Large-scale culture of largemouth bass (LMB, Micropterus salmoides) has increased in recent years, and it remains a critical species for sportfish restoration and has shown promise as a food fish species. LMB are susceptible to numerous diseases (bacterial, viral, and parasitic), which can lead to massive economic losses for production facilities. Various bacterial diseases are responsible for LMB mortality, but Aeromonas spp. infections, including Aeromonas veronii and Aeromonas hydrophila, are associated with rapid, high-level mortality rates. These pathogens may be linked to increased rearing stressors in aquaculture systems, whether it be poor water quality parameters, increased temperature, or previous infections, since Aeromonas spp. are often considered opportunistic pathogens.
Due to limited information on the ontogeny of the LMB immune response and susceptibility to bacterial infections, developing new disease management techniques is critical. To elucidate these aspects of LMB health, we conducted an experiment to evaluate the long-term disease susceptibility and characterize LMB immune parameters during the critical juvenile rearing period. We hypothesized that the susceptibility to experimental Aeromonas spp. infection and immune response would be dynamic over this period.
To complete this study, A. veronii (ARS-LMB-32-2018) and A. hydrophila (ARS-LMB-9-2022) strains have been used to conduct monthly immersion pathogens challenge using juvenile LMB (~1 to 15 g) for six months. In addition to the challenge trials, the LMB will be sampled monthly (pre- and post-challenge) for spleen, kidney and distal intestine tissue to examine targeted immune gene expression and characterize microbial communities in the gut. Results from the month 1 challenge indicate that LMB are susceptible to Aeromonas spp., mortality rates observed at month 1 followed a similar trend with preliminary virulence assessments of the isolates. A. veronii produced a higher cumulative percent mortality (CPM) compared to A. hydrophila (27% and 10%; respectively). This long-term experiment will discern a more complete understanding of LMB health and disease susceptibility. An increased understanding of opportune times for treatment interventions will also be elucidated through this increased understanding of pathogen and host immune dynamics.