Determining the safety and suitability of fluorescein dye for characterization of skin ulcerations in cultured Nile tilapia (Oreochromis niloticus) and African sharptooth catfish (Clarias gariepinus)

There is a need to identify the presence of lesions in fish skin as soon as they erupt. Fish skin lesions are either macroscopic (can be visualized by the naked eye) or microscopic (difficult to detect with the naked eye). Skin wounds resulting in loss of the epithelium (superficial or deep ulcers) are serious as they may interfere with osmoregulation and open portals for opportunistic pathogens. Herein, we report on the use of a fluorescein dye for the detection of skin ulcers that cannot be seen by the naked eye. Due to their importance in aquaculture endeavors in Egypt, this study focused on two indigenous species, the Nile tilapia (Oreochromis niloticus) and the scale-less African sharptooth catfish (Clarias gariepinus). Fluorescein dye was tested for safety to fish without interfering with microbiological analysis. Parallel to the use of the flourescein dye, the detected ulcers were examined for the presence of bacteria or tissue alterations. Further, we experimentally induced the formation of skin ulcers in O. niloticus physically or by injecting Aeromons hydrophila, and then assessed the utility of fluorescein dye in detecting the induced skin lesions. Results obtained in this study demonstrated that fluorescein dye application is harmless to Nile tilapia at concentrations up to 0.5 mg fluorescein/ml water for up to 15 min. Indeed, a low dose of fluorescein (0.10 mg/ml for 5 min) could identify very minute skin abrasions. We highly recommend the use of fluorescein dye for the evaluation of skin health in farmed fish species and the visualization of minute skin abrasions.