Tolerance to white spot syndrome virus (WSSV) in the freshwater prawn Macrobrachium rosenbergii is associated with low VP28 envelope protein expression

Abstract

The freshwater prawn Macrobrachium rosenbergii was experimentally infected with white spot syndrome virus (WSSV) by intramuscular injection. Infection was confirmed by positive, single-step, WSSV polymerase chain reaction (PCR) assays targeting the VP28 gene from Day 2 up to Day 90 post injection (p.i.). Although no mortality of WSSV-infected prawns was observed, bioassays with the black tiger shrimp Penaeus monodon using hemolymph from Day 90 PCR-positive prawns resulted in white spot disease (WSD). Transcriptional analysis of the VP28 gene of WSSV by reverse transcriptase (RT)-PCR assays and Western blot assays revealed transient expression of the VP28-specific transcript in DNase-treated total RNA from hemolymph, gills, head soft tissue and eyestalks at 2 d p.i. By 3 d p.i., the VP28 transcript could no longer be detected in eyestalks and hemolymph but was still lightly detectable in head soft tissue and gills. It became undetectable there from 5 d p.i. onwards, despite the undiminished presence of the virus shown by single-step PCR targeting of the VP28 gene. VP28 was not detected by the less sensitive Western blot in hemolymph at any time during the study period, but it was detectable in all other tested tissues from Days 2 to 4 p.i. Our results demonstrated that M. rosenbergii is tolerant to a relatively constant level of persistent WSSV infection characterized by a low expression of VP28 and, possibly, other virion proteins. Despite this, M. rosenbergii can carry a level of infectious WSSV sufficient to represent a feasible threat to cultivated penaeid shrimp such as P. monodon. It remains to be seen whether a very low virion protein expression relative to the viral copy number may constitute a general decapod characteristic for persistent viral infections that produce no signs of disease.