Pigs increase their rearing density, their genetic potential is improved, their living environment deteriorates, live pigs circulate frequently, and large amounts of antibiotics and drugs are added to cause the following factors: On the one hand, the pig’s immunity declines; on the other hand, the pathogen’s virulence and Increased variation. China's pig-breeding industry is facing increasingly serious disease challenges, and the losses caused by the disease are increasing. We have never encountered such a serious challenge. This challenge does not seem to disappear. The disease will continue to attack our country's pig-breeding industry in different forms, making it difficult to prevent it. what should we do? By better medicine? Better vaccine? Improve the pig's living environment? Breeding disease-resistant varieties? It is very important, but we must not overlook the significance of nutrition in immunity. Nutrition is the material basis of pig immunity.
The following focuses on the impact of the epidemic challenge on the nutritional needs of pigs, the role of nutrition on pig immunity, and provides some new ideas for pig-raising peers in the fight against the disease. The paper draws on a large number of research results of other animals (including humans) because of the universal significance of nutrition on the role of immunity.
1 Disease Challenges and Animal Responses
Pathogenic microorganisms and animals continue to struggle and adapt during the long evolutionary process. Animals have evolved complex immune systems that can quickly mobilize in the face of disease challenges and suppress the invading pathogens until they are eliminated. Animals survive the battle with the disease. Simple pathogenic microorganisms have also developed a set of weapons to deal with animals in the course of evolution, and they can change themselves to break through the animal's immune defenses. Some animals are defeated by the disease. This kind of path is one step higher, and the struggle of the enemy will continue.
According to the study of Le Floc'h et al. (2004), the challenge of the disease activates the animal's immune system, producing a large number of cytokines (such as interleukins) and chemical factors. These immune factors work together in the body to produce a series of metabolic changes, and nutrients, especially amino acids, preferentially supply the tissues involved in the immune response. In the challenge of the disease, the phenomenon of animal anorexia is very obvious. In this case, muscle degradation becomes an important source of amino acids.
Activation of immune-related metabolic pathways results in a change in the requirements of certain particular amino acids.
The disease challenge causes the animal to slow or even stop growing. Sandberg et al. (2007) believe that the main reason is the drastic reduction in the intake of diseased animals, but the increase in animal maintenance needs due to the epidemic challenge (the need for immunity is also attributed to maintenance) is also an important cause, as evidenced by the pair-feeding test. Whether it is parasites, germs, or viruses, the animal's nutritional needs are greatly increased. Disease challenges can lead to a substantial increase in the maintenance of all nutrients. Based on the severity of the infection, the increased maintenance energy needs of the epidemic challenge are estimated to be 5% to 35%, and the increased protein needs to be estimated at 0.57 to 1.2 g (kg) per kg of body weight. This shows that the infection substantially increases the need for protein.
After the challenge of the animal, due to the activation of the immune system, the body synthesizes a large number of cytokines, chemical factors, acute-phase proteins, immune antibodies, etc. The metabolic rate is obviously enhanced, and the increase in thermogenesis and increase in body temperature require large amounts of nutrients. First of all, a large number of amino acid synthesis acute phase proteins, antibodies and other active substances, loss of body protein, increase in nitrogen emissions. In the challenge of the disease, the amino acid supply is mainly due to the degradation of the body protein due to the substantial reduction in the appetite and feed intake of the animal and even the hunger strike. In the event of an epidemic challenge, increased animal metabolism will inevitably increase the demand for vitamins and trace elements. On the other hand, the epidemic challenge leads to oxidative stress in the animals, which produces large amounts of free radicals and increases the consumption of antioxidants (VE, VC, Se, etc.).
It is now basically clear that in the challenge of diseases, the metabolism of animals has increased, the need for nutrients has increased, and the nutrient distribution of animals has shifted from growth to immunity. These metabolic reactions of animals are to survive the disease and survive as much as possible, which is the result of long-term evolution or natural selection. However, under artificial selection, the metabolic pattern of the pig at the time of disease challenge has a certain deviation from the natural selection of the orbit. In recent years, the progress of pig breeding has led to a significant increase in the growth potential of pigs and a significant increase in the growth rate of lean meat. Once infected, such pigs can use nutrient allocation patterns that change to some extent: nutrients distributed to the immune system are reduced, and nutrients distributed to grow are increased. In healthy conditions, this is naturally beneficial to improve production performance (pigs are bred under very healthy conditions), but when the disease is challenged, such pigs have low immunity and the mortality rate is higher than that of old breeds (China). The local pigs grow slowly but the disease resistance is much higher than that of modern exotic pigs. Continually focusing on improving growth performance options genetically alters the distribution of nutrients and must sacrifice functions beyond growth. Therefore, raising high growth potential lean-type pigs must provide a high level of nutrition, especially in the event of disease challenges to ensure that the supply of nutrients in order to have enough nutrients for immunization, pigs can overcome the disease.
Although there is an increased need for animal nutrition to meet the challenges of nutrition, it is difficult to understand that the animal's response is the opposite and feed intake is reduced. The mechanism may have to be understood from an evolutionary perspective. This reaction must have an evolutionary basis, is incorporated into the animal's DNA genetics, and is conducive to animal survival under natural conditions. In evolution, natural pigs have little chance of getting nutrition after being ill, and the immunity needs high nutrition. The only reliable nutrient source is the body reserve, and a large amount of mobilized body reserves becomes inevitable for sick animals. This may be caused by a series of metabolic changes caused by the secretion of nerves, endocrine, and active substances. If corticosteroids rise, glucagon rises, catecholamine hormone levels rise, and insulin levels drop. Animal oxygen consumption increases, glycogen breakdown increases, and mitochondrial oxidative metabolism produces a large amount of ATP, which may feedback suppress the animal's appetite. This is still a hypothesis that needs to be confirmed by research. Under the rearing conditions, even if the disease occurs, we need to get as many pigs as possible from the feed to get enough nutrients (try to feed the pigs), especially the nutrients needed for immunization, to ensure that adequate nutrients are used for immunization, and to minimize decomposition of body tissues. Once body tissues are depleted, death is the inevitable outcome.
The disease challenge leads to a reduction or even cessation of animal growth. This decline in food intake and growth is not caused by the damage of the animal itself to the animal, but rather caused by the animal's immune response to the disease challenge. A series of metabolic changes produced by the activation of the immune system and the mass production of inflammatory cytokines also cause serious damage to the animal's production performance during the elimination of the pathogen. Not only pathogens, but other immune responses, such as vaccination, also reduce the animal's growth performance to some extent. Although the animal's immune response is necessary for the animal to maintain its health (or survival), the animal also pays for it at the expense of reduced feed intake and growth. The immune system may sometimes overreact, especially after encountering a viral infection, producing excessive amounts of inflammatory cytokines that damage the animal's own tissues. The immune response is out of control and causes animal death in severe cases. For example, leading to the death of SARS patients is mainly the immune response is over, at this time properly suppress the immune response but let the patient have the opportunity to survive. It seems that immunity is a double-edged sword.
2 Animal nutrition under epidemic challenges
The current nutritional requirements for pigs and feeding