Is Astragalus Extract Benefits for the Immune System?

Antibiotics are widely used in animal husbandry for their disease-fighting and growth-promoting effects. However, they can have adverse effects on animals, causing toxicity to the liver, kidneys, nervous system and hematopoietic system, as well as disrupting the flora in the animal's body and causing antimicrobial resistance. The most significant impact on humans is that it can cause bacteria to develop drug resistance and even form superbugs, which not only threatens human health, but also seriously affects food safety, environmental protection and ecological balance. The “anti-ban” was officially implemented in July 2020, which means that feed manufacturers will stop producing commercial feeds containing growth-promoting drug feed additives (except traditional Chinese medicine). The era of a complete ban on antibiotics has arrived, stimulating the research, development and promotion of new feed additives that are environmentally friendly, safe and efficient.

Chinese herbal medicine is a natural feed additive that not only has multiple functions such as disease prevention, health care, improving production performance and animal quality, but also has the characteristics of being diverse, natural, with low residues and not prone to drug resistance. Chinese herbal medicine can not only directly inhibit and kill pathogenic microorganisms, but also improve the immunity and disease resistance of animals. The main active ingredients of Chinese herbal medicine are polysaccharides, glycosides, alkaloids, essential oils and organic acids [1]. The main immunologically active substance in many Chinese herbal medicines is polysaccharides. Astragalus polysaccharides (APS) is a new type of green feed additive that can effectively improve the disease resistance of young animals. This paper reviews the research progress on the effects of astragalus polysaccharides on animal immune function, with the aim of providing a reference for researchers.

1 What is astragalus polysaccharide?

Astragalus has the effects of tonifying qi, strengthening the body's defenses, diuresis and promoting the elimination of toxins and pus. It mainly contains bioactive substances such as alkaloids, glycosides, glucuronic acid, flavonoids and polysaccharides. Among them, astragalus polysaccharide is the most immunologically active substance, composed of water-soluble heteropolysaccharides such as glucose, galactose, fructose and arabinose. Astragalus polysaccharides have immunomodulatory, antioxidant, antitumor, antidiabetic, cardiovascular system protective, anti-inflammatory and animal growth promoting activities, and do not cause drug residue problems. They can be used as feed additives and immune enhancers [2.4].

2. The mechanism of the immune enhancing effect of astragalus polysaccharide

2.1. Enhancing the immune function of immune organs

Immune organs are an important part of an animal's immune function. The central immune organs include the bone marrow and thymus of livestock and the bursa of Fabricius of poultry, while the peripheral immune organs include the spleen and lymph nodes. Different animals have different immune organs. The gill head kidney is an important immune organ in fish. A large number of studies have shown that astragalus polysaccharides can increase the relative weight of the thymus, spleen and lymph nodes, which is important for improving the body's immune function [5.10]. Wu Qiaowan found that adding 600 to 1200 mg/kg astragalus polysaccharide to the feed can significantly increase the head kidney index of young fish [11]. Other studies have shown that astragalus polysaccharide can increase the relative weight of the spleen and thymus in mice with lung cancer [12, 13].

2.2 Enhancing the action of immune cells

Immune cells include lymphocytes, dendritic cells, macrophages, granulocytes, mast cells, etc. On the one hand, astragalus polysaccharide can enhance the proliferation and differentiation of specific immune cells, further promote the secretion of antibodies by plasma cells, balance T cell subsets, and achieve the purpose of enhancing the body's immune function. On the other hand, it can also promote the function of non-specific immune cells, such as natural killer cells, macrophages, red blood cells, and dendritic cells [14, 15].

Xu et al. reported that astragalus polysaccharide can enhance the proliferation of mouse splenic lymphocytes [16]. Tao Yanhua et al. showed that Astragalus polysaccharide can promote lymphocyte transformation and improve cellular immune function in piglets within a certain dose range [17]. A large number of studies have reported that Astragalus polysaccharide can significantly enhance the activity of phagocytes in fish, thereby achieving the purpose of enhancing the immune regulatory effect of fish [11, 18.20].

2.3 Effect on immunoglobulins

Immunoglobulins are antibody-like globulins that include IgA, IgG, IgM, IgD and IgE. Astragalus polysaccharide can improve the immune function of animals by increasing the serum levels of IgA, IgG and IgM. Meng et al. showed that astragalus polysaccharide can significantly increase the serum immunoglobulin levels of mice [21]. . Shan et al. showed that oral administration of astragalus polysaccharide to chicks with Newcastle disease can increase the concentration of IgA in the serum, thereby enhancing the immune function of the jejunal mucosa and improving the ability to fight the Newcastle disease virus [22].

