White Spot Syndrome Virus

White spot disease (WSD) is a highly contagious viral infection of decapod crustaceans that can cause high levels of mortality in cultured shrimp. Since its first outbreak in 1992-93, this disease has caused serious economic losses. The causative agent of WSD is white spot syndrome virus (WSSV), which is a large, nonoccluded, enveloped, rod-shaped to elliptical DNA virus with a tail-like extension at one end. WSSV multiplies in the nucleus and has a very broad host range among crustaceans. Most of the predicted open reading frames in its genome (~300 kilobase pairs (kbp)) encode polypeptides that show no homology to known proteins, while the identifiable genes are mainly involved in nucleotide metabolism and DNA replication. Nine homologous regions of highly repetitive sequences are found in the genome. Functional studies have identified several WSSV genes, including three immediately early genes, three latency-related genes and an anti-apoptosis gene. WSSV genes are classified as early genes or late genes according to their expression patterns. These early and late genes are regulated as coordinated cascades under the control of different promoters. Based on genetic analyses and morphological features, WSSV was recently classified as the sole species of a new monotypic family called Nimaviridae (genus Whispovirus).