Genetic variability of Edwardsiella piscicida isolates from Mississippi catfish aquaculture with an assessment of virulence in channel and channel × blue hybrid catfish

Abstract

The bacterium Edwardsiella piscicida causes significant losses in global aquaculture, particularly channel ( Ictalurus punctatus ) × blue ( I . furcatus ) hybrid catfish cultured in the south‐eastern United States. Emergence of E . piscicida in hybrid catfish is worrisome given current industry trends towards increased hybrid production. The project objectives were to assess intraspecific genetic variability of E . piscicida isolates recovered from diseased channel and hybrid catfish in Mississippi; and determine virulence associations among genetic variants. Repetitive extragenic palindromic sequence‐based PCR (rep‐PCR) using ERIC I and II primers was used to screen 158 E . piscicida diagnostic case isolates. A subsample of 39 E . piscicida isolates, representing predominant rep‐PCR profiles, was further characterized using BOX and (GTG) 5 rep‐PCR primers, virulence gene assessment and multilocus sequence analysis (MLSA) targeting housekeeping genes gyrb , pgi and phoU . The MLSA provided greater resolution than rep‐PCR, revealing 5 discrete phylogroups that correlated similarly with virulence gene profiles. Virulence assessments using E . piscicida representatives from each MLSA group resulted in 14‐day cumulative mortality ranging from 22% to 54% and 63 to 72% in channel and hybrid fingerlings, respectively. Across all phylogroups, mortality was higher in hybrid catfish ( p E . piscicida is an emerging threat to hybrid catfish aquaculture in the south‐eastern United States.

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