Morphological and molecular characterization of Acanthostomum species with Plesiomonas shigelloides coinfection: Immunological and histopathological appraisal in Bagrus bajad

The complex interactions between parasitic and bacterial infections in wild fish populations present significant challenges for aquatic health management. This study investigated the coinfection of acanthostomid trematodes (Acanthostomum absconditum and Acanthostomum spiniceps) and the bacterium Plesiomonas shigelloides in wild caught Bagrus bajad from Egypt. Using a combination of morphological, molecular, immunological, and histopathological approaches, we examined 200 fish specimens for parasitic infection and subsequent bacterial colonization. Parasitological examination revealed prevalence rates of 22.5 % for A. absconditum and 30 % for A. spiniceps, with distinct morphological features characterized through light microscopy. Bacterial isolation and molecular identification through 16S rRNA sequencing identified P. shigelloides in 66.7 % of parasite-infected fish. Ten bacterial isolates showed high genetic similarity (98.06–99.85 %) and phylogenetic clustering with known P. shigelloides strains. Antibiotic susceptibility testing revealed multidrug resistance patterns, with all isolates resistant to trimethoprim, and penicillin. PCR-based screening detected virulence gene astA, actP, and ahpA in 80 %, 90 %, and 70 % of isolates, respectively. Immunological assessment showed significantly elevated serum lysozyme activity in coinfected fish (430–450 ± 25.00 μg/mL) compared to those with parasitic infection alone (200–260 ± 27.00 μg/mL) and uninfected controls (85–120 μg/mL). Histopathological examination revealed extensive intestinal damage, including villous destruction and parasitic penetration into the submucosa. These findings highlight the complex host-parasite-bacteria interactions in B. bajad and underscore potential health risks associated with P. shigelloides coinfection, emphasizing the need for comprehensive monitoring and management strategies in wild fish populations.