2.4 Antiviral effect

Astragalus polysaccharides can stimulate interferon formation, induce secretion of interleukin, hinder the synthesis of viral protein, produce antiviral protein, and thus produce a high antiviral effect [23]. Zhang et al. reported that astragalus polysaccharides can prevent infection with avian infectious bronchitis virus by inhibiting its replication [24].

3 Effects of Astragalus extract Astragalus polysaccharide on animal immune function

3.1 Effects of Astragalus polysaccharide on poultry immune function

Astragalus polysaccharide can enhance the immune function of poultry by increasing the immunoglobulin index of the immune organs. Wang Haifeng and Wu et al. showed that feeding young broilers with astragalus polysaccharide can significantly increase serum immunoglobulin levels and immunity [25, 26]. Wang Tao et al. added an astragalus polysaccharide preparation fermented by lactic acid bacteria to the feed, and found that the IgG and IgA concentrations in the serum of broiler chickens increased, thereby enhancing the body's immunity [27].

 Duan Junying et al. found that feeding broilers with 0.1% astragalus polysaccharide significantly increased the spleen, thymus and bursa indices of broilers [5]. Peng Ting et al. reported that the addition of astragalus polysaccharide liposomes to broiler diets significantly increased the index of immune organs. It is recommended that the addition be continued for 28 to 42 days for more significant results [7]. Other studies have also shown that feeding astragalus polysaccharides can enhance the immune capacity of broilers under immunosuppressive conditions, and that the immune activity of astragalus polysaccharides after γ-ray irradiation is even stronger [28.30].

Astragalus polysaccharides can significantly increase the antibody levels of chickens against Newcastle disease and avian influenza, enhance the immune effect of vaccines, and play a role in reducing the chance of immune failure in chickens. This shows that astragalus polysaccharides have certain promotional significance as immune enhancers in poultry production. Li Chenghe, Lu Decheng and others found that feeding broilers with different doses of astragalus polysaccharide can effectively improve the immune effect of the Newcastle disease vaccine and increase the serum IgM and IgG content [31, 32]. Miao Zhipeng and others reported that one of the ways to significantly enhance the immune effect of the Newcastle disease and H9 subtype avian influenza vaccines in broiler chickens is to feed an appropriate amount of astragalus polysaccharide [33]. Xiang Shuangyun et al. showed that a higher dose (1.0%) of astragalus polysaccharide can more significantly increase the level of Newcastle disease antibodies, promote the development of immune organs in chicks and the proliferation of T cells [8]. Li Guiqin et al. showed that astragalus polysaccharide has a significant effect on alleviating immune stress and improving immunity in ducks immunized against duck viral hepatitis [34].

3.2 Effects of Astragalus extract Astragalus polysaccharide on immune function in pigs

Feeding Astragalus polysaccharide to lactating sows can significantly increase the antibody levels in their milk, and piglets can increase their own antibody levels by sucking on their mothers' milk. Xu Songqi et al. found that adding astragalus polysaccharides to the diet of lactating sows can increase the concentration of IgG and IgA in the serum of sows and improve the immunity of lactating sows and piglets [35]. Tan et al. found that astragalus polysaccharides can significantly increase the concentration of IgG and IgM in the colostrum of sows and increase the level of specific antibodies to swine fever virus [36]. Jiang Guangzhi reported that pregnant sows naturally infected with porcine reproductive and respiratory syndrome virus (PRRSV) fed with astragalus polysaccharide for 60 days could significantly increase the PRRSV antibody level in the late stage of pregnancy and reduce the PRRSV viral load in the umbilical cord blood [37].

Weaned piglets have an underdeveloped intestinal system and immune function, and factors such as the stress caused by weaning lead to a decrease in resistance and a high susceptibility to disease, resulting in stunted growth. However, feeding astragalus polysaccharides can significantly improve the immunity of weaned piglets function, protecting them from disease. Yang et al. pointed out that adding astragalus polysaccharide to the feed can increase the levels of IgA and IgM in piglets' serum, and can improve growth performance, liver function and intestinal villus morphology [38]. Lü Debao et al. reported that astragalus polysaccharides can improve the immunity and survival rate of early weaned piglets [39]. The combined use of astragalus polysaccharides and bacterial lipopolysaccharides can increase the amount of histamine in the small intestine, enhance the small intestine's ability to resist pathogen infection and the body's immune function [40].

Astragalus polysaccharides can also improve the immunosuppressive effect of diseased pigs.

