Edwardsiella ictaluri bacteraemia elicits shedding of Aeromonas hydrophila complex in latently infected channel catfish, Ictalurus punctatus (Rafinesque)

Edwardsiella ictaluri is a primary bacterial pathogen of channel catfish, Ictalurus punctatus, and the causative agent of enteric septicaemia of catfish . Edwardsiella ictaluri is known to gain entry to the host by infection of the nares, gastrointestinal tract, and gills, and to disseminate to organs via an as yet uncharacterized acute bacteraemia. In this study, fluorescent microscopy showed E. ictaluri on the gill within 5 min of immersion challenge and E. ictaluri could also be isolated from the circulation within 5 min. When removed to clean water, catfish cleared circulating bacteria within 15 min and the blood remained free of E. ictaluri until its reappearance at the 12 h post‐infection sampling. However, Aeromonas hydrophila , the aetiological agent of motile aeromonad septicaemia, appeared within the circulation 7 h post‐challenge with E. ictaluri and was detected in all fish at 12 h post‐infection. Only 20% of fish carried A. hydrophila in the trunk kidney that could be detected by plate culture on Rimler–Shotts agar; however, 100% of challenged and stress‐control fish were A. hydrophila complex positive at 24 h post‐challenge. These results suggest that although the catfish is capable of clearing its circulation of E. ictaluri , superinfection with latent A. hydrophila may enhance clinical signs of edwardsiellosis. This is the first report of a bacterial superinfection appearing in fish.