Immunohistochemical localization of lysozyme in the prelarvae of tilapia, Oreochromis mossambicus

Catfish (Clarias gariepinus) were vaccinated against a virulent (PPD 134/91) and an avirulent strain (L37) of formalin-inactivatedAeromonas hydrophila. The specific antiserum against the avirulent strain,in vitro, showed a significant inhibition in the growth of the avirulent strain as compared to that treated with control serum. In contrast, no significant difference was found in the growth rate of the virulent strain (PPD 134/91) which was treated with either the specific antiserum or control serum. The specific antibodies against PPD 134/91 showed an increased opsonizing effect. However, opsonization did not enhance the bactericidal activity of phagocytic cells against the virulent strain. The specific antiserum did not show any opsonizing effect on the serologically different virulent strain (PPD 122/91). The specific antiserum against PPD 134/91 could inhibit,in vitro, the invasive ability of the same virulent strain ofA. hydrophilato epithelial cells. However, no significant protection was encountered in naive fish when they were intramuscularly injected with the virulent strain ofA. hydrophilapretreated with the specific antiserum. Hence, the presentin vitrofindings suggest that the specific antiserum in the mucus might play an important role in fish disease resistance by preventing serologically similar virulent strains ofA. hydrophilafrom attaching and penetrating into the integument at the initial stage of infection.