Peracetic acid treatment of Ichthyophthirius multifiliis (Ciliophora: Ichthyophthiriidae) and Trichodina spp. reduces the infection by Aeromonas hydrophila and improves survival in Nile tilapia (Oreochromis niloticus)

Abstract

Co-infections with ectoparasites and bacteria commonly occur in fish farms and are associated with high mortality. In March 2020, a private Nile tilapia (Oreochromus niloticus) farm in Sharkia Governorate, Egypt, experienced high fish mortality, and the clinically diseased fish were characterized by lethargy, abnormal swimming behavior, loss of appetite, skin hemorrhages, exophthalmia, and abdominal distension. Laboratory parasitological and bacteriological examinations of the diseased fish revealed concurrent infections with Ichthyophthirius multifiliis, Trichodina centrostrigeata, and Aeromonas hydrophila. Pathological findings indicated severe gill and skin infections with I. multifiliis and T. centrostrigeata associated with necrosis, hemorrhages, and depletion of the hemopoietic organs, as well as cardiac inflammation. Peracetic acid (PAA) was applied as a strong oxidizing antimicrobial disinfectant, and in vitro evaluation showed that 5 ppm achieved 5-log reductions in the viable colony count of A. hydrophila after a contact time of 5 min and 1.076 ppm represented the 4-h 50% lethal concentration (LC50) for the infective stage of I. multifiliis (theronts). The continuous exposure of culture water to 1 mg L−1 PAA for 1 week followed by periodical use of PAA at the same concentration twice per week was effective in eradicating the ciliated protozoans, reducing the aerobic bacterial density in water, ameliorating the pathological alterations, restoring the fish health, and reducing the mortality rate by up to 15%. Thus, the use of PAA to treat I. multifiliis and T. centrostrigeata reduces the infection by A. hydrophila in Nile tilapia and improves their survivability.