Aeromonas spp. suggested as the causative agents of red spot disease in northern Vietnamese grass carp Ctenopharyngodon idella

In northern Vietnam, a disease called ‘red spot disease’ has been causing high morbidity and mortality in populations of farmed grass carp Ctenopharyngodon idella for about 2 decades. The name ‘red spot disease’ refers to a condition characterised by haemorrhagic lesions, reddening and ulceration of the skin. Eight different bacterial isolates, namely Aeromonas hydrophila, A. sobria, Staphylococcus epidermidis, Vibrio alginolyticus, Pseudomonas fluorescens, P. luteola, Citrobacter freundii and P. putida, were isolated from diseased grass carp and used for experimental infection of the same species. Fish were challenged with the different bacterial isolates both by immersion and intramuscular injection. Different concentrations of bacteria were tested to evaluate their pathogenicity. Injection with 1 × 105 CFU of A. hydrophila and A. sobria resulted in clinical signs identical to those of red spot-diseased grass carp in Vietnam. None of the other bacterial isolates tested caused any morbidity or mortality in fish challenged either intramuscularly (1 × 106 CFU) or by bath immersion (1 × 106 or 1 × 108 CFU ml-1).