First Report and Morphological Characterization of the Protozoan Ciliate Tetrahymena pyriformis (Ciliophora: Oligohymenophorea) Infecting Oreochromis niloticus from the Godavari Basin, India.

PURPOSE: This study reports the first confirmed occurrence of the ciliated protozoan Tetrahymena pyriformis infecting Nile tilapia (Oreochromis niloticus) in the Godavari Basin, India. It evaluates its pathological impact on fish health. METHODS: Between May 2024 and March 2025, a total of 314 fish were examined from Nanded and Parbhani districts. Infections were diagnosed through light microscopy using Giemsa (1:10) and silver nitrate (2%) staining, with ultrastructural features confirmed by scanning electron microscopy (SEM). Histopathological examinations of gills and skin were performed to assess tissue damage associated with parasitism. RESULTS: Out of 314 examined fish, 96 (30.5%) were infected, primarily in the gills and skin. As a facultative parasite, T. pyriformis became opportunistically pathogenic under stress conditions, leading to tetrahymenosis. Histopathology revealed severe epithelial disruption, excessive mucus secretion, hemorrhagic lesions, and necrosis, indicating substantial pathological effects of the infection. CONCLUSION: This first record of T. pyriformis in Nile tilapia from India highlights its pathogenic potential. It underscores the importance of routine parasitological monitoring in both wild and cultured populations to ensure fish health and sustainable aquaculture management.