A case of severe mortality in farmed Labeo rohita was investigated to characterize the causative agent. We identified the bacterial strain as Aeromonas veronii isolated from the gut of infected L. rohita by biochemical assay, scanning electron microscopy and 16S rRNA gene sequence analysis. The in vivo challenge experiment showed that the LD₅₀ of A. veronii was 2.2 × 10⁴ CFU/fish. Virulence gene investigation revealed that the isolated A. veronii possesses Aerolysin, Cytotoxic enterotoxin, Serine protease, Dnase and Type III secretion system genes. The isolated strain was resistant to two antibiotics (ampicillin and dicloxacillin) while susceptible to 22 other antibiotics. The study further revealed that A. veronii induced both stresses along with non-specific and specific immune responses marked by elevated cortisol HSP70, HSP90 and IgM levels in the treated L. rohita fingerlings. Although the bacterial pathogen enhances the immune response, the negative effect on fish, including stress, and high mortality, create concern and a need for A. veronii management in L. rohita farms. The knowledge gained from this study would facilitate future research aimed at assessing the pathogenicity of A. veronii, with an emphasis on microbial disease management in other farmed fish species.

Aphanomyces invadans is an aquatic oomycete pathogen and the causative agent of epizootic ulcerative syndrome (EUS) in fresh and brackish water fish, which is responsible for severe mortalities and economic losses in aquaculture. Therefore, there is an urgent need to develop anti-infective strategies to control EUS. An Oomycetes, a fungus-like eukaryotic microorganism, and a susceptible species, i.e., Heteropneustes fossilis, are used to establish whether an Eclipta alba leaf extract is effective against the EUS-causing A. invadans. We found that treatment with methanolic leaf extract, at concentrations between 50–100 ppm (T4–T6), protects the H. fossilis fingerlings against A. invadans infection. These optimum concentrations induced anti-stress and antioxidative response in fish, marked by a significant decrease in cortisol and elevated levels of superoxide dismutase (SOD) and catalase (CAT) levels in treated animals, as compared with the controls. We further demonstrated that the A. invadans-protective effect of methanolic leaf extract was caused by its immunomodulatory effect and is linked to the enhanced survival of fingerlings. The analysis of non-specific and specific immune factors confirms that methanolic leaf extract-induced HSP70, HSP90 and IgM levels mediate the survival of H. fossilis fingerlings against A. invadans infection. Taken together, our study provides evidence that the generation of anti-stress and antioxidative responses, as well as humoral immunity, may play a role in protecting H. fossilis fingerlings against A. invadans infection. It is probable that E. alba methanolic leaf extract treatment might become part of a holistic strategy to control EUS in fish species.

The experiment was conducted to evaluate the effect of dietary supplementation of carotenoid (marigold oleoresin) on growth, survival and total body carotenoid of Koi carp (Cyprinus carpio). The experiment was carried out in 12 fiber aquarium tanks of size 12"×6"×6"(20l capacity). Each tank was stocked with15 fishes of uniform size. The marigold oleoresin were as dietary supplement at levels 60, 120, 180, 240, 300 ppm/kg of the feed and designated as treatment T1, T2, T3, T4 and T5 respectively and diet without marigold oleoresin supplementationserved as a control (T0). The result of the experiment showed that there was significant difference found in absolute growth rate and specific growth rate of the fish (P >0.05). However, 180ppm marigold oleoresin fed fishes showed higher mean weight gain of 3.98±0.22g and lower feed conversion ratio (FCR) of 2.81±0.04 than the other treatment groups. The body coloration and total carotenoid concentration of muscle tissue (30.16±0.60?g/g) was significantly higher in fish fed with 180 ppm marigold oleoresin diet. The study showed that incorporation of 180ppm of marigold oleoresin in diet was found better to enhance the growth and coloration in C. carpio.