Biopesticidal Efficacy and Safety of Azadirachtin: Broad-Spectrum Effects on Ectoparasites Infesting Goldfish, <i>Carassius auratus</i> (Linn. 1758)

Ectoparasites are a serious concern for the ornamental fish industry, causing severe pathological and behavioral changes that diminish the aesthetic value of fish and adversely affect trade. Recently, phytotherapy, application of plant extracts or their active biomolecules, has gained attention for the control and treatment of various diseases of humans and animals, including fish. The present study evaluated the broad-spectrum antiparasitic activity of azadirachtin, a neem-based bioactive molecule, under <i>in vitro</i> condition against adult protozoan (<i>Ichthyophthirius multifiliis</i>) and adult metazoans ectoparasites, including <i>Dactylogyrus</i> sp. and <i>Argulus</i> sp., as well as juvenile stages and eggs of the latter, for its prevalence in infesting goldfish (<i>Carassius auratus</i>). Azadirachtin was tested at concentrations of 9.52-47.61 mg L^-1 against <i>I. multifiliis</i> theronts; 1-30 mg L^-1 for <i>Dactylogyrus</i> sp.; and 25-125 mg L^-1 for <i>Argulus</i> sp. The median effective concentration (EC₅₀), antiparasitic efficacy (AE), relative antiparasitic efficacy, and therapeutic index (TI) were estimated for azadirachtin. EC₅₀ of azadirachtin against different parasites was estimated to be in the range of 6.08 to 61.29 mg L^-1. Relative antiparasitic efficacies were found to be 25 mg L^-1 for 4 h, 50 mg L^-1 for 6 h, and 100 mg L^-1 for 12 h against <i>Dactylogyrus</i> sp., <i>I. multifiliis</i> theronts, and <i>Argulus</i> sp., respectively. <i>Argulus</i> egg hatching was inhibited above 100 mg L^-1, with eggs at 125-175 mg L^-1 becoming unclear by day 14. The 15 h median lethal concentration (LC₅₀) of azadirachtin (EC = 21.5%) against <i>C. auratus</i> was 20.48 mg L^-1. The TI indicates a safe dose in <i>C. auratus</i> against <i>Dactylogyrus</i> sp. (11.22 for 4 h) and <i>I. multifiliis</i> theronts (2.11 for 12 h) while showing a critical value for <i>Argulus</i> (0.65 for 15 h). The present findings provide evidence for the broad-spectrum antiparasitic activity of azadirachtin against protozoan and metazoan ectoparasites, suggesting its potential as an agent for controlling a wide range of ectoparasites in ornamental fishes.