Severe Outbreak of Saprolegnia Spp. Infection in Spotted Snakehead (Channa punctata, Bloch 1793): Clinical Assessment, Histopathology, Haemato-Biochemical Indices and Insights Into Therapeutic Effects.

Saprolegniasis, caused by Saprolegnia spp., is one of the most lethal oomycete infections affecting freshwater fish. This study investigated a severe fungal infection in Channa punctata (n = 150) fish obtained from a pen culture in the Tanguar haor region of the Sunamganj District. The infection was identified by its characteristic fluffy white appearance, with fungal hyphae detected in multiple organs. Diagnosis was confirmed as Saprolegnia spp. through clinical signs and wet mount examination. Infected fish exhibited notable symptoms, including drowsiness, head-down floating, cloudy eyes, skin burns and deep lesions, with broken caudal fins in some cases. The infection prevalence was calculated at 85%. For further growth analysis, Czapek-Dox agar (CDA) medium was used to culture fungal hyphae. The average weight and length of the fish were 74.41 ± 1.32 g and 14.38 ± 0.84 cm, respectively. Subsequently, 120 infected fish were allocated into 4 treatment groups, each placed in glass aquaria (90 × 45 × 45 cm) with 120 L of water and treated for a 10-day experimental period. The groups included T1 (control), T2 (ivermectin, 2.5 ppm + NaCl, 2%), T3 (CuSO 1 ppm + Viodin 2.5 ppm) and T4 (aquarium heater set to 30°C + NaCl, 2%). Significant recovery was noted in T4, where the cotton-like structures disappeared within 10 days. Haematological and biochemical indices also revealed significant differences between T1 and T4. Histopathological analysis identified several cellular abnormalities, such as granuloma formation, fungal hyphae presence, dermal degeneration, gill and liver haemorrhages, kidney necrosis and tubular degeneration and splenitis with intracellular oedema. Post-treatment, histopathological anomalies were notably reduced in T4, with fewer fungal hyphae observed, whereas moderate alterations remained in T3 and T2 compared to the control group. Kaplan-Meier survival analysis indicated that C. punctata achieved the most favourable survival rate (p < 0.05) in T4 by combining elevated temperature with NaCl, resulting in a 70% survival rate, whereas the lowest survival rate of 23.33% was observed in T1. This study offers valuable insights for effective treatment and management of Saprolegnia spp. infections in C. punctata.