turkeys

One of the first and most cited reports on the immunomodulatory effects of yeast products in poultry was published in 1996 and concerned turkey diet supplementation with MOS (Savage et al., Reference SAVAGE, COTTER and ZAKRZEWSKA1996a; Reference SAVAGE, ZAKRZEWSKA and ANDERSEN1996b). In this study significant stimulatory effects of MOS (1.1 g/kg of diet) on plasma IgG and bile IgA antibody levels, body weight gain and intestinal villus height in turkey poults were reported. Similarly, in the subsequent experiment dietary MOS supplementation (1 g/kg) increased serum IgG and IgM levels and decreased T lymphocyte percentage in the peripheral blood (Cetin et al., Reference CETIN, GUCLU and CETIN2005).

Huff et al. (Reference HUFF, HUFF, FARNELL, RATH, SOLIS and DONOGHU2010) evaluated the ability of commercial yeast extract product (YE, Alphamune) to modulate the immune response in male turkey poults challenged with E. coli. In this experiment the numbers and percentages of heterophils in peripheral blood were increased and their oxidative burst activity was stimulated by YE. As an effect of these changes in the innate (non-specific) immune response, E. coli bacteria were isolated from the air sac and liver of a lower percentage of birds provided with YE. Diet supplementation with YE also had a positive effect on blood haematology. In addition, YE modulated the dramatic increase in oxidative burst and serum triglycerides due to transport stress. In conclusion, the authors suggested that YE, due to its immunomodulatory properties, has potential as a non-antibiotic alternative for decreasing pathogenic bacteria in turkey production (Huff et al., Reference HUFF, HUFF, FARNELL, RATH, SOLIS and DONOGHU2010). These same authors (Huff et al., Reference HUFF, DUTTA, HUFF and RATH2011) indicated that YE can positively modulate an innate immune response, measured as the heterophyle oxidative burst activity, improve the body weight gain at the first week of age and decrease the mortality in stressed female turkeys. There was also a tendency for increased heterophyle/lymphocyte ratio after diet supplementation with YE due, the authors suggested, to the fact that yeast may function as a mild stressor, which enables the animal to adapt more easily to other stressors (Huff et al., Reference HUFF, DUTTA, HUFF and RATH2011).

broiler chickens

The effects of yeast product supplementation were studied in broiler chickens by Gao et al. (Reference GAO, ZHANG, YU, WU, YOON, QUIGLEY, GAO and QI2008). They found that yeast culture (Diamond V XP Yeast Culture) positively modified immune functions, both the innate and acquired immune response. As part of innate immunity, the activity of serum lysozyme was increased, which suggested that more phagocytes were activated and better protection against infections was provided when the yeast culture was added to the diet. As part of an acquired, humoral immune response, enhanced antibody titres to Newcastle disease virus and serum IgM were reported. Likewise, the mucosal immunity, measured as a secretary IgA concentration in the duodenum, was increased in broilers fed the diet supplemented with yeast culture. It should also be noted that yeast product, used in this experiment at 2.5 g/kg, significantly improved body weight gain, feed conversion, digestibility of Ca, P and energy, retention of protein, and villus height to crypt depth ratio in the duodenum, jejunum and ileum (Gao et al., Reference GAO, ZHANG, YU, WU, YOON, QUIGLEY, GAO and QI2008).

In a subsequent study, the above authors evaluated the immunostimulative efficacy of the Saccharomyces cerevisiae fermentation product in Eimeria tenella-infected broilers (Gao et al., Reference GAO, ZHANG, WU, YU, YOON, MOORE, GAO, YAN and QI2009). They reported that body weight gains were significantly decreased by coccidial infection and improved by yeast supplementation. The positive effect of yeast product on immune response was reflected in increased CD3+, CD4+ and CD8+ T-lymphocyte counts, the ratio CD4+/CD8+ in blood and the spleen, and ileum intraepithelial lymphocyte count and caecal tonsil secretory IgA counts. The authors concluded that the yeast product used could improve the immune function and growth performance in coccidia-infected chickens (Gao et al., Reference GAO, ZHANG, WU, YU, YOON, MOORE, GAO, YAN and QI2009). The beneficial effects of whole yeast cell product, in chickens challenged with Eimeria sp. were confirmed recently by Shanmugasundaram et al. (Reference SHANMUGASUNDARAM, SIFRI and SELVARAJ2013). They reported that whole yeast cell product significantly increased macrophage nitric oxide and inflammatory cytokine production, improved body weight gain and feed efficiency, and decreased the faecal coccidial oocyst count during postcoccidial challenge.

