The aim of this study was to investigate the effects of nettle (
Urtica dioica) hydroalcoholic extract (NHE) on saprolegniasis control, serum biochemistry, immunological and hematological indices of rainbow trout infected with
Saprolegnia. Rainbow trout (n=180, weight=22±0.27g) were randomly distributed in six treatments. Experimental groups were first made prone to the disease by
scaling the caudal peduncle, and then were exposed to bathing with
S. zoospores (3×10
5/L) for 4 hours. After infection, fish were stocked in 100 L tanks and received four treatments (50, 100, 150, and 200 mg/L of NHE) daily for 1 hour during 1 week. At the one-week bathing with NHE (first week), and a week after the bathing (second week), parameters of hematology [red blood cells (RBCs), white blood cells (WBCs), hematocrit, and hemoglobin], serum biochemistry (albumin, globulins, and total protein), immunity parameters (lysozyme activity, complement system, and respiratory burst activity), and mortality rate were examined in all treatments. Results showed that 100 mg/L of NHE significantly increased hematological, serum biochemical and immunity parameters in rainbow trout infected with saprolegnia (p< 0.05). Also, doses 50, 150 and 200 mg/L of NHE significantly reduced mortality rate with increasing fish resistance to saprolegniasis compared the control. Hence, the use of NHE at a dose of 100 mg/L with bath therapy is recommended to improve hematological and immunological parameters, and to control of saprolegniasis in this species.