effects of discontinuous administration of dietary mannan oligosaccharide and β-1,3-glucan on hematological and blood serum biochemical parameters of rainbow trout (oncorhynchus mykiss)

introduction bans and restrictions of antibiotics as feed additives in fish culture in many countries have resulted in the increase in studies on alternative dietary supplements such as prebiotics to enhance the health and production of cultured fish. prebiotics are nondigestible food ingredients that beneficially affect the host by selectively stimulating the growth and/or activity of one or a limited number of bacteria in the colon. in addition, prebiotics promote the growth of lactic acid bacteria that are beneficial to health and lessen the density of pathogenic microorganisms. because of the conservation of immune system, it is suggested that alternate administration of different immunostimulants may activate different parts of immune system of shrimps and take advantage of different immunostimulants to solve the problem of immunity fatigue and enhance the immunity continuously. however, there is no report on the discontinuous administration of immunostimulants for rainbow trout. thus, considering the strategic importance of this species, the goal of this study was to assess the effects of discontinuous administration of dietary mannan oligosaccharide and β-1,3-glucan on hematological and blood serum biochemical parameters of rainbow trout. materials and methods a 6-week trial was conducted to compare the effects of discontinuous administration of dietary mannan oligosaccharide and β-1,3-glucan on hematological and blood serum biochemical parameters of rainbow trout (oncorhynchus mykiss). four feeding strategies were set, including feeding prebiotic-free diet continuously (control), feeding dietary prebiotic continuously (1.5 g kg-1), feeding dietary alternately (one week prebiotic+one week control diet and 2 day prebiotic+5 day control diet). juveniles were fed the experimental diet in rate of 4% of the body weight per day, spread across two feeding time. at the end of the experiment, three fish were sampled randomly from each tank and were anesthetized with clove solution, and blood was drawn from the caudal vein, using a syringe. then, blood samples were introduced to both heparinized and non-heparinized tubes in order to perform haematological and biochemical studies, respectively. blood sera were obtained by centrifuging blood samples at 3000 rpm for 5 min, and the sera were removed with a disposable transfer pipette and stored at -20ºc until analysis for biochemical and hematological studies. the determined parameters were the number of red blood cells (rbc), white blood cells (wbc), hematocrit (pcv), and hemoglobin (hb). differential white blood cell counts, including neutrophiles (heterophiles), lymphocytes and monocytes were also identified. statistical analysis was carried out using one-way analysis of variance using spss. differences between means were determined using duncan’s multiple test (p<0.05). results and discussion the results showed that wbc, hb, pcv, lymphocyte, monocyte, heterophyl, protein, albumin were higher but the level of cholesterol and triglyceride were lower in continuously administration of prebiotic than other feeding strategies and control group, although there were no significant effect. there were significant increase in rbc in continuously strategy. it seems that the improvement in hematological and blood serum biochemical parameters in feeding dietary prebiotic continuously may be attributed to the stimulation of growth of beneficial bacteria such as lactic acid bacteria which improved microvilli alignment and increased resistance against pathogens. the reasons for the different results are not clear yet. phagocytic activity measurement was not performed in this study, but it may be proposed that elevated leucocyte levels could have been responsible for the increase in the activity enhancing defense mechanism during feeding. the increase in the serum protein and albumin levels in feeding dietary prebiotic continuously is considered to be associated with a stronger innate response in fish. however, the decreased albumin and total protein in other strategy and control group in the current study would be indicative of an abnormality in liver or kidney functions. however, unlike current study, shrimps fed with dietary β-glucan discontinuously 2 days followed by the basal diet for 5 days showed the highest immunity. it appears that the different basal diet, level of prebiotic supplementation, adaptation period, animal characteristics (species, age, stage of production), microbiota loading, period and hygienic conditions of the experiment can cause these differences. conclusion the result indicated that continuously applying mannan oligosaccharide and β-1,3-glucan at the level 1.5 g kg-1 into the diet caused improved hematological and blood serum biochemical parameters of rainbow trout. this study encourages further research on different aspects of prebiotic continuously administration in rainbow trout as well as immunological studies to determine the effects of this strategy on the immune system and disease resistance.