Dactylogyrus extensus and Pseudomonas fluorescens dual infection in farmed common carp (Cyprinuscarpio).

Dactylogyrus extensus and Pseudomonas fluorescens are serious pathogens in Cyprinus carpio aquaculture causing severe impacts and substantial economic losses. During the early spring of 2021, abnormal mortalities were reported among farmed C. carpio. Moribund fish showed anorexia, respiratory distress, dermal ulcers, and septicemia. The water analysis revealed low dissolved oxygen (3.4 mg/L), and high un-ionized ammonia levels (0.65 mg/L). Seventy moribund C. carpio specimens were collected and subjected to parasitological and bacteriological examinations. The monogenetic trematode D. extensus was discovered in wet mounts from the gills of all the examined fish samples (100%). The identity of recovered parasites was confirmed by sequencing and alignment of the 28S rDNA gene. P. fluorescens was concurrently identified in the infested fish samples (58.5%) based on phenotypic characteristics using the API20 E. The identity of bacterial isolates was confirmed further by sequencing and alignment of 16S rRNA gene. The IL-1β and MHCII were upregulated in infested fish in tandem with the severity of infections. P. fluorescens isolates displayed high resistance to most of the tested antibiotics. The study is one of the earlier reports on D. extensus and P. fluorescens co-infections in farmed C. carpio and highlights the need of effective control programs to protect fish health and minimize losses.