Background. Herbal medicines are increasingly used for their effects on the immune system. Silymarin, a mixture of flavonolignans from the seed of milk thistle (Silybum marianum), is used in treatment of liver disease, food and drug poising, and foetal diseases such as viral hepatitis, diabetes, and ischemia. Although, immunostimulant effects of the dietary silymarin were studied on different experimental animals, no information is available about the effects of silymarin on immune parameters of fish. The presently reported study was conducted to investigate the immunomodulatory effects of silymarin on some immunological and haematological parameters of rainbow trout. Materials and methods. Juvenile rainbow trout, Oncorhynchus mykiss (Walbaum, 1792), were maintained in 1000 L fiberglass tanks at 15 ± 2ºC supplied with systems of water recirculation and aeration. Silymarin extract incorporated into diets (0.0, 0.1, 0.4, and 0.8 g of per 1 kg of feed) of fish.  The trout were fed silymarin-supplemented diet for 30 days. Haematological parameters, such as red blood cell count (RBC), white blood cell count (WBC), haematocrit (Hct), haemoglobin (Hb), differential leukocyte- and immunological parameters such as peroxidase, lysozyme, and complement activities, total protein, albumin and globulin levels were measured on day 7, 15, and 30 days of silymarin treatment. Results. The results indicated that oral administration of silymarin in fish, after 15 and 30 days of experimental periods, might enhance haematological and immunological parameters including lysozyme and complement activities, total protein and globulin levels, compared to the controls. Conclusion. The results suggest that oral administration of silymarin may be useful to strengthen the immune system in rainbow trout.

silymarin, rainbow trout, haematological parameters, immunological parameters