Emergence of <i>Aeromonas dhakensis</i>‐linked motile <i>Aeromonas</i> septicemia in <i>Pangasius hypophthalmus</i> biofloc systems in Pakistan

Abstract This study reports on an outbreak of motile Aeromonas septicemia (MAS) that caused significant mortalities in iridescent shark catfish ( Pangasius hypophthalmus ) in biofloc systems in the Punjab province of Pakistan. Clinical symptoms observed on the affected fish included anal fin hemorrhaging, mouth area bleeding, skin ulcerations, interstitial nephritis, and focal fusion of secondary lamellae. According to necropsy findings and a molecular phylogenetic analysis based on gyrB gene sequences, the etiologic agent of the disease was identified as Aeromonas dhakensis Ali.Pk01. The new isolate displayed resistance to multiple antibiotics such as ampicillin, cefuroxime sodium, meropenem, and penicillin, but remained susceptible to several antibiotics including amikacin, ceftazidime, chloramphenicol, ciprofloxacin, levofloxacin, and oxytetracycline. To fulfill Koch's postulates, fish were challenged by intraperitoneal injection with A. dhakensis Ali.Pk01, revealing its high virulence. Also, this isolate was found to encode multiple virulence genes including act , ela , alt , lip , and hyl . This study offers crucial information for local catfish farmers, highlighting the potential spread of virulent multidrug‐resistant pathogens and the need for better biosecurity and disease control measures without reliance on antibiotics.