Prophylactic and therapeutic effects of a nanocomposite (silver nanoparticle plus Terminalia catappa) against Saprolegnia parasitica in tambaqui

Infectious diseases have been the main limiting factor for international fish farming, especially saprolegniosis, which causes large economic losses. Therefore, this study evaluated the prophylactic and therapeutic effect of a nanocomposite (Silver nanoparticle plus extract of Terminalia catappa) to control Saprolegnia parasitica in tambaqui Colossoma macropomum. Two tests were carried out: 1) a therapeutic assay including long and short-term baths, containing two concentrations of nanocomposite each, on infected tambaqui and 2) a prophylactic assay where the zoospore and nanocomposite (four different concentrations) were added at the same time into water for 72 h. Mortality, prevalence index, hematology, and infected areas with S. parasitica on the fish body were evaluated in both tests. In the therapeutic test, all fish from the control group (without nanocomposite) had increased infected area, as well as lethargy and hemorrhage, resulting in 100% mortality. They also had reduced red blood cells, lymphocytes, neutrophils, monocytes, and thrombocytes. In group exposed to nanocomposite, higher concentrations affected mycelial growth, especially in the highest concentration (4.33 mg L−1), reducing infected areas on the fish body by 98.70% and achieving 100% survival. The treatment with long-term bath at the lowest concentration (0.54 mg L−1) had increased values of neutrophils. In the prophylactic test, fish groups without handling stress and the methylene blue had no clinical signs or mortality. However, the fish group submitted to handling stress presented the highest prevalence and infected areas resulting in 100% mortality. However, the increasing of nanocomposite concentration promoted less oomycete prevalence, pathology intensity, and mortality. The most effective concentration in the prophylactic assay was 0.87 mg L−1, preventing the infection without blood alterations. Then, the nanocomposite as prophylactic measured at concentration of 0.87 mg L−1 is the best strategy to prevent S. parasitica infection in fish.