Incidence and prevalence of Edwardsiella piscicida has increased in Mississippi farm-raised catfish in recent years. Edwardsiella piscicida affects mostly market-sized catfish during the final stages of the production cycle resulting in significant economic losses. The objectives of this study were to determine the genetic variability of E. piscicida, assess virulence in channel and hybrid catfish, and evaluate the capacity of a live-attenuated E. ictaluri vaccine to protect channel and hybrid catfish against heterologous E. piscicida isolates. This work identified five discrete E. piscicida lineages, along with group specific associations of several virulence related genes. In general, E. piscicida was shown more virulent in hybrids than channel catfish, in line with previous work. Further, a live-attenuated E. ictaluri vaccine was shown to confer cross-protective immunity in channel and hybrid catfish against E. piscicida.

The U.S. aquaculture industry has become well established in the last three decades, and channel catfish aquaculture is the most significant component of this industry. Virulent Aeromonas hydrophila has been a serious disease problem since 2009 in the U.S. catfish aquaculture, and Aeromonas veronii and Edwardsiella piscicida are emerging pathogens of catfish. Therefore, this study aims to address fundamental questions on virulence mechanisms of these three fish pathogens, which I expect to support the development of control measures for preventing these diseases. In this study, E. piscicida and virulent Aeromonas hydrophila (vAh) genomes were sequenced, and comparative analyses were conducted using the genome sequences. Average nucleotide identity (ANI) calculations showed that E. piscicida strains share high sequence identity, yet they are from diverse host species and geographic regions. vAh isolates share very high sequence identity, while the other A. hydrophila genomes are more distantly related to this clonal group. We applied several comparative genomics approaches to evaluate E. piscicida genomes and E. ictaluri genomes, providing valuable information about unique and shared features of these two important pathogens in the Edwardsiella genus. Comprehensive secretion system analysis of 55 A. hydrophila genomes and deletion of tssD and tssI core elements of T6SS from vAh isolate ML09-119 has provided new knowledge. We sequenced the genome of virulent Aeromonas veronii strain ML09-123 from catfish indicated that it was highly similar to an A. veronii strain from China. Evaluation of all 41 A. veronii genomes available in the National Center for Biotechnology Information (NCBI) provides a base platform to investigate in detail the molecular mechanism of A. veronii biology and virulence. Lastly, we constructed deletion mutants vAhΔsia, vAhΔent, vAhΔcol, vAhΔhfq1, vAhΔhfq2, and vAhΔhfq1Δhfq2 to determine roles of A. hydrophila secreted proteins and regulatory proteins on virulence in catfish. Results showed that sialidase (vAhΔsia) and enterotoxin (vAhΔent) mutants were significantly attenuated.

Microcystin-LR is a hepatotoxin produced by cyanobacteria. Aeromonas hydrophila and Edwardsiella piscicida infections are leading causes of losses in market-sized channel catfish (Ictalurus punctatus). These older fish should have natural immunity in place and a predisposing factor is likely a prerequisite for these disease outbreaks. While microcystin-LR rarely causes mortality in warm-water aquaculture, we believe it may be a predisposing factor that leads to bacterial disease outbreaks during the summer months due to its ability to damage the liver. Our study investigated microcystin-LR’s effects on channel catfish susceptibility to these pathogens. We found that a sublethal dose of microcystin-LR induced substantial damage to multiple immune organs. In our challenges with both the toxin and bacteria, we saw a significant increase in mortality of fish. Our findings suggest that microcystin-LR increases channel catfish susceptibility to Aeromonas hydrophila and Edwardsiella piscicida infections.