Unique lesions and viral-like particles found in growth retarded black tiger shrimp Penaeus monodon from East Africa

The problem of growth retardation in cultured black tiger shrimp, Penaeus monodon was observed at one commercial shrimp farm in East Africa in mid 2004. The mean body weight of the shrimp at the sixth month in affected ponds was only 19 ± 4 g, which was 30% less than the expected size of normal shrimp grown within the same period at this farm. A preliminary examination for pathogenic bacteria, parasites and viruses revealed mild bacterial infections in a few specimens but all were negative for 7 known pathogenic shrimp viruses, either by assays using polymerase chain reaction (PCR) technology or by histological examination. A subsequent examination of a second lot of samples prepared for transmission electron microscopy (TEM) revealed the presence of previously undescribed lesions in the lymphoid organ and gills. In semithin sections stained with toluidine blue, these were apparent as large cytoplasmic inclusions that were not visible in normal tissue sections stained with hematoxylin and eosin. By TEM, the cytoplasmic inclusions contained large numbers of non-enveloped, icosahedral virions of approximately 25 nm diameter. Given the unique nature of the lesions and the negative results with molecular tests for known shrimp viruses, it is possible that the particles seen constitute a new viral pathogen of shrimp. The retarded growth problem in East Africa resembles a similar problem called monodon slow growth syndrome (MSGS) that has been problematic with P. monodon in Thailand since 2001. Whether the slow growth in Africa and Thailand are due to the same or related pathogens or some other factor(s) remains to be explored.