Genetic Variability of Edwardsiella piscicida from Mississippi Catfish Aquaculture from 2013-2020

1,2Marsha Lewis, 1,2 Cyndi Ware, 1,2Lester Khoo, 3Adrián López-Porras, 1,2,4 Noor-Ul-Huda,

5Geoff Waldbieser, 1,2Divya Rose, 1,2James Steadman, 1,2Matt Griffin

1Department of Pathobiology and Population Medicine, College of Veterinary Medicine, Mississippi State University, Stoneville, MS 38776; 2Thad Cochran National Warmwater Aquaculture Center, Delta Research and Extension Center, Mississippi State University, Stoneville, MS 38776; 3Department of Biosciences, University of Oslo, Oslo, Norway; 4Department of Pathobiology, St. George’s University, School of Veterinary Medicine, True Blue, Grenada, West Indies;  5Warmwater Aquaculture Research Unit, Agriculture Research Service, U.S. Department of Agriculture, Stoneville, MS 38776

ABSTRACT

Catfish farming plays a significant role in the United States agricultural economy, particularly in Mississippi, Alabama, and Arkansas. In 2022, the U.S. catfish industry generated sales exceeding $400 million (USD).  Mississippi is responsible for nearly 60% of all U. S. catfish production, with more than 36,000 water surface acres devoted to catfish farming. The “Delta” region, located in the western part of the state, is where most Mississippi catfish production occurs. Recent industry trends indicate a shift from channel catfish (Ictalurus punctatus) to channel x blue catfish (I. furcatus) hybrids over the past decade. Hybrids have demonstrated superior performance over channel catfish in several aspects of production, which has contributed to their growing popularity as a culture fish.  Recent reports indicate that hybrids now make up nearly 60% of the total catfish production in the United States.   Although hybrids are generally more resistant to several pathogens that affect channel catfish, there is one agent, the gram-negative enteric bacterium Edwardsiella piscicida, which demonstrates increased virulence in hybrids.  Disease case submissions to the Aquatic Research and Diagnostic Laboratory (ARDL) at the Thad Cochran National Warmwater Aquaculture Center in Stoneville, MS, show increased incidence and prevalence of E. piscicida infections in recent years, with hybrids accounting for more than 90% of E. piscicida diagnoses Results from previous studies assessing intraspecific genetic variability of E. piscicida isolates from catfish and other fish species have identified six discrete phyletic groups, with some host and virulence associations. The current study employed multilocus sequence typing (MLST) targeting select housekeeping genes (gyrase B [gyrB], pgi [glucose-6-phosphate isomerase], phoU [phosphoglucomutase]), to assess genetic variability of >300 E. piscicida isolates from ARDL case submissions from 2013-2020. Initial analysis confirmed previous findings, identifying the five previously described groups from Mississippi catfish aquaculture, as well as other groups from non-ictalurid fish, with the majority of catfish isolates falling into two main clades.   Sequence typing of individual housekeeping genes indicates multiple sequence types are present in catfish aquaculture, indicating a greater variability than was previously thought.  Moreover, the frequency of antibiotic resistance among these isolates was assessed, revealing 51 isolates with reduced sensitivity to one or more approved antibiotics (Aquaflor® [florfenicol], Terramycin® [oxytetracycline], Romet® [sulfadimethoxine/ormetoprim].  The significance of these findings in relation to disease outbreaks is currently unclear, but suggests Edwardsiella piscicida isolates from catfish farms are genetically heterogenous. Further research is necessary to determine the role of these genetic variants in disease outbreaks.