May 15, 2025
May 15, 2025
Ethan T. Woodyard1, Alvin C. Camus2, Caitlin E. Older3, Bradley M. Richardson3, Thomas G. Rosser4, Caroline Coussens4, David J. Wise3,5, Charles C. Mischke5,6, Michael E. Colvin7, Matt J. Griffin1
1Department of Pathobiology and Population Medicine, College of Veterinary Medicine, Mississippi State University, Mississippi State, Mississippi 39762; 2 Department of Pathology, College of Veterinary Medicine, University of Georgia, Athens, GA 30677; 3Warmwater Aquaculture Research Unit, USDA-ARS, Stoneville, Mississippi 38776; 4Deparement of Comparative Biomedical Sciences, College of Veterinary Medicine, Mississippi State University, Mississippi State, Mississippi 39762; 5Department of Wildlife, Fisheries and Aquaculture, Mississippi State University, Mississippi State, Mississippi 39762; 6Thad Cochran National Warmwater Aquaculture Center, Delta Research and Extension Center, Mississippi State University, Stoneville, Mississippi 38776; 7U.S. Geological Survey, Columbia Environmental Research Center, Columbia, Missouri 65201
Proliferative gill disease, caused by the myxozoan parasite Henneguya ictaluri, is a significant parasitic disease affecting commercial catfish production, causing significant morbidity and mortality in channel (Ictalurus punctatus) and channel x blue (I. furcatus) hybrids. In addition to their favorable growth characteristics and disease resistance, field and laboratory studies have suggested hybrid catfish do not perpetuate the life cycle of H. ictaluri. However, multi-batch production of channel catfish is still utilized depending on market conditions. These factors incentivize the use of a “crop rotation” strategy whereby production alternates between channels and hybrids. To determine effects of crop rotations on myxozoan communities, gill tissue from sentinel channel catfish and water samples were collected from ponds under rotation strategies including: newly stocked either with channels or hybrids, transitioned from channels to hybrids or vice versa, stocked with channels or hybrids for 2 years, or stocked with hybrids for 5 years. Fecal samples of piscivorous birds from commercial catfish farms were also included. DNA was extracted from these samples and subjected to targeted amplicon sequencing of a diagnostic variable region of the myxozoan 18S rRNA gene using Illumina MiSeq. Sequence homology and phylogenetic analyses were used to identify myxozoan sequences. Reads matching H. ictaluri, H. exilis, and H. mississippiensis were present in gill and water samples from ponds in all rotations. Similarly, gill clip wet mounts from sentinel channel catfish exhibited cartilage breaks consistent with PGD in all ponds regardless of rotation strategy. While rotation strategy affected myxozoan community composition, it did not eliminate H. ictaluri from ponds. The absence of competent fish hosts, detection of catfish-infecting Henneguya spp. from gill and water samples in all rotations and bird feces implicate birds as potential mechanical vectors for myxozoans between ponds. Future work will assess infectivity of infectious stages passed in bird feces, as well as species-level identification of unknown myxozoans detected in this present study and experimental elucidation of their life cycles.
1Maural Sowlat, Lester H. Khoo1, 2Bradley M. Richardson, 1Marsha A. Lewis, 1James M. Steadman, 1Cynthia C. Ware, 3David J. Wise and 1Matt J. Griffin
1Aquatic Research and Diagnostic Laboratory, Department of Pathobiology and Population Medicine, College of Veterinary Medicine, Mississippi State University, 127 Experiment Station Rd., Stoneville, MS 38776; 2Warmwater Aquaculture Research Unit, Agricultural Research Service, United States Department of Agriculture, 141 Experiment Station Rd., Stoneville, MS 38776; 3Mississippi Agriculture and Forestry Experiment Station, Delta Research and Extension Center, Mississippi State University, 127 Experiment Station Rd., Stoneville, MS 38776
Antibiotic resistance is a pressing concern for the global aquaculture industry. Monitoring trends in key bacterial species can help preserve antibiotic effectiveness and sustain treatment success. At the Aquatic Research and Diagnostic Laboratory (ARDL) in Stoneville, MS, the susceptibility of bacterial pathogens to approved antibiotics is estimated by disc diffusion assays. Susceptibility records, documented as zone of inhibition (ZOI) diameters (mm), along with antibiotic feed directives from 2007-2024, were assessed to monitor changes in the in vitro susceptibility of Edwardsiella ictaluri, the causative agent of Enteric Septicemia of Catfish or ESC. Responsiveness to florfenicol (FFC), oxytetracycline (OTC), and sulfadimexothine-ormetoprim 5:1 (SMO) is assessed for all case submissions to the ARDL. According to ARDL standards, which are based on client feedback regarding the effectiveness of antibiotic treatments, isolates are considered susceptible when the ZOI measures 20 mm or greater. A total of 3,911 diagnostic records were included in this retrospective, with mean ZOI diameters (+/- SD) of the susceptible populations of 47.98 mm +/- 7.42 for FFC, 36.54 mm +/- 7.79 for OTC, and 35.53 mm +/- 6.23 for SMO. For FFC, ZOI diameters were distributed bimodally, showing a population divide and potential epidemiological cut-off value above ARDL’s clinical breakpoint of 20 mm. OTC displayed a high rate (16.18%, 625/3870) of unresponsive (0 mm ZOI) isolates, which also expressed smaller FFC ZOIs. A correlation was established between the ZOI diameters for E. ictaluri against FFC and OTC. Resistance genes relating to FFC, OTC, and sulfonamides, but not diaminopyrimidines, were identified on plasmids of isolates non-responsive to OTC with reduced sensitivity to FFC, supporting the relationship between FFC and OTC susceptibility and the rarity of resistance to SMO. Over the nine years from 2014 to 2022, there was a gradual rise in the proportion of case isolates showing reduced susceptibility to one or more antibiotics. The trend was associated with increased targeted antibiotic use beginning in 2013, followed by a return to historical prevalence levels. This recovery was likely driven by reduced antibiotic use after the 2019 introduction of an experimental oral E. ictaluri vaccine developed by Mississippi Agriculture and Forestry Experiment Station (MAFES) scientists at Mississippi State University’s Delta Research and Extension Center. The vaccine has been administered under veterinary prescription on participating catfish farms, with oversight from the State Veterinarian of Mississippi. These farms, which also receive diagnostic services from the ARDL, offer MAFES and CVM researchers a unique opportunity to assess the vaccine’s impact beyond production records. By integrating diagnostic data with field observations, researchers can evaluate the vaccine’s potential to reduce antibiotic use and combat antimicrobial resistance (AMR). This study highlights how antibiotic use affects susceptibility trends, reveals the complex genetic factors influencing phenotypic resistance, and underscores the potential for aquaculture to address AMR through vaccination and the judicious use of antibiotics.