Green silver nanoparticles as a potential control strategy against Ichthyophthirius multifiliis: Efficacy and toxicity evaluation in goldfish, Carassius auratus.

Ichthyophthirius multifiliis, the causative agent of white spot disease, poses a major challenge to ornamental fish culture. Seasonal surveillance of goldfish (Carassius auratus) from Galiff Street Fish Market, Kolkata, identified I. multifiliis as the dominant ectoparasite, with prevalence rates of 79.3 % (post-monsoon), 71.0 % (pre-monsoon), and 53.84 % (monsoon). The green synthesized silver nanoparticles using neem leaf (n-AgNPs) were confirmed by a characteristic reddish-brown coloration, UV-Vis absorption peak at 421 nm, zeta potential (-23.5 mV), and spherical, uniformly dispersed particles with an average size of 25 ± 5 nm. FTIR analysis indicated the presence of functional groups involved in phytochemical-mediated reduction and stabilization. In vitro assays showed an antiparasitic efficacy (AE) of 100 % against trophont stage of I. multifiliis at 10, 7.5, 5, and 2.5 mg/L with median effective concentrations (EC₅₀) 5.15, 3.30, 2.07, and 0.34 mg/L at 2, 4, 6, and 8 h, respectively. Further, the microscopic examination revealed complete immobilization of trophonts accompanied by membrane rupture and extrusion of cellular contents could lead to parasite death. The acute toxicity assays of n-AgNPs in goldfish indicated 96 h LC₅₀ values of 10.22 mg/L. Overall, the findings highlight neem-mediated green n-AgNPs as a potent and promising strategy for controlling I. multifiliis; however, further in vivo studies are required to validate their practical application and assess long-term host safety.