Mass Kills Associated with Pseudomonas fluorescens and Saprolegnia parasitica Concurrent Infection in Earthen Pond-Farmed Nile tilapia (Oreochromis niloticus)

Poor water quality is considered the most critical predisposing factor for disease eruptions in aquaculture facilities. Abrupt decrease or fluctuation in water temperature is a detrimental factor in the occurrence of some low temperature dependent microbes. The ubiquitous pathogen Pseudomonas fluorescens and the saprotrophic water mold Saprolegnia parasitica are ideal examples of this disease pathway. In the current study, the two ubiquitous microbes were isolated from Nile tilapias at an acre-sized earthen pond that was overstocked with more than 25,000 fingerlings during an episode of mass mortalities at mid-February 2023. The peak of mass mortalities occurred after heavy rains and wind storms that hit the country during this time of the year. Most of the dead and moribund fishes were suffering from generalized skin ulcers, fin rot, gill congestion, and cottony wool-like hyphae intimately attached to skin ulcers and fins. Internally, almost all internal organs including the kidney, spleen, and liver were congested. Histopathological examination of affected fish showed hyalinization and necrosis of fin tissues together with inflammatory cell infiltration. Moreover, fungal hyphae were detected within the muscle bundles. The final identity of the retrieved Pseudomonas fluorescens was confirmed using 16S rRNA gene sequence analysis, while Saprolegnia parasitica isolates were confirmed using ITS rDNA gene sequence analysis. All supportive accession numbers for retrieved isolates were published through the Genbank official website.