Interesting results were obtained when yeast (Saccharomyces cerevisiae) products, i.e. yeast cells, yeast protein concentrate (YPC) or YPC-pellets, were added to the diet (1 g/kg) of heat-stressed broilers orally challenged with Salmonella enteritidis (Haldar et al., Reference HALDAR, GHOSH, TOSHIWATI and BEDFORD2011). Humoral immune response against Newcastle disease was significantly increased by YPC and YPC-pellets at 14d, 28d and 35d, and yeast cells supplementation at 35d of age. These beneficial, immunomodulatory effects of YPC and YPC pellets were confirmed by a significant improvement of performance indices, i.e. body weight at 21 and 35d and FCR in 35d. Yeast products, mainly YPC and YPC-pellets, were also effective in a decreasing of Salmonella numbers in digesta and excreta and E. coli numbers in digesta, reducing of serum cortisol and increasing of serum T3 concentration (Haldar et al., Reference HALDAR, GHOSH, TOSHIWATI and BEDFORD2011).

The positive effects of yeast on the immune functions of broilers were found when the yeast cell wall (YCW) was evaluated. Thus, a dietary addition of YCW derivative containing 25% MOS and 30% β-glucan enhanced innate immunity response, measured as phagocytic activity of macrophages, in chickens inoculated with Sephadex G-50 (Ferreira et al., Reference FERREIRA, MURAKAMI, SILVEIRA, DOS SANTOS and FERNANDES2011). Macrophages are not only the elements of innate immunity, where they destroy bacteria by phagocytosis but, by secretion of cytokines and presentation of antigens, they are also important for acquired (specific) immune response. However, in the study of Ferreira et al. (Reference FERREIRA, MURAKAMI, SILVEIRA, DOS SANTOS and FERNANDES2011), the response in immunity was found at one level of YCW product addition (2 g/kg of diet) and did not beneficially affect the performance parameters of chickens. Recently the efficacy of Saccharomyces cerevisiae YCW dietary supplementation for protecting the integrity of intestinal mucosa in broilers vaccinated against coccidiosis was evaluated (Luquetti et al., Reference LUQUETTI, FURLAN, ALARCON and MACARI2012). It was found that the villi of birds supplemented with yeast cell wall (2 g/kg) were higher, irrespective of a vaccination against coccidiosis. When YCW was added to the diet of vaccinated chickens, a slight reduction in epithelial cell loss percentage was noted. The authors concluded that S. cerevisiae YCW supplementation may be a useful management tool to maintain the intestinal integrity of broilers vaccinated against coccidiosis (Luquetti et al., Reference LUQUETTI, FURLAN, ALARCON and MACARI2012). Results of the study of Ghosh et al. (Reference GHOSH, HALDAR, BEDFORD, MUTHUSAMI and SAMANTA2012) revealed that YCW can provide greater immunity against Newcastle disease than AGP (bacitracin methylene disalicyte), may ensure greater protection against intestinal colonisation by Salmonella pullorum bacteria, and improve feed efficiency (Ghosh et al., Reference GHOSH, HALDAR, BEDFORD, MUTHUSAMI and SAMANTA2012).

