Studies on the pathogenesis of Vibrio spp infection in Penaeus monodon Fabricius

Vibrio infections are one of the principal final causes of serious production losses in penaeid shrimp farming. The pathogenesis of experimental Vibrio spp infections in Penaeus monodon has been investigated using an immunohistochemical technique. \nA Vibrio spp isolate was obtained from a disease outbreak in Thailand and identified using biochemical tests and SDS-PAGE. The isolate was identified as gelatine negative Vibrio vulnificus biotype I with variable urease activity. A polyclonal antiserum was raised in rabbits against the bacterium and its specificity tested by means of agglutination, immunoblot and ELISA tests. This antiserum was used to identify the 31KD protein and a 90-99KD LPS fraction responsible for the immune response using a Western-blot technique. P. monodon juveniles were experimentally infected with the bacterium either by injection, ingestion or bath challenge method. In a time course study, an indirect immunoperoxidase technique was employed to trace the presence of the bacterium in the tissue of the challenged shrimps. \nThe penetration of the bacteria differed between the three challenge methods, the clearance mechanisms of bacteria converged into common routes. Bacterial cells penetrated through damaged gill cuticle, body cuticle or through the insertion of the cuticular setae. Haemocytes were observed to phagocytose the bacterial cells and migrate through connective tissue and be transported with the haemolymph, accumulating around hepatopancreas, midgut caecum, gills and antennal gland. Once the phagocytic haemocytes reached the heart, they were distributed to lymphoid organ. Bacterial material appeared to accumulate in heart and lymphoid organ. \nBacteria entering through the mouth were broken down and only soluble material filtered through gastric sieves passed into the hepatopancreas. This soluble material then diffused through the hepatopancreatic tubules into the haemolymph and then to gills and antennal gland. Once this material reached the heart it was distributed to lymphoid organ accumulating in the heart and lymphoid organ as previously described. \nFinally, bacterial material was released to the exterior in two steps. Initially through the gills and later through hepatopancreatic B-cell vacuoles and branchial and subcuticular podocytes. \n \nThis mechanism and its implications are discussed.