Abdominal segment deformity disease (ASDD) of the whiteleg shrimp Penaeus vannamei reared in Thailand

From 2004-2006, shrimp farmers in Thailand, Malaysia and Indonesia have complained of cultured whiteleg shrimp Penaeus vannamei that grow and survive normally but have abdominal segment deformity disease (ASDD). This is characterized by abdominal segments that are enlarged or twisted laterally and/or dorso-ventrally, sometimes accompanied by opaque muscles. Histological analysis revealed abdominal muscle necrosis and degeneration, with haemocytic infiltration. To study the cause, affected shrimp were tested by polymerase chain reaction (PCR) methods for the presence of known viruses including infectious hypodermal and haematopoietic necrosis virus (IHHNV), infectious myonecrosis virus (IMNV), P. vannamei nodavirus (PvNV), Macrobrachium rosenbergii nodavirus (MrNV), Taura syndrome virus (TSV) and Laem Singh virus (LSNV). Except a few shrimp that were positive for IHHNV, TSV and LSNV, the diseased shrimp were negative for all of the viruses tested. Their negative status was confirmed by in situ hybridization specific for IHHNV, IMNV, MrNV, TSV and LSNV. By transmission electron microscopy (TEM), 20-22 nm, non-enveloped viral particles were observed in the cytoplasm of the muscle lesions, connective tissue and glial cells of neural tissues and gills. Affected neural tissues included the optic lobe, brain, thoracic ganglia, abdominal ganglia and ventral nerve cord. The findings suggest that ASDD in P. vannamei may be caused by a new virus that primarily invades neural tissue and results in neuromuscular dysfunction and abnormal morphology.