DAO prepress abstract  -  doi: 10.3354/dao01907

ABSTRACT: A standardized inoculation model was used in two separate studies to gauge the virulence of three White Spot Syndrome virus isolates (WSSV Thai-1, WSSV Thai-2 and WSSV Viet) in Penaeus vannamei juveniles. Mortality patterns (Study No. 1) were compared and WSSV-positive cells quantified (Study No. 2) in tissues following intramuscular inoculation of shrimp with the most (WSSV Thai-1) and least (WSSV Viet) virulent isolates as determined by Study No. 1. The results of Study No. 1 demonstrated that mortalities began at 36 hours post inoculation (hpi) for both Thai isolate groups and at 36-60 hpi for the Viet isolate group. Cumulative mortality reached 100% 96 to 240 h later in shrimp challenged with the WSSV Viet isolate as compared to shrimp challenged with the Thai isolates. WSSV infection was verified in all groups by indirect immunofluorescence. In Study No. 2, WSSV-infected cells were quantified by immunohistochemical analysis of both dead and time course sampled shrimp. WSSV-positive cells were detected in tissues of Thai-1 inoculated dead and euthanized shrimp from 24 hpi onwards and from 36 hpi onwards in shrimp injected with the Viet isolate. Significantly more infected cells were found in tissues of dead shrimp inoculated with the Thai-1 as compared to Viet isolate-inoculated shrimp. In these studies, substantial differences in virulence were demonstrated between the WSSV isolates. The Vietnamese isolate induced a more chronic disease and mortality pattern than was found for the Thai isolates, possibly because it infected fewer cells. This difference was most pronounced in gills.