Objective: The present study was designed to evaluate the toxic effect of sublethal concentration of mercuric chloride (0.3 mg/L HgCl₂) on histopathological lesions in the kidney of Nile tilapia (O. niloticus) and the protective effect of microalgae, Nannochloropsis oculata (N. oculata) against the induced tissue alterations for 3 weeks.
Design: Randomized controlled study
Animals: Nile tilapia
Procedures: Fish were randomly assigned to 4 groups, group1: control (basal diet), group 2 (Hg/ exposed to HgCl₂ at a dose of 0.3 mg/L (1/4 of LC₅₀), and fed basal diet), group 3: (Hg+N5, similar to group2, but fed diet supplemented with N. oculata 5% and group 4 (Hg+N10, similar to group2, but fed diet supplemented with N. oculata 10 %. Two fish from each aquarium tank (6 fish/group) were sampled at weeks 1, 2, and 3 of the experiment. The posterior kidney was dried out in a graded ethanol series and then embedded in paraffin. Each block of tissue was cut into serial sections (5 μm thick) and stained with hematoxylin and eosin (H&E).
Results: Histopathological alterations were induced following mercuric exposure in a time-dependent manner. The kidney showed congestion, hemosiderosis, and hemorrhage with vacuolated tubular epithelium, hyaline droplet degeneration, and necrosis of the tubular epithelium. Supplementation with N. oculata, particularly at 10 %, succeeded in alleviating the histopathological induced lesions in the kidney.
Conclusion and clinical relevance: Our findings demonstrate thatHgCl₂ has nephrotoxic properties that led to severe histopathological alterations in the kidney of Nile tilapia, while dietary supplementation with N. oculata was able to alleviate the induced kidney alterations.