Saprolegniosis poses a significant challenge in the aquaculture industry, leading to substantial losses, including an estimated one egg loss for every thirty eggs in cold-water fish hatcheries (Masigol et al., 2021; Chanu et al., 2022). The Saprolegnia genus, classified under the Saprolegniaceae family, comprises water molds that thrive in freshwater environments. They are recognized as a critical fungal group affecting both farmed and wild fish.

This fungus is naturally found in fresh water and moist soil and is responsible for Saprolegniosis. Recently, several ornamental koi fish exhibiting symptoms such as white patches, weakness, lethargy, skin lesions, fin deterioration, and swimming incoordination were referred to the microbiology department at the Faculty of Veterinary Medicine, Tehran University. Initially, the symptoms raised suspicions of Cort syndrome; however, further testing confirmed Saprolegniosis.

Skin samples from the affected fish were collected and cultured on Sabouraud dextrose agar (SDA) at 20°C. Phenotypic methods were used to determine the genus of the fungus, followed by genotypic methods, specifically PCR, for species identification. The PCR product was sequenced by Sina Clone Company and deposited in a gene bank, ultimately confirming the presence of Saprolegnia salmonis.

Given the disease's direct correlation with the fish's living conditions, owners were advised to improve water quality, ensure appropriate living conditions, and manage dietary needs. Additionally, enhancing public awareness of Saprolegniosis and improving hygiene standards in endemic areas is recommended to mitigate its impact on the aquaculture sector.