-2), although viruses causing very similar diseases in other fish may range from approximately 140 to 380  nm in ultrathin section. The capsid may show a fringe of fibril-like external protrusions which may be short (2.5  nm) or much longer and a double-layered outer enveloped may be present. The core appears to contain a helical tubular filament 13  nm in diameter assumed to be nucleoprotein(s).

Physicochemical and Physical Properties

Virions are heat labile. Infectivity is sensitive to treatment with ether or glycerol.

Nucleic Acid

By restriction endonuclease analysis, the genome length is 102.6  kbp for LCDV-1 and approximately 98  kbp in LCDV-2. Contour length measurements by electron microscopy indicate the DNA molecule to be 146  kbp; the degree of terminal redundancy is approximately 50% but varies considerably among virions. The GC content is 30.7%. Like FV-3, the genome is highly methylated. The presence of 5-methylcytosine occurs at 74% of CpG, 1% of CpC and 2-5% of CpA giving an overall level of methylation of 22%. The complete DNA sequence is known for LCDV-1.

Protein

PAGE analysis under denaturing conditions revealed the presence of 33 polypeptides in LCDV-1 virions isolated directly from flounder tumors; these ranged in weight from Mr 4-220 10³. Analysis of LCDV-2 virions gives a discernibly different pattern of polypeptides supporting the idea that they are distinct species. The major capsid protein is Mr 51.4 10³ comprising 459 amino acid residues. Enzymatic activities associated with purified virions include a viral encoded adenosine triphosphate hydrolase located between core and envelope, a protein kinase and a thymidine kinase. Genome sequence analysis indicated the presence of 38 putative proteins with significant amino acid homologies to proteins of known function in other species. These include a DNA polymerase, a DNA methyltransferase, a type II restriction endonuclease and a putative pyrophosphate-releasing NTPase (with similarity to homologous proteins from African swine fever virus and poxviruses). Coding regions related to viral transcription included two large and one small subunit of DNA dependent RNA polymerase, a putative DNA puff protein, and others. Genes coding for putative enzymes involved in capping or polyadenylation of viral RNA were not detected with the exception of a putative dsRNA-specific ribonuclease. Other regions coded for putative proteins involved in protein processing and modification, disruption of host nucleic acid metabolism, and possible virus-host interaction proteins including a nonfibrillar collagen like protein possibly secreted to form an extracellular capsule and an insulin like growth factor which may be responsible for infected cell hypertrophy. Genome sequence analysis failed to confirm that the virion-associated thymidine kinase is virus encoded.

Lipids

Non-enveloped particles contain up to 17.1% lipid which is readily digested by treatment with phospholipase A₂, suggesting high levels of phospholipid as seen in other members of the family.

Genome Organization and Replication

The LCDV-1 genome contains 195 potential ORFs of which 110 are largely non-overlapping and 38 of these show significant homology to proteins of known function. These 38 ORFs represent 43% of the coding capacity of the genome. The presence of DNA methyl transferase and methyl-sensitive restriction endonuclease with specificity for a CCGG target site may be indicative of restriction-modification system in LCDV-1 capable of degrading host genomic DNA while protecting viral DNA by specific methylation. LCDV-1 DNA contains numerous short direct, inverted and palendromic repetitive sequence elements. Lack of a suitable cell line has hindered studies of LCDV replication. Virus assembly occurs in and around virogenic stroma in the cytoplasm. Crescent shaped capsid precursors develop into fully formed capsids followed by condensation of the core structures. The production and assembly of LCDV-1 has been reported to be seasonally affected.

Antigenic Properties

Not known.

Biological Properties

LCDV-1 infects flounder and plaice while the tentative species LCDV-2 infects dab. Infection results in common, chronic and benign lesions comprising grossly hypertrophied cells occurring mostly in the skin and fins. The disease has been observed in a large number of teleost species although the etiological agents may be other virus species. The duration of infected cell growth and viral proliferation is highly variable (5 days to 9 months, probably temperature dependent) and is followed by rapid degeneration and liberation of infectious virus. Transmission is achieved by contact; external sites, including the gills are the principal portals of entry by the virus. High host population densities and external trauma are believed to enhance transmission. Implantation and injection are also effective routes of transmission. The incidence of disease may be higher in the presence of certain fish ectoparasites. The viruses are considered to be of economic importance in fisheries and fish farms. Both LCDV-1 and LCDV-2 are difficult to culture in vitro although limited growth and plaquing has been reported in several fish cell lines. Many other presumed iridoviruses have been detected in fish populations, sometimes causing important levels of disease and mortality. However, most have not been sufficiently characterized to merit formal inclusion in the family.

List of Species Demarcation Criteria in the Genus

Not applicable.

List of Species in the Genus

Official virus species names are in italics. Tentative virus species names, alternative names ( ), strains or serotypes are not italicized. Virus names and assigned abbreviations ( ) are:

Species in the Genus

Lymphocystis disease virus 1	[L63545]	(LCDV-1)
(Flounder lymphocystis disease virus (FLDV))
(Flounder virus)

Tentative Species in the Genus

Lymphocystis disease virus 2	(LCDV-2)
(Dab lymphocystis disease virus)

List of Unassigned Species in the Family

Goldfish virus 1	(GFV-1)

Phylogenetic Relationships within the Family

Amino acid sequence analysis of the MCP supports the concept of the three genera: Iridovirus, Ranavirus and Lymphocystivirus within the family. No sequence information for species in the Chloriridovirus genus is available. See 4.

Similarity with Other Taxa

The MCP of several members of the family Iridoviridae shares amino acid sequence similarities to African swine fever virus (Asfarviridae) and Paramecium bursaria Chlorella virus 1 (Phycodnaviridae). However, the genetic organization of these families is quite distinct from that of the Iridoviridae.

Derivation of Names

Chloro: from Greek chloros, meaning “green”.

Cysti: from Greek kystis meaning “bladder/sac”.

Irido: from Greek iris, iridos, goddess whose sign was the rainbow, hence iridescent “shining like a rainbow” from the appearance of patently infected invertebrates and centrifuged pellets of virions.

Lympho: from Latin lympha, meaning “water”.

Rana: from Latin rana, meaning “frog”.