Family: Sedoreoviridae

Genus: Cardoreovirus

Distinguishing features

The cardoreovirus genome consists of 12 segments of linear dsRNA. Eirocheir sinensis reovirus (ESRV), a virus of the only species in the genus, Eriocheir sinensis reovirus, was isolated from a Chinese mitten crab (Eirocheir sinensis H. Milne-Edwards, 1853). Two unclassified but clearly related viruses Macropipus depurator reovirus (MDRV) and Carcinus mediterraneus reovirus (CMRV), were also isolated from crabs (Montanie et al., 1993). The relationship between these three viruses is uncertain. All three viruses have genome electrophoretic profiles that are distinct from those of other members of the order Reovirales that have 12-segmented genomes, namely phytoreoviruses (plant viruses) and seadornaviruses (insect-borne arboviruses). Comparison of the ESRV RNA-directed RNA polymerase with that of other reoviruses reveals only a distant relationship with seadornaviruses (amino acid identity 27%).

Intact virions have capsomeric structures by negative-staining and transmission electron microscopy (TEM). Replication is accompanied by production of filaments, tubules and viral inclusion bodies (VIB).

Virion

Morphology

Purification of ESRV or CMRV particles on CsCl, and analysis by TEM and negative-staining, reveals intact icosahedral particles about 70 nm in diameter with a morphology typical of non-­turreted reovirus cores. In particular, the morphology of ESRV is similar to that observed for the seadornaviruses and orbiviruses. Negative-staining shows subunits of 8 to 9 nm in diameter on the particle surface. Empty particles have a hexagonal electron-dense (stained) centre, 50 nm in diameter, surrounded by an 8 to 10 nm thick capsid. Some of the empty capsids appear as two concentric layers of similar thickness. The double layered core is surrounded by a further outer-capsid layer. Cores of particles containing RNA are about 55 nm in diameter.

Physicochemical and physical properties

The buoyant density of ESRV viral particles in CsCl is 1.39 g cm⁻³ for full particles and 1.29 g cm⁻³ for empty particles.

Nucleic acid

The genome consists of 12 segments of linear dsRNA that are numbered in order of reducing Mr, or increasing electrophoretic mobility during agarose gel electrophoresis (AGE). The genome comprises approximately 23,300 bp. The AGE electropherotype follows a 3-4-2-3 pattern. The terminal sequences of ESRV Seg1 are 5′-GGAUUUAAAA ….AUAACAGAC-3′.

Proteins

The structural proteins of CMRV can be separated by SDS-PAGE and six proteins have been identified by silver staining, with masses of 120, 94, 76, 44, 32 and 24 kDa.

Genome organization and replication

Replication of cardoreoviruses occurs in the cytoplasm of crab cells. During the early stages of infection, virions appear within viral inclusion bodies (VIB), which are dispersed in several regions of the cytoplasm, and around which viruses accumulate in rosettes. Filaments and tubular structures have also been observed in the cytoplasm of infected cells. Infected tissues appear to be progressively destroyed and their normal structures are replaced by disorganized cell debris and cells derived from blood or connective tissues.

Biology

Cardoreoviruses have only been isolated from diseased crabs, these mostly being of marine species. Experimental inoculation of viruses into healthy crabs causes signs of infection that include absence of aggressiveness, increasing weakness and lack of appetite. Infected crabs survive from 7 to 20 days. Connective tissue of many organs including the hepatopancreas, digestive tract, gills and hematopoietic organs show severe damage. The most obvious lesions are observed in connective tissue surrounding tubules of the hepatopancreas, although all cellular types are progressively destroyed. Connective tissue is replaced by isolated cells, degenerative cells and debris which does not show any organization. Inside these necrotic areas, nodules appear which consist of debris surrounded by aggregated haemocytes. Epithelial cells of the digestive tract, gills and hepatopancreas do not seem to be affected by the virus. No cell culture system that supports virus replication is available.

Species demarcation criteria

There is only one species in the genus and species demarcation criteria have not been defined.

Related, unclassified viruses

Virus names and virus abbreviations are not official ICTV designations.