水産養殖研究開発ジャーナル

概要

Aeromonas Septicemia Infection in Cultured Nile tilapia, Oreochromis niloticus L.

Bekele Lema, P.Natarajan, L.Prabadevi, Kassaye Belkew Workagegn

The study was conducted at Gudar Experimental Aquaculture farm, from November 2011 to March 2012. The main objectives of the present study were to investigate host-parasite relationship of Aeromonas bacteria in Nile tilapia (Oreochromis. niloticus) in aquaculture production system and evaluate pathogenicity and hematological change through experimental infection. The bacterial isolates were identified according to the biochemical reactions scheme. The pathogenicity test was performed for Aeromonas bacteria through intra-peritonium injection (IP) using 21/gauge sterile needle at ventral parts of the body of experimental fish. After 24 hours post injection of bacteria (1.4 x 106 CFU ml-1), all fish fingerlings became less active, showed decreased feeding rate and erratic swimming behavior, and remained at the bottom of the aquarium. External clinical signs observed were darkened dorsal part of body and mild hyperemia of the pectoral and ventral fin bases. Significant hyperemia on the base of the fins, severe fin rot and fin erosion were observed but there was no loss of scales, no sore development on the body part, and no excess mucus secretion around the gills and fin parts that are some of the clinical symptoms of Aeromonas infection. The results indicated that fish fingerlings developed apparent symptoms of Aeromonas bacterial infection and the intensity increased due to stress factors like local irritation, disturbances in the aquaria, handling and crowding and there by developed the clinical symptoms of the disease. In this study, all clinical findings except free mucus defecation, scale protruding, edema within scale pockets, skin ulceration and abdominal distension were observed. Macroscopic findings revealed the presence of pale gills, serious fluid accumulation in intestine, pale gonads with matured eggs, pale liver and enlarged gall bladder filled with emerald-black secretion. Fish injected with Aeromonas bacteria (1.4x106 CFU ml-1) showed increased WBC which was believed to be caused by migration of white blood cells from the spleen to the blood circulation and cause leukocytosis. This fact shows more production of leucocytes in the bacterial injected fish that enhances the defense mechanism of the fish.