Infections of MrNV (Macrobrachium rosenbergii nodavirus) in cultivated whiteleg shrimp Penaeus vannamei in Asia

Macrobrachium rosenbergii nodavirus (MrNV) is well-known as a major pathogen that causes whitened muscles and high mortality in the freshwater prawn Macrobrachium rosenbergii. Recently, it has also been reported to cause white muscles and high mortality in postlarvae of the marine shrimp Penaeus (Penaeus) monodon and Penaeus (Fenneropenaeus) indicus in India. The latter report stimulated us to re-examine specimens from Asian shrimp farms that had experienced high mortality in Penaeus (Litopenaeus) vannamei with white muscles but tested negative for infectious myonecrosis virus (IMNV) and Penaeus vannamei nodavirus (PvNV) by RT-PCR analysis. Some of these specimens were indeed positive for MrNV by RT-PCR. Sequences of the two single-stranded RNA fragments in the MrNV genome were amplified from these P. vannamei specimens and found to share ~97% nucleic acid sequence identity with the MrNV sequences deposited at GenBank (NC_005094 and NC_005095NC_005094NC_005095). Extra small virus (XSV) usually associated with MrNV, was detected in some but not all of the samples. Infectivity tests were performed by feeding P. vannamei with minced tissues from MrNV-infected M. rosenbergii. The assays were preformed at low salinity of 2ppt and at two different water temperatures of approximately 22°C and 28°C. It was revealed that shrimp exhibited a higher infectivity and mortality at the lower temperature. Our findings suggested that P. vannamei is an additional species that is susceptible to MrNV and that low temperature together with low salinity of rearing water may increase the severity of infections leading to significant mortality. ► IMNV and PvNV are the only viruses that cause white muscle disease in P. vannamei. ► We found diseased P. vannamei positive for MrNV by RT-PCR and genome sequencing. ► MrNV previously known to cause white muscle in M. rosenbergii but not P. vannamei. ► Bioassay indicated that MrNV may cause mortality in P. vannamei at low temperature.