The present study was carried out to evaluate the effect of oxytetracycline (OTC)dosing at five different concentrations, viz., 0 mg (0X), 80 mg (1X), 240 mg (3X), 400 mg (5X) and 800 mg (10X) kg -1 biomass day -1 for 30 consecutive days on the growth, safety and intestinal histology of Nile tilapia, Oreochromis niloticus (L.) juveniles. The OTC-residues in the edible muscle were detected at scheduled intervals by LC-MS/MS. A dose-dependent decline in feed intake, biomass and survival were recorded in OTC-dosed fish. The OTC-residue levels were 0, 204.75±45.75, 318.00±0.00, 778.50±145.50 and 684.00±18.00 ng g -1 in 0X, 1X, 3X, 5X and 10X groups, respectively on day 30 OTC-dosing, which reduced subsequently. Relatively mild histopathological lesions including degeneration of epithelial layer, loss of absorptive vacuoles, necrotized intestinal villi, mucinous degeneration, and necrotized absorptive region were observed in the intestine of OTC-dosed fish. Lamina propria swelling was the characteristic change observed in the 10X group on day 15. The observed data revealed that OTC-dosing is reasonably safe at the therapeutic dose of 80 mg kg -1 biomass day -1 . However, the precise dose for safe usage of OTC is to be determined according to the culture conditions and species cultured.
Studies on gut microbiome in aquatic animals is a field that has grown equivalently with the increase in aquaculture industry. Research in this field is progressing quickly focussing on nutritional manipulation, developments in microbiome research and modification of the gut microbiota. The outcome of the research is helpful in fish farming as well as to maintain host health and welfare. However, clear understanding of the specific role played by the different gut microbiota on diversified fish species is still lacking. A number of important topics have been highlighted here which would be beneficial for further research at potent areas.
To develop vaccine for rohu (Labeo rohita), efficacy of three antigenic preparations from Aeromonas hydrophila were evaluated. Thirty six tanks with ten rohu were divided in quadruplicates (R1 to R4) with nine tanks (G1 to G9). Rohu of G1 to G6 tanks were given intraperitoneal vaccine with outer membrane protein, somatic protein and formalin-inactivated whole cell itself and along with Incomplete Freund's Adjuvant @ 200 µg/fish, G7 and G8 tanks were injected with Incomplete Freund's Adjuvant (100 µl/fish) and normal saline (100 µl/fish) respectively and G9 tanks were kept as control. After 28 d, rohu of R3 and R4 were subjected to intramuscular A. hydrophila challenge (LD50) @ 2.85×10 6 cells/fish for 7 d and RPS (%) was calculated. Specific cellular and humoral immune responses were determined for rohu of R1 and R2. Results showed that rohu immunized with outer membrane protein along with adjuvant could offer an appropriate vaccine strategy.
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