First case of Aeromonas schubertii infection in the freshwater cultured snakehead fish, Ophiocephalus argus (Cantor), in China.

An epizootic in snakehead fish, Ophiocephalus argus, in earthen ponds in Xianning, Hubei Province, central China, from June to August 2009 was found to be caused by Aeromonas schubertii. The cumulative mortality within 40 days was 45%, and the diseased fish were 18 months old and 35-45 cm in length. Multiple, ivory-white, firm nodules, 0.5-1 mm in diameter, were scattered throughout the kidney. Blood clots, 3-5 mm in diameter, were found in the liver. This is a disease frequently found in cultured snakehead throughout China. Isolated bacteria were Gram negative, facultatively anaerobic, motile, short rod-shaped, with a length of 0.3-1.0 μm. Morphological and biochemical tests, as well as phylogenetic analysis derived from 16S rRNA, gyrB, rpoD and dnaJ gene sequencing all strongly indicated that these snakehead isolates are identical to A. schubertii. In addition, the isolates possessed two plasmids: 5.0 kb and 10.0 kb. Antibiotic sensitivity testing of the isolates was carried out by the standard Kirby-Bauer disc diffusion method. Experimental infection assays were conducted, and pathogenicity (by intraperitoneal injection) was demonstrated in snakehead fingerlings and zebrafish, Brachydanio rerio (Hamilton).