Treatment of Trichodina sp reduced load of Flavobacterium columnare and improved survival of hybrid tilapia

Bacterium Flavobacterium columnare and protozoan Trichodina spp are common pathogens of cultured fish. Studies of parasite–bacterium interaction show evidence that concurrent infections increase severity of some infectious diseases, especially bacterial diseases. The effect of parasite treatment on F. columnare infection in tilapia is currently unknown. This study evaluated whether treatment of Trichodina sp parasitized hybrid tilapia (Oreochromis niloticus × Oreochromis aureus) with formalin would improve fish survival and reduce F. columnare infection in fish after F. columnare exposure. Hybrid tilapia parasitized by Trichodina sp were divided into 3 treatment groups. The first group of fish received no parasite treatment. The second group of fish were bath treated with 150 mg L−1 formalin for 1 h. The third group of fish treated twice with 150 mg L−1 formalin bath for 1 h each at 2 day intervals. All fish were then exposed to F. columnare by immersion challenge. The tilapia not treated with formalin showed significantly higher mortality (37.5%) than those treated with formalin (≤16.7%) after exposure to F. columnare. Fish treated twice showed lower mortality (6.37%) than those treated only once (16.7%). The non-treated fish showed significantly higher load of F. columnare in gill, kidney and liver compared to those treated with formalin following exposure to F. columnare. The bacterial load of non-treated fish was 27075 genome equivalents per mg of gill tissue (GEs/mg), 12 fold higher than those treated once with formalin (2250 GEs/mg) or 39 fold higher than those treated twice with formalin (699 GEs/mg) after exposure to F. columnare. This study demonstrated that formalin treatment for Trichodina sp parasitism reduced bacterial infection as suggested by reduced loads of bacteria in fish tissues and subsequently decreased fish mortality.