A. S. Sahul Hameed​, A. S. Ninawe, T. Nakai, S. C. Chi and K. L. Johnson

Edited by Nick J. Knowles and Stuart G. Siddell

Chapter contents

Posted September 2018

Sarthroviridae: The family

Member taxa

Supporting information

  • Authors - corresponding author: Sahul Hameed, A.S. (
  • Resources - sequence alignments and tree files
  • References


A summary of this ICTV Report chapter has been published as an ICTV Virus Taxonomy Profile article in the Journal of General Virology, and should be cited when referencing this online chapter as follows:

Sahul Hameed, A.S., Ninawe, A.S., Nakai, T., Chi, S.C., Johnson, K.L., and ICTV Report Consortium, 2018, ICTV Virus Taxonomy Profile: Sarthroviridae, Journal of General Virology, 99: 1563–1564.


Extra small virus (XSV) is a satellite virus of Macrobrachium rosenbergii nodavirus (MrNV), an unclassified virus related to members of the family Nodaviridae (Bonami et al., 2005) (Table 1.Sarthroviridae). Both viruses have isometric, spherical virions (Figure 1.Sarthroviridae), infect giant fresh water prawns, and together cause white tail disease, which is responsible for mass mortalities and important economic losses in hatcheries and farms (Bonami and Sri Widada 2011). XSV replication is dependent on that of MrNV and the two viruses are always found together (Bonami and Sri Widada 2011, Zhang et al., 2006).  

Table 1.Sarthroviridae. Characteristics of members of the family Sarthroviridae.



Typical member:

extra small virus (AY247793), species Macrobrachium satellite virus 1, genus Macronovirus


Spherical, about 15 nm in diameter


Linear positive-sense ssRNA with a 3-poly(A) tail


Dependent on co-infection with Macrobrachium rosenbergii nodavirus


A single ORF is translated into two capsid proteins, one an N-terminally truncated version of the other

Host range

Giant fresh water prawns


One genus with a single species



XSV virions are non-enveloped, spherical, about 15 nm in diameter and serologically unrelated to those of MrNV. They have icosahedral symmetry (Longyant et al., 2012, Qian et al., 2003).

Figure 1.Sarthroviridae. Transmission electron micrographs of negatively-stained (left) extra small virus (bar = 50 nm) and (right) Macrobrachium rosenbergii nodavirus (bar = 100 nm) virions purified by differential ultracentrifugation.

Physicochemical and physical properties

XSV virions have a density of 1.325 g mL-1 in 25–40% CsCl gradients. Viruses are inactivated at 50 °C for 10 minutes but stable to pH between 2 and 8.     

Nucleic acid

The XSV genome is a linear positive-sense RNA molecule of 796 nucleotides which, unlike other satellite viruses, contains a short poly(A) tail of 15–20 nucleotides at the 3-end (Widada and Bonami 2004) (Figure 2.Sarthroviridae).


Protein analysis by SDS-PAGE showed that the protein composition of XSV is simple and composed of two polypeptides of about 17 and 16 kDa which are the major protein components that form the viral capsid.





Genome organization and replication

The positive-sense genome contains a single open reading frame from which two capsid proteins are independently translated from different initiation codons. These capsid proteins, CP17 (17 kDa) and CP16 (16 kDa), are present in nearly equimolar amounts in the virion (Widada and Bonami 2004, Wang et al., 2008). It has been reported that the 3′-untranslated region plays an important role in selective encapsidation of the XSV genome (Liang et al., 2014). The capsid proteins are not recognizably similar to proteins in the public sequence databases (Widada and Bonami 2004). Replication is dependent upon the RNA-dependent RNA polymerase of the helper virus MrNV.

Figure 2.Sarthroviridae. Genome organization of the Macrobrachium rosenbergii nodavirus and extra small virus genomes. RdRP: RNA-dependent RNA polymerase, CP: capsid protein. Numbers indicate nucleotide positions.


XSV and MrNV have been detected in aquatic insects of several species, which were collected from nursery ponds containing freshwater prawn (Macrobrachium rosenbergii) infected with MrNV and XSV (Sudhakaran et al., 2008a). Both viruses could also replicate in mosquito cell lines, suggesting that aquatic insects serve as vectors for the transfer of XSV and MrNV (Sudhakaran et al., 2008a).


XSV virions are serologically unrelated to those of MrNV (Longyant et al., 2012, Qian et al., 2003).

Derivation of names 

Sarthro from small arthropod.

Macronovirus from Macrobrachium rosenbergii the host species, and nodavirus, the group of viruses to which the helper virus Macrobrachium rosenbergii nodavirus is most closely related.

Relationships within the family

Several XSV isolates from geographically remote locations, including French West Indies, Thailand, Taiwan, China and India, have been reported (Bonami and Sri Widada 2011, Sudhakaran et al., 2008b). The isolates display 96–99% nucleotide identity in their capsid protein genes (Sudhakaran et al., 2008b).

Relationships with other taxa

The capsid protein is not recognizably similar to proteins in the public sequence databases (Widada and Bonami 2004).


Genus: Macronovirus

Since only one genus (Macronovirus) is currently recognized in the family Sarthroviridae, the family description above corresponds to the genus description. For clarity, the additional information that can be found on the genus page is also presented below.

Member species

Exemplar isolate of the species
SpeciesVirus nameIsolateAccession numberRefSeq numberAvailable sequenceVirus Abbrev.
Macrobrachium satellite virus 1extra small virusAY247793NC_043197Complete coding genomeXSV
Macrobrachium satellite virus 1extra small virusChinaDQ174318Complete coding genomeXSV-Ch
Macrobrachium satellite virus 1extra small virusM308EU150132Complete coding genomeXSV-M308
Macrobrachium satellite virus 1extra small virusM299EU150131Complete coding genomeXSV-M299
Macrobrachium satellite virus 1extra small virusM298EU150130Complete coding genomeXSV-M298
Macrobrachium satellite virus 1extra small virusM23EU150133Complete coding genomeXSV-M23
Macrobrachium satellite virus 1extra small virusTaiwanDQ521573Complete coding genomeXSV-Th
Macrobrachium satellite virus 1extra small virusIndiaEF159731Partial genomeXSV-In

Virus names, the choice of exemplar isolates, and virus abbreviations, are not official ICTV designations.