Abstract
BackgroundFrom 2001-2003 monodon slow growth syndrome (MSGS) caused severe economic losses for Thai shrimp farmers who cultivated the native, giant tiger shrimp, and this led them to adopt exotic stocks of the domesticated whiteleg shrimp as the species of cultivation choice, despite the higher value of giant tiger shrimp. In 2008, newly discovered Laem-Singh virus (LSNV) was proposed as a necessary but insufficient cause of MSGS, and this stimulated the search for the additional component cause(s) of MSGS in the hope that discovery would lead to preventative measures that could revive cultivation of the higher value native shrimp species.ResultsUsing a universal shotgun cloning protocol, a novel RNA, integrase-containing element (ICE) was found in giant tiger shrimp from MSGS ponds (GenBank accession number FJ498866). In situ hybridization probes and RT-PCR tests revealed that ICE and Laem-Singh virus (LSNV) occurred together in lymphoid organs (LO) of shrimp from MSGS ponds but not in shrimp from normal ponds. Tissue homogenates of shrimp from MSGS ponds yielded a fraction that gave positive RT-PCR reactions for both ICE and LSNV and showed viral-like particles by transmission electron microscopy (TEM). Bioassays of this fraction with juvenile giant tiger shrimp resulted in retarded growth with gross signs of MSGS, and in situ hybridization assays revealed ICE and LSNV together in LO, eyes and gills. Viral-like particles similar to those seen in tissue extracts from natural infections were also seen by TEM.ConclusionsICE and LSNV were found together only in shrimp from MSGS ponds and only in shrimp showing gross signs of MSGS after injection with a preparation containing ICE and LSNV. ICE was never found in the absence of LSNV although LSNV was sometimes found in normal shrimp in the absence of ICE. The results suggest that ICE and LSNV may act together as component causes of MSGS, but this cannot be proven conclusively without single and combined bioassays using purified preparations of both ICE and LSNV. Despite this ambiguity, it is recommended in the interim that ICE be added to the agents such as LSNV already listed for exclusion from domesticated stocks of the black tiger shrimp.
Highlights
From 2001-2003 monodon slow growth syndrome (MSGS) caused severe economic losses for Thai shrimp farmers who cultivated the native, giant tiger shrimp, and this led them to adopt exotic stocks of the domesticated whiteleg shrimp as the species of cultivation choice, despite the higher value of giant tiger shrimp
Density gradient separations from tissue homogenates Based on previous work suggesting that the causative agent(s) of MSGS were viral in nature [2], the gills of shrimp from MSGS ponds were subjected to homogenization and sucrose density gradient centrifugation that yielded several bands in the range of 10-50% (w/w) sucrose
The nature of ICE Here we have shown the presence of an integrase containing element (ICE) that is located together with Laem-Singh virus (LSNV) in the cytoplasm of cells of the lymphoid organ (LO), gills and the sub-retinal, fasciculated zone of the eyes of black tiger shrimp exhibiting gross signs of disease referred to as monodon slow growth syndrome (MSGS)
Summary
From 2001-2003 monodon slow growth syndrome (MSGS) caused severe economic losses for Thai shrimp farmers who cultivated the native, giant tiger shrimp, and this led them to adopt exotic stocks of the domesticated whiteleg shrimp as the species of cultivation choice, despite the higher value of giant tiger shrimp. It has been suggested that an infectious agent may be the cause [1] This contention was supported by the rapid spread of the problem and preliminary laboratory challenge tests showing that daily weight gain of less than 0.1 g/day at 4 months, 3) unusually bright yellow markings, 4) “bamboo-shaped” abdominal segments, and 5) brittle antennae. These features distinguish MSGS from stunted growth caused by monodon baculovirus (MBV) ( called Penaeus monodon polyhedrovirus [4]) or HPV [5,6]. Clones identified by Blast analysis to contain sequences of shrimp or previously screened pathogens were discarded, and remaining clones were used for in situ hybridization assays with shrimp from MSGS ponds and from normal growth ponds
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