Surface characters and extracellular toxins involved in the pathogenesis of Aeromonas hydrophila.

A small number of serotypically distinct strains of A. hydrophila obtained from diseased freshwater fish were examined for their pathogenic properties comprising of cell surface characteristics and extracellular toxins. Test strains exhibited homogeneity in their cell surface characteristics despite being serologically heterogeneous. Studies on extracellular biological activities revealed qualitative and quantitative differences in production of toxins, probably explaining their antigenic diversity. Three distinct proteases, namely heat stable metallo protease, heat labile serine protease and heat labile metallo protease were identified from the strains.

Immunostimulatory effect of azadirachtin in Oreochromis mossambicus (Peters).

The effect of azadirachtin, a triterpenoid derived from Azadirachta indica on the immune response was studied in the freshwater teleost, O. mossambicus. Bovine serum albumin (BSA) and sheep erythrocytes (SRBC) were used as antigens to evoke immune response. The immune responses in fish were measured by quantifying antibodies produced and counting the peripheral blood leucocytes in control and experimental fish. In general, azadirachtin significantly enhanced the antibody response and leucocyte count in a dose dependent manner. An inverse relationship was observed between the dose of azadirachtin and the degree of immunostimulation. Timing of azadirachtin administration in relation to immunization revealed that the maximum enhancement of antibody response was observed when the stimulant was given two days prior to immunization. The observed immunostimulatory property of azadirachtin has an implication in the maintenance of finfish health in freshwater intensive aquaculture practices.