In vitro and in vivo activity of herbal and chemical treatments against Saprolegnia ferax—a causative agent for saprolegniasis

Saprolegniasis is a widespread fungal-like disease, causing serious damage to cultured and wild fish populations and their eggs, with subsequent economic losses. Many existing therapeutics for this disease prophylaxis are toxic with negative impacts on the environment. This study compared the efficacy of three medicinal plant extracts and a chemical compound readily available in Ghana to assess their potential effects on controlling saprolegniasis. The fungistatic and fungicidal properties of three plant leaf extracts, namely neem leaf, bitter leaf, Indian almond leaf and the chemical, potassium permanganate, were investigated under in vitro conditions. Different concentrations ranging from 5 to 20 mg/ml were tested. The average radial growth of the fungi in the presence of the treatments and controls was assessed. The efficacy of bitter leaf extract was further evaluated under in vivo conditions by exposing 200 juvenile Nile tilapia to S. ferax spores and treatment administered at different concentrations. Fish mortality and water quality parameters were monitored. Results from the in-vitro test showed that KMnO₄ and Indian almond leaf extract completely suppressed mycelial growth at the lowest tested concentration. The in-vivo treatment with bitter leaf extract at a concentration of 15 mg/ml demonstrated high efficacy in controlling S. ferax infection in Nile tilapia with a survival rate of 95%. It was concluded that the Indian almond extract was the most effective at the lowest tested concentration under the in vitro conditions. At the same time, bitter leaf was recommended for its safety for fish and widespread availability to fish farmers in Ghana.