A systematic review and meta-analysis of the potential effect of medicinal plants on innate immunity of selected freshwater fish species: its implications for fish farming in Southern Africa

Abstract

One of the major bottlenecks affecting the expansion of the freshwater aquaculture sector in developing countries is the outbreak of diseases. Fish farmers have traditionally relied on antibiotics and synthetic chemicals to control diseases. However, antibiotics and synthetic chemicals are associated with negative effects on the environment and consumers and their excessive use has resulted in antibiotic-resistant pathogens. In addition, the majority of freshwater fish farmers, especially small-scale farmers, have limited access to antibiotics and synthetic chemicals due to lack of resources. Medicinal plants have been reported to be suitable replacements in aquaculture, but their usage in aquaculture is still limited. The aim of this paper is to undertake a meta-analysis on the effect of medicinal plants on innate immune response and disease resistance in commonly farmed freshwater aquaculture fish species, namely Mozambique tilapia (Oreochromis mossambicus), African catfish (Clarias gariepinus), trout (Oncorhynchus mykiss), and cyprinids (Labeo rohita and Cyprinus carpio). The analysis showed that the mean effect size for respiratory burst (− 1.90 (95% CI − 2.40; − 1.40), I2 = 100%, P = 0); lysozyme activity (0.05 (95% CI − 0.38; 0.48), I2 = 97%, P < 0.01); white blood cells (− 0.69 (95% CI − 1.16; − 0.23), I2 = 89%, P < 0.01); and phagocytic activity (− 1.21 (95% CI − 2.08; − 0.35), I2 = 91%, P < 0.01) was significantly different. Mean effect size for survival rates was not significantly different (2.56 (95% CI 0.10; 5.01), I2 = 0%, P = 1). The funnel plots for all parameters were asymmetrical, which indicates possible publication bias or the presence of systematic differences or inconsistencies among studies. Overall, the meta-analysis showed that medicinal plants could enhance immunity and disease resistance in tilapia, African catfish, carp, and trout. These findings reinforce the observation that plant supplements can be used to prevent disease outbreaks in aquaculture. It is therefore recommended that freshwater fish farmers in African regions such as Southern Africa learn to produce their own diets supplemented with plants at their farms in order to prevent diseases in cultured fish.