White Spot Syndrome – What We Have Learned about the Virus and the Disease

One of the lessons ultimately learned from the first dramatic outbreaks of white spot syndrome virus (and other shrimp viral diseases) in the early 1990’s was that aquaculture management practices needed to be improved. Subsequent research revealed that critical factors included: broodstock sourcing, postlarva sourcing, screening techniques and strategies, diverse transmission pathways, critical infection levels, disease susceptibility, and stressors. In Taiwan, Penaeus monodon brooders are usually captured from the wild. In this wild populations WSSV prevalence runs at about 58-67%, and the infected brooders may pass the virus on to their offspring (via transovum transmission). Nauplii infection status appears to be a key indication in predicting the subsequent outcomes of culturing in grow-out ponds, but it relatively inconvenient and expensive to measure nauplii infection status directly. We have found, however, that only heavily infected brooders are likely to produce heavily infected nauplii, and these brooders can be recognized because they are WSSV positive even before spawning. Conversely, lightly infected brooders (i.e. brooders that only become WSSV positive after spawning; the stress of spawning triggers rapid replication of the virus) and WSSV-free brooders produce nauplii that are, at worse, only lightly infected. These lightly infected nauplii are still able to perform well in grow-out ponds. This paper reviews the work done in the last decade by several research groups, and shows how studies on the key aspects of the biology of WSSV infection have led to improved disease management solutions that are now widely used by the shrimp aquaculture industry. BACKGROUND: THE VIRUS ITSELF White spot syndrome virus (WSSV) was initially described as a non-occluded baculovirus, but even while the molecular data were still limited, (the preliminary WSSV-DNA sequence analysis), the morphological characteristics and the general biological properties of the virus had already highlighted its uniqueness (Lo et al., 1996a; 1997; Wongteerasupaya et al., 1996). Recent data, including the genome sequence and phylogenies based on DNA polymerase and protein kinase, suggest that WSSV is a member of a new virus family tentatively named as Nimaviridae (Liu et al., 2001; van Hulten et al., 2001a; Vlak et al., 2001; Yang et al., 2001; Chen et al., 2002a). The size of the WSSV genome has been differently reported for different isolates: 305107 bp (GenBank Accession No. AF332093; Yang et al., 2001), 292967 bp (GenBank Accession No. AF369029; van Hulten et al., 2001a), Lo, C-F., S-E Peng, Y-S Chang and G-H Kou. 2005. White Spot Syndeome What we have learned about the virus and the disease. In P. Walker, R. Lester and M.G. Bondad-Reantaso (eds). Diseases in Asian Aquaculture V, pp. 421-433. Fish Health Section, Asian Fisheries Society, Manila. Diseases in Asian Aquaculture V