Retinopathy in stunted black tiger shrimp Penaeus monodon and possible association with Laem-Singh virus (LSNV)

Monodon slow-growth syndrome (MSGS) is a condition of Penaeus monodon cultivation ponds characterized by abnormally slow growth and coefficients of size variation greater than 35%. Shrimp from these ponds often show the presence Laem-Singh virus (LSNV) in the lymphoid organ, heart and other tissues. We compared various tissues of abnormally small and normal size (large) P. monodon from 3 MSGS ponds and from 2 reference ponds with normal growth (control ponds) both histologically and for the presence of LSNV. Small and large shrimp from the MSGS ponds were positive for LSNV by reverse transcriptase polymerase chain reaction (RT-PCR) tests while shrimp from one normal-growth pond were positive and from another negative. The LSNV infections were confirmed by transmission electron microscopy (TEM) and in situ hybridization (ISH). The only striking difference between the small and large LSNV-positive shrimp was retinopathy exclusively in the small shrimp from the MSGS ponds. Retinopathy comprised abnormally enlarged haemolymphatic vessels, haemocytic infiltration and rupture of the membrane that separated the fasciculated zone from the overlying row of retinular cells. By TEM and ISH, LSNV was detected in the fasciculated zone and in onion bodies of the organ of Bellonci in the small shrimp from the MSGS ponds, but not in those tissues and cells of the large shrimp from the MSGS pond or from the normal-growth ponds, whether LSNV-positive or not. The results suggested that retinopathy associated with LSNV may be linked causally to stunting of P. monodon in MSGS ponds.