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

Abstract

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 2. ppt 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. © 2012 Elsevier B.V.