Liu Xinping et al. showed that adding astragalus polysaccharides to the feed of sows infected with swine fever can prolong the duration of antibody titers against swine fever, significantly increase the concentration of peripheral T cells, increase the CD4+/CD8+ ratio, and promote the immune function of pigs [41]. Liu Fenghua et al. showed that Astragalus polysaccharide can significantly increase the percentage of peripheral T cells and NBT positive cells in pigs infected with porcine circovirus type 2 [42]. Jiang Guangzhi's [37] research showed that feeding astragalus polysaccharides can increase the level of IFN-γ in the peripheral blood of piglets rapidly and for a sustained period of time, promote the cellular immune response mediated by the porcine blue ear disease attenuated vaccine, thereby enhancing the cellular immune effect of the vaccine and prolonging the time against infection with field virus.

3.3 Effect of astragalus polysaccharide on the immune function of aquatic animals

Astragalus polysaccharide has a significant immune-enhancing effect on many types of fish. Adding astragalus polysaccharide to aquatic feeds can improve the immunity of aquatic animals. It has been reported that the addition of astragalus polysaccharides to fish feed can significantly promote the development and maturation of immune organs in fish, thereby improving the immune function of the body [9, 10]. Liu Chunhua et al. showed that moderate feeding of astragalus polysaccharides can improve the non-specific immune function of grass carp and reduce the infection of Aeromonas hydrophila [43].

The addition of astragalus polysaccharides to the feed can increase the phagocytic activity of Nile tilapia, young bluegill sunfish and large yellow croaker [11, 18, 20]. Zhang et al. showed that astragalus polysaccharides enhanced the activity of head kidney macrophages in large yellow croaker and improved the non-specific immunity of the body. Among them, astragalus polysaccharide liposome had the best effect and was considered to be a good drug delivery route, but its exact mechanism still needs to be studied [19]. Wu et al. showed that oral administration of astragalus polysaccharide to crucian carp can improve its innate immune status, survival rate and growth performance, and the optimal dose in the feed is 100 mg/kg [44]. Wang Yu et al. showed that Astragalus polysaccharides can improve the immunity and disease resistance of hybrid snakeheads, and the optimal addition amount is 1.50 g/kg [45].

In addition Astragalus polysaccharides can also enhance the immune function of other aquatic animals to varying degrees. Chen Yajun et al. found that adding astragalus polysaccharides to the feed can significantly enhance the non-specific immune activity of loaches [46]. Zhou's research confirmed that astragalus polysaccharides can significantly enhance the vitality of the phagocytes of sea turtles [47]. Wang Yong Jie et al. showed that astragalus polysaccharide has a certain regulatory effect on the immune function of hybrid grass carp [48]. Yang Yibin et al. reported that astragalus polysaccharide has the effect of enhancing the immunity and disease resistance of the Chinese soft-shelled turtle and promoting its growth [49].

3.4 Astragalus extract The effect of astragalus polysaccharide on the immune function of other animals

Tang Jiao Yu et al. added astragalus polysaccharides to the feed of weaned rabbits at different concentrations to increase the spleen and thymus index, IgA and IgG content and T cell positive rate [50]. Li Nan et al. showed that astragalus polysaccharides can significantly enhance the immune level and improve the health of rabbits [51]. Bao Yu Lin et al. showed that astragalus polysaccharide can significantly enhance the immune function and growth performance of Tibetan sheep raised indoors [52]. Feng Shibin et al. proved through experiments that astragalus polysaccharide can increase the immunoglobulin content in the serum of Tibetan lambs, thereby significantly improving the body's immunity [53]. Xu Duan et al. found that feeding lambs with different doses of astragalus polysaccharide can reduce the incidence of lambs and improve the immunity of the flock [54]. Some studies have found that astragalus polysaccharide can reduce the harm caused by transportation stress in cattle and improve immunity against stress [55, 56].

4 Summary and outlook

As a green additive, astragalus polysaccharide has an important role in enhancing animal immune function and has the potential to replace antibiotics. In current research, the main ways to improve the utilization of astragalus polysaccharide are γ-ray treatment and ultrasound treatment, astragalus polysaccharide liposomes, lactic acid bacteria fermentation of astragalus polysaccharide and the combined use of astragalus polysaccharide and microecological agents. However , the application of astragalus polysaccharides still requires further research and discussion, such as the exact mechanism of action of astragalus polysaccharide liposomes and the development of other protective coating materials for astragalus polysaccharides, the optimal dosage of astragalus polysaccharides, the pathological mechanism of astragalus polysaccharides in animals, and the optimal extraction and purification methods of astragalus polysaccharides. In the future

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