The antibiotic treatment of cartilaginous fish, including Siberian sturgeon, is relatively poorly understood. The lack of research into simultaneous use of antibiotic and immunomodulation in the Acipenseridae has created the need for effective methods of prevention and treatment of the diseases affecting these species, especially in relation to intensive rearing and breeding with adapted technologies. The Siberian sturgeons administered oxytetracycline intraperitoneally @ 100 mg/kg BW and 24 hours post-injection the fish were immersed in lysozyme dimer @ 100 µg/l water for 30 minutes. The impact of oxytetracycline used together with or separately from lysozyme dimer on the morphology of head kidney in Siberian sturgeon was determined. The results of microscopic evaluation of this organ revealed the formation of morphological lesions resulting from the administration of the antibiotic: hyperaemia and regressive lesions in the epithelium of renal tubules - parenchymatous and vacuolar degeneration, necrosis and hyaline casts in the lumen of renal tubules. The domination of hematopoietic cells over lymphocytes, necrosis of hematopoietic cells, proliferation of lymphoid cells, presence of melano-macrophages and less-frequent macrophages were observed. It was shown that the immersion of Siberian sturgeons in lysozyme dimer had an immunomodulatory effect and decreased the severity of morphological lesions in the head kidney induced by the administration of oxytetracycline.

Key words: Head kidney, Immunomodulation, Oxytetracycline, Pathomorphology, Siberian sturgeon