Evaluating the impact of ionic and nano silver on the reproductive dynamics of Macrogyrodactylus congolensis: host-dependent and independent effects.

Ectoparasitic infections pose significant challenges in aquaculture, often leading to high fish morbidity and mortality. Macrogyrodactylus congolensis, a viviparous monogenean parasite infecting Clarias gariepinus, is of particular concern due to its rapid reproduction and transmission rates. This study investigates the effects of ionic silver (AgNO3) and engineered nano-silver (nAg) on the reproductive dynamics and spatial distribution of M. congolensis, both on and off its host. Parasite reproduction off the host was assessed under unexposed then varying concentrations of AgNO3 (0.1-100 µg/L) and nAg (0.1-100 mg/L), while host-parasite interactions were conducted using the LC10 of the parasite for AgNO3 and nAg. Results indicate that silver-based treatments exhibit concentration-dependent inhibitory effects on parasite reproduction. At lower concentrations (≤ 2 µg/L AgNO₃ and ≤ 1 mg/L nAg), minimal effects were observed. In contrast, higher concentrations (≥ 50 µg/L AgNO3 and ≥ 20 mg/L nAg) significantly suppressed reproduction. Parasite mortality increased with silver exposure, particularly in off-host conditions. Spatial distribution heatmaps illustrated that M. congolensis primarily occupied the host's head region, i.e. preference for microhabitats that support survival and transmission. Additionally, scanning electron microscopy (SEM) analysis of parasites exposed to both forms of Ag showed tegument disruption. The host integument showed minimal punctures when exposed to AgNO3 and no damage in the control and exposure to nAg. While AgNO3 and nAg show potential as antiparasitic agents in aquaculture, their broader ecological and physiological impacts on host fish and the environment require further investigation. The study highlights the need for optimised dosing strategies to balance efficacy and environmental and host safety, offering valuable insights into developing alternative parasite management approaches for aquaculture.