Ichthyobodo necator infection as a first record in cultured Dicentrarchus labrax: morphomolecular characterization and infestation patterns in correlation to water quality parameters

Ichthyobodo necator is a damaging and economically important protozoan parasite; however, its clinical patterns and incidence, correlated to environmental factors, are still poorly understood. This study provides the first report of Ichthyobodo necator infestation among Dicentrarchus labrax. We recorded significant mortalities in the cultured D. labrax of average size (5.92 g weight, and 11.55 cm length) in El-Max Station, National Institute of Oceanography and Fisheries, NIOF, Egypt, during the period extending from early summer to early fall (June to October) 2023. Behavioral and gross lesions of the infested fish were recorded; besides, dead and moribund fish were collected. Wet smears of the skin and gill of the affected fish were examined by compound light microscopy. Moreover, water samples from the affected ponds were collected and examined for their physicochemical parameters. The bodonid biflagellated Ichthyobodo necator (Henneguy) C. Pinto, 1928 was isolated from the skin and gills and identified based on specific morphological characteristics. DNA sequencing of PCR products and phylogenetic analysis confirmed identification, with clustering of the current isolates with I. necator isolates from different fish species. The multivariate analysis revealed a positive correlation between the prevalence and infestation intensity of Ichthyobodo, increased temperature levels, decreased dissolved oxygen DO, and also correlated with higher ammonia, and pH values. Additionally, the elevated concentrations of iron, calcium, magnesium, zinc, and copper in pond water were positively correlated with infestation and mortalities. These water parameter levels promote respiratory distress, accelerated prevalence, and infestation intensities of I. necator, particularly during the infestation peak in August and September. This study bridges the knowledge gap regarding I. necator parasite infestations and their correlation with water quality parameters.