Since the first outbreak of motile Aeromonas septicemia (MAS) in farmed catfish in southeastern United States in 2009, recurring outbreaks of MAS had resulted in the loss of millions of pounds of food-size fish annually. Fish mortalities in ponds ranged from 5 to 100%. A new pathotype of Aeromonas hydrophila, named virulent A. hydrophila (vAh), was identified and attributed to the cause of the outbreak. However, information is limited as for what nourished vAh population in pond water to reach a threshold level that caused acute outbreaks of MAS in fish.
In this study, the effects of nutrients in water on
proliferation of vAh and severity of MAS in channel catfish were investigated. Results of the study revealed that both nutrient-rich tryptic soy broth powder (TSBp, the microbiological growth medium) and the commercial fish feeds supported vigorous growth of vAh in water (Figure 1). By addition of 6 g TSBp or 6 g of fish feed to 15 L of water, vAh multiplied from 2.4 x 105 CFU/mL to approximately 4.7 x 108 CFU/ml and 2.0 x 108 CFU/mL, respectively, at 24 h post inoculation (hpi). Challenge of fish in the vAh-propagated water at 24 hpi resulted in approximately 96% and 73% mortality, respectively. Finding s of this study suggest that, when fish were under environmental stresses and had poor appetite for feed supplied in the pond, the unconsumed feed provided vAh ample nutrients to propagate instantly and rapidly, which would result in outbreaks of MAS. Methods of culture of vAh in situ performed in this study was also shown to be useful for mimicking vAh growth dynamics in response to nutrient status in relatively natural environment.