Zhang et al. (Reference ZHANG, LIAO, LI, LI, MA and YAN2012) determined the effects of dietary YCW (Saccharomyces cerevisiae) on the performance and immune function of cyclosporine A-treated (immunosuppressed) chickens. The results showed that supplementation of YCW (3 g/kg) significantly improved daily weight gain compared with the control. YCW significantly mitigated the cyclosporine A-induced decrease of peripheral blood lymphocyte blastogenic response. In addition, YCW significantly increased the relative weights of the bursa of Fabricius and thymus, and up-regulated the splenic expression of pro-inflammatory cytokines IL-1β and IL-6 in immunosuppressed birds. The authors indicated that YCW has a beneficial influence in attenuating the immunosuppressive effects of cyclosporine. The challenge, therefore, is improving the growth performance of broiler chickens (Zhang et al., Reference ZHANG, LIAO, LI, LI, MA and YAN2012). Similar results were found in the study of Muthusamy et al. (Reference MUTHUSAMY, HALDAR, GHOSH and BEDFORD2011), where YCW positively affected body weight gain, feed conversion and villus height in the jejunum, and enhanced humoral immune response, measured as antibody level against the Newcastle disease vaccine (NDV). It was also reported that YCW supplementation (2 g/kg) could mitigate the negative effect of Fusarium mycotoxins on the immune system, including the relative weight of immune organs, antibody titres of Newcastle disease and splenic mRNA expression of IL-1β, IL-6, and IFN-γ (Li et al., Reference LI, YANG, YANG, WANG, JIANG and WU2012).

Silva et al. (Reference SILVA, DELLA TORRE DA SILVA, TORRES, DE FARIA FILHO, HIROTA HADA and BARBOSA DE MORAES2009) studied the immunomodulatory effects of two Saccharomyces cerevisiae products (MOS and yeast extract) added to pre-starter broilers diet (1-7 d of age). The obtained results indicated that the dietary addition of yeast products in the pre-starter phase did not have any significant effect on the humoral immune response, i.e. serum IgG titres against Newcastle disease virus or infectious bursal disease or lacrimal IgA titres, throughout the production cycle of the broilers (Silva et al., Reference SILVA, DELLA TORRE DA SILVA, TORRES, DE FARIA FILHO, HIROTA HADA and BARBOSA DE MORAES2009).

In a recent experiment on non-challenged chickens, supplementation of yeast-derived carbohydrates down regulated the expression of certain genes in the ileum and caecal tonsils, suggesting anti-inflammatory response, and increased the H:L ratio in the blood and number of germinal centres in the caecal tonsils, without any effects on immune organs weights, gut morphology and production parameters (Munyaka et al., Reference MUNYAKA, ECHEVERRY, YITBAREK, CAMELO-JAIMES, SHARIF, GUENTER, HOUSE and RODRIGUEZ-LECOMPTE2012). Yeast-derived carbohydrates, used in-feed for chickens infected with Clostridium perfringens, did not affect performance and gut morphology-associated response, whereas they altered innate immunity, supporting pro-inflammatory response via T-helper cell associated pathways to control early stages of infection (Yitbarek et al., Reference YITBAREK, ECHEVERRY, BRADY, HERNADEZ-DORIA, CAMELO-JAIMES, SHARIF, GUENTER, HOUSE and RODRIGUEZ-LECOMPTE2012).

The effect of different levels of dietary MOS supplementation (0-3 g/kg) on antibody response to infectious bronchitis, infectious bursal disease and Newcastle disease vaccines was examined in an early study of Shafey et al. (Reference SHAFEY, MUFAREJ, SHALABY and JARELNABI2001). They did not find any changes in the immune response to IB, IBD and ND vaccines in chickens fed supplemented diets. More recently Khalaji et al. (Reference KHALAJI, ZAGHARI and NEZAFATI2011) reported that dietary MOS supplementation (0.5-1.5 g/kg) beneficially affected the acquired immune function, measured as a response to phytohaemagglutinin-P and SRBC, and performance of broilers. Similarly, MOS at 1, 2 or 3 g/kg in feed was effective in stimulating humoral immune response and increased the antibody titre against Avian Influenza virus in vaccinated chickens (Tohid et al., Reference TOHID, HASAN and ALIREZA2010). Recently, the immunomodulating activity of MOS was confirmed at the transcriptional level, since it was found that dietary MOS altered the intestinal expression of genes important for cell immune response, including β 2-microglobulin, lysozyme, lumican, apoliprotein A-1, and fibronectin (Xiao et al., Reference XIAO, POWER, MALLONEE, ROUTT, SPANGLER, PESCATORE, CANTOR, AO, PIERCE and DAWSON2012).

