Aeromonas veronii caused bilateral exophthalmia and mass mortality in cultured Nile tilapia, Oreochromis niloticus (L.) in India

Aeromonas spp. cause significant mortality and high economic losses in Nile tilapia, Oreochromis niloticus culture worldwide. In the present study, we isolated and confirmed the involvement of four bacteria from diseased Nile tilapia showing signs of bilateral exophthalmia in Tamil Nadu, India. The bacterial isolates were identified as Citrobacter freundii, Pseudomonas aeruginosa, Acinetobacter juvenii and Aeromonas veronii based on biochemical and 16S rRNA gene sequence analysis. All the four isolates failed to produce positive CAMP reaction on 5% sheep blood agar and no cytopathic effect was observed in OnIL cell line after inoculating filtrate of the tissue homogenates prepared from the affected Nile tilapia. Experimental challenge infection assays revealed that out of the four, A. veronii isolate alone killed 100% of experimental fish within 120h and the bacteria could be subsequently re-isolated from the dead fish. The LD50 values of A. veronii were found to be 10(5.35) CFU/fish. The bacterial isolate exhibited cytotoxic effects on onIL cells and haemolytic activity on 5% sheep blood agar and was found positive for virulence genes such as enterotoxin act and haemolytic toxin aerA. This study described a virulent A. veronii isolated from Nile tilapia with bilateral exophthalmia and eye lesions as that exhibited by the well known outbreak causing pathogen, Streptococcus agalactiae. In this study, A. veronii isolates were also found sensitive to cefixime, ciprofloxacin, chloramphenicol, kanamycin, cifixime/clavulanic acid, suggesting the possible application of these antibiotics in aquaculture.