The changes in the immune function of birds obtained during decades of intensive selection are rather disadvantageous. Today's chickens and turkeys are more susceptible to different infectious and metabolic diseases and exhibit a high mortality rate. Subsequently in recent years there is growing interest in the use of feed additives with immunomodulatory properties in intensive poultry production. This article reviews and discusses the results of current studies on the effects of yeast cell products on the different mechanisms of the immune system in poultry. The majority of the experiments presented in this review indicate that the use of yeast derivatives may have a beneficial influence on the immune system and resistance to the colonisation of pathogenic microorganisms. However, the positive effect of yeast derivatives on the immune system is not always reflected in an improvement in production parameters, especially when the experiments are conducted on healthy (non-challenged) birds.

A total of 200 weaned (35 days) hybrid Hyla rabbits were randomly divided among five groups housed in bicellular cages (20 cages per group). Between 35 and 60 days of age, the groups were submitted to the following treatments: group ANT (positive control) fed a basal diet supplemented with antibiotics (colistin sulphate, 144 mg/kg; tylosin, 100 mg/kg; and oxytetracyclin, 1000 mg/kg); groups MOS_0.5, MOS_1.0 and MOS_1.5 fed the basal diet supplemented with 0.5, 1.0 and 1.5 g/kg mannanoligosaccharides (MOS), respectively; another group fed the basal diet without antibiotics or mannanoligosaccarides supplementation (negative control). Along the trial, an episode of epizootyc rabbit enteropathy occurs so that in the control group mortality rate was very high (78%) and survivor rabbits showed severe symptoms of disease (diarrhoea). Thus, the control group was discarded from the trial. At 60 days of age, samples of caecal content were collected from 10 rabbits per group and used as inocula for an in vitro gas production trial. At the end of fermentation (120 h of incubation), organic matter digestibility (OMd), cumulative gas production, fermentation kinetics, pH, volatile fatty acid (VFA) and NH3 productions were measured. Inoculum from MOS_1.0 rabbits showed the significant higher values of OMd (64.21%, P < 0.05), gas production (262.32 ml/g, P < 0.05), acetate (96.99 mmol/g OM, P < 0.05) and butyrate (26.21 mmol/g OM, P < 0.05) than the other groups. Slight differences were recorded among the groups ANT, MOS_0.5 and MOS_1.5. In addition, branched chain acids, in proportion to total VFAs, were significantly higher in MOS_1.0 inoculum (0.04, P < 0.05). MOS are able to affect fermentation activity of caecal micro-organism, but their activities seem not proportional to their level in the diet.

Since in-feed antibiotics (IFAs) are being taken out of broiler diets around the world, beginning in Sweden in the year 1986, the search for alternatives to replace IFAs has gained increasing interest in animal nutrition. Gut microflora appears to be the target for IFAs and alternatives to exert health benefits and some growth-promoting effects. In this review the effect of six kinds of alternatives to IFAs on gut microflora is discussed and their working mechanisms and growth-promoting effects are reviewed. This review focuses on mannanoligosaccharides (MOS) as alternative to antibiotics using research results obtained by various authors in recent years.