Immune assessment of farm-reared Penaeus vannamei shrimp naturally infected by IMNV in NE Brazil

Farm-reared shrimp production was growing intensely in Brazil until 2002, when an outbreak of a new disease, caused by the infectious myonecrosis virus (IMNV), disrupted the production in the northeast region. The aim of this study was to evaluate some hemato-immunological parameters in Penaeus vannamei shrimp during the progress of the disease caused by IMNV. Affected shrimp were separated into three groups, according to disease severity: asymptomatic shrimp (Group AS), shrimp with initial symptoms (Group IN) and with advanced symptoms (Group AD). The parameters examined were: hemograms, percentage of apoptotic hemocytes (Hoechst staining), anion superoxide production (NBT reduction): phenoloxidase (PO) activity (L-DOPA oxidation), agglutinating titre (dog erythrocytes) and antimicrobial activity of the hemolymph. Significant changes in immunological parameters were only seen in Group AD shrimp, where increases in apoptotic hemocytes (8-fold), agglutinating titre (16 fold), anion superoxide hemocyte production (50%) and hemolymph antimicrobial activity (21-fold against Micrococcus luteus) were observed when compared to shrimp of Groups AS and IN. The group AD shrimp also showed significant reductions in total hemocyte Counts (30%), percentage of circulating granulocytes (7%) and PO activity (50%). These results suggest that the immune system of IMNV-infected shrimp responds only at a late stage of the disease, when animal recovery is unlikely. This is the first study to examine the immune response of P. vannamei during the progress of disease caused by IMNV in Brazilian farms where the virus first appeared. (C) 2009 Elsevier B.V. All rights reserved.