The potency of seven medicinal plant crude extracts against helminth parasites of Clarias gariepinus; Wenyonia sp. and Procamallanus sp. with less toxicological effect on host fingerlings were investigated. The activities of crude extracts of Allium sativum, Cymbopogon citrates, Ocimum sanctum, Thymus vulgaris, Vernomia amygdalina, Zingiber officinale, and Zanthoxylum zanthoxylones were investigated at 96 hours exposure. Relative Toxicity Factors (RTFs) of the extracts increase at host fingerlings' Low Observed Effect Concentration (LOEC) - 0.1 LC50 g/L in the trend; A. sativum (RTF, 1.0) < C. citrates (RTF, 2.0) < O. sanctum (RTF, 2.0) < T. vulgaris (RTF, 3.0) < V. amygdalina (RTF, 4.0) < Z. officinale (RTF, 5.0) < Z. zanthoxylones (RTF, 10.0). As the concentration/response increases in the intervals; 1.0 LC50 g/L and (1.5) LC50 g/L, there were no change in relative toxicities except for Z. officinale, that showed decreased RTFs, while Z. zanthoxylones showed tremendous increase in RTFs (RTFs, 10.0, 16.0 and 27.0). Average Survival Time (AST) and Percentage Reduction in Survival Time (%RST) relative to saline solution for the intestinal parasites exposed to the extracts at the least interval; 0.1 LC50 g/L host fingerling, showed the similliar trend in susceptibility. Z. zanthoxylones had shown the highest potency against helminth parasites compared to the other extracts. The effects of plant
extracts on parasites and the sensitive lifestage of their fish host is dose-dependent and to avoid overdosage, the need for determination of suitable concentration is of great importance.