The immunomodulatory properties of MOS were investigated in broiler chickens during natural exposure to coccidiosis (Gomez-Verduzco et al., Reference GOMEZ-VERDUZCO, CORTES-CUEVAS, LOPEZ-COELLO, AVILA-GONZALEA and NAVA2009). Dietary supplementation with MOS (0.5 g yeast cell wall/kg of diet) positively altered mucosal, humoral and cell-mediated immune responses, i.e. increased mucosal IgA secretion, antibody titre against NDV and basophilic hypersensitivity after intradermic inoculation of phytohaemagglutinin (determination of T-cell response). These effects on immunity were connected with improved body weight gain and feed conversion, and reduced parasite excretion in faeces of infected chickens fed the diet supplemented with MOS. The authors indicated that MOS, used in the form of a yeast cell wall derivative, enhanced the immune response against coccidial infections in chickens and has a potential for modulating mucosal immune responses (Gomez-Verduzco et al., Reference GOMEZ-VERDUZCO, CORTES-CUEVAS, LOPEZ-COELLO, AVILA-GONZALEA and NAVA2009).

β-glucan is the next yeast cell wall compound whose immunomodulatory properties have been intensively studied in broilers chickens. In one of the earlier experiments, Cheng et al. (Reference CHENG, LEE, WEN and WENG2004) found that β-glucan dietary supplementation (0.2 or 0.5 g/kg) stimulated some aspects of cell-mediated immune response, i.e. increased chemotaxis activity of macrophages. However, antibody titres against Newcastle disease vaccine (NDV), lymphocyte proliferation after stimulation with difference mitogens and growth performance were not affected by dietary β-glucan (Cheng et al., Reference CHENG, LEE, WEN and WENG2004). The addition of β-glucan to the diet of male chickens significantly enhanced innate immune responses, i.e. increased phagocytosis, bactericidal killing, and oxidative burst in isolated heterophils (Lowry et al., Reference LOWRY, FARNELL, FERRO, SWAGGERTY, BAHL and KOGUT2005). In this study β-glucan provided significant protection against Salmonella enterica organ invasion in young chickens (Lowry et al., Reference LOWRY, FARNELL, FERRO, SWAGGERTY, BAHL and KOGUT2005). Huff et al. (Reference HUFF, HUFF, RATH and TELLEZ2006) found that β-glucan supplementation protected broiler chickens against a decrease in growth after challenge with Escherichia coli. This beneficial influence was observed when β-glucan was fed for only the first seven days prior to challenge, but not with continuous supplementation. β-glucan also modulated the effects of E. coli challenge on the relative weights of the liver, heart and the bursa of Fabricius (Huff et al., Reference HUFF, HUFF, RATH and TELLEZ2006).

Zhang et al. (Reference ZHANG, GUO and WANG2008) found a significant, quadratic increase in serum cytokines (IL-1, IL-2, IFN-γ and TNF-α), plasma globulin, serum IgG and intestinal IgA concentrations, and lymphoid organs weights following β-glucan supplementation (25-125 mg/kg). The authors explained these results by the fact that β-glucan may join to the MHC compound, in this way activating macrophages, which leads to the subsequent immunostimulation and suggested that observed changes in cytokine levels increase the preparedness of the body to respond to internal challenges (Zhang et al., Reference ZHANG, GUO and WANG2008). Additionally, in this experiment, performance of broiler chickens, i.e. body weight gain and feed conversion, were improved by β-glucan (Zhang et al., Reference ZHANG, GUO and WANG2008). In contrast, in the recent study of Elrayeh and Yildiz (Reference ELRAYEH and YILDIZ2012), β-glucan added to the diet alone or simultaneously with inulin did not affect humoral immune response, measured as a specific antibody titre NDV, and the performance indices of broiler chickens.

The immunomodulatory properties of yeast β-glucan were also thoroughly evaluated in broiler chickens challenged with coccidiosis (Cox et al., Reference COX, SUMNERS, KIM, McELROY, BEDFORD and DALLOUL2010). The obtained results indicated that dietary yeast β-glucan (0.2 or 2.0 g/kg) had no effect on growth performance, but altered immune-related genes (inducible nitric oxide synthase, IL-4, IL-13, IL-14, IL-18, IFN-γ, mucin-1, mucin-2) expression, and significantly reduced intestinal lesion severity in infected birds. Generally, these changes in cytokine profiles modulated the Th1/Th2 balance, favouring an enhanced innate and Th1-mediated cell response, and down regulating the Th2-mediated immunity. The authors indicated that the increased pro-inflammatory response, that was obtained in birds fed with β-glucan, enhanced protection against the coccidiosis as seen by the decreased lesion severity. They emphasised that augmentation of the inflammatory response by β-glucan was not chronic and gave no negative results in performance parameters, further suggesting that the immune system was not stimulated enough to be detrimental to growth (Cox et al., Reference COX, SUMNERS, KIM, McELROY, BEDFORD and DALLOUL2010).

The number of experiments on the potential effect of yeast nucleotides on poultry immune functions is far more limited. In a 12-week study with one day old ISA-Brown male chicks, the experimental diet was supplemented with yeast RNA (5 or 10 g/kg) and fed for four weeks (Deng et al., Reference DENG, WONG and NOLAN2005). There was no effect of nucleotides on growth performance during the supplementary period, however birds previously fed the diet with RNA (10 g/kg) grew faster than in control treatment during weeks four to eight. Humoral and cell-mediated immune response, measured as serum antibody level against SRBC and cutaneous reactivity of toe webs to phytohaemagglutinin, were not altered by dietary yeast RNA, but chicks fed the experimental diet (5 g/kg) had a higher relative spleen weights than the control birds at week four (Deng et al., Reference DENG, WONG and NOLAN2005). In a recent study (Jung and Batal, Reference JUNG and BATAL2012), dietary yeast nucleotides did not affect the performance indices of broilers reared under normal conditions, however, when birds were exposed to stress conditions such as high stocking density, nucleotides had a beneficial effect on body weight gain, feed conversion ratio, villus height and villus height to crypt depth ratio. The authors concluded that dietary nucleotides may be important for maintaining maximum growth performance when chickens are exposed to stress conditions, such as high stocking density combined with dirty litter (Jung and Batal, Reference JUNG and BATAL2012).

laying hens

One of the first experiments on the immune response of layers fed a diet supplemented with yeasts product was conducted with heat-stressed hens (Asli et al., Reference ASLI, HOSSEINI, LOTFOLLAHIAN and SHARIATMADARI2007). In this study, immunised Hy-Line White hens fed the diet supplemented with Saccharomyces cerevisiae culture (1 g/kg) had significantly higher antibody titres against SRBC and numerically higher egg production than the control birds. The authors indicated that yeast culture during heat stress improves a layer's humoral immune response and may lead to increased laying performance and egg quality parameters (Asli et al., Reference ASLI, HOSSEINI, LOTFOLLAHIAN and SHARIATMADARI2007). These positive results were confirmed more recently by Yalcin et al. (Reference YALCIN, YALCIN, CAKIN, ELTAN and DAGASAN2010), who evaluated the effects of dietary yeast (Saccharomyces cerevisiae) autolysate on egg production and quality, some blood parameters and humoral immune response of Hy-Line Brown hens. Yeast autolysate supplementation significantly increased antibody titre to SRBC, positively affected egg production, egg weight and feed efficiency, and decreased egg yolk cholesterol and blood serum levels of cholesterol and triglyceride. There were also some beneficial changes in yolk fatty acid composition after yeast culture addition to the diet. The authors concluded that dietary yeast autolysate (2, 3 or 4 g/kg) had positive influences not only on humoral immune response, but also on performance and egg cholesterol concentration in laying hens (Yalcin et al., Reference YALCIN, YALCIN, CAKIN, ELTAN and DAGASAN2010). Similar beneficial effects of yeast autolysate in terms of humoral immunity and laying performance were found in their subsequent study (Yalcin et al., Reference YALCIN, YALCIN, UZUNOGLU, DUYUM and ELTAN2012).

In the recent study of Lensing et al. (Reference LENSING, VAN DER KLIS, YOON and MOORE2012) the efficacy of a Saccharomyces cerevisiae fermentation product (YP) was evaluated in hens infected with Eimeria maxima. Supplementation of YP had no effect on humoral immunity and performance however it significantly reduced the incidence and severity of intestinal lesions. According to the authors this effect of YP may be partially attributed to its antioxidant and anti-inflammatory properties (Lensing et al., Reference LENSING, VAN DER KLIS, YOON and MOORE2012). Moreover Gurbuz et al. (Reference GURBUZ, BALEVI, KURTOGLU and OZNURLU2011) reported that YCW positively altered egg production, feed intake and villus height in Hy-Line layers.

A study on Ross breeder hens fed a diet contaminated with aflatoxin (1 g/kg) evaluated the influence of YE from Saccharomyces cerevisiae on immune function, blood haematological parameters and the antioxidant defence system (Matur et al., Reference MATUR, ERGUL, AKAYAZI, ERASLAN, INAL, BILGIC and DEMIRCAN2011). The obtained results showed that YE reduced some of the adverse effects of aflatoxin on heterophils and lymphocytes, and improved functions of the non-specific immune system, measured as a phagocytic activity and oxidative burst of heterophils. The CD5⁺ cell percentage decreased in hens fed the aflatoxin contaminated diet, however this negative effect was minimised in YE supplemented hens. The beneficial effect of YE was reflected in increased white blood cell counts and monocyte percentages, and decreased plasma malondialdehyde concentration. Therefore, the authors concluded that YE might be useful for modulating some of the negative effects of aflatoxin on the immune functions in birds (Matur et al., Reference MATUR, ERGUL, AKAYAZI, ERASLAN, INAL, BILGIC and DEMIRCAN2011). Similarly, in an earlier experiment, Stanley et al. (Reference STANLEY, WINSMAN, DUNKLEY, OGUNLEYE, DALEY, KRUEGER, SEFTON and HINTON2004) found that Saccharomyces cerevisiae culture significantly enhanced the performance and hatchability, and decreased embryonic mortality in broiler breeders fed the diet contaminated with aflatoxin.

The efficacy of yeast derivatives in boiler breeder hens was evaluated using MOS as a potential immunomodulator (Shashidhara and Devegowda, Reference SHASHIDHARA and DEVEGOWDA2003). The obtained results showed that supplemental MOS (0.5 or 1.0 g/kg) positively affected acquired immune response, i.e. it significantly enhanced antibody titres against infectious bursal disease virus (IBDV) both in the hens and in their progeny. Importantly, besides this effect on immunity, MOS improved the sperm density, and hatchability and fertility of eggs. The authors speculated that the effect of MOS on antibody titres might have been due to the direct influence of MOS on the immune system or improved intestinal absorption of such microelements as Zn, Cu and Se which are important for immune function. They also stated the hypothesis that yeast cell wall components, such as MOS and β-glucans, can be recognised and bound to defence cells in the gut-associated lymphoid tissue (GALT), and, in turn, activate immune defences such as phagocytosis, the alternative complement pathway and the lectin pathway (Shashidhara and Devegowda, Reference SHASHIDHARA and DEVEGOWDA2003). Corresponding findings were obtained in the earlier study of Cotter et al. (Reference COTTER, MALZONE, PALUCH, LILBURN and SEFTON.2000), where MOS positively affected humoral immune response in commercial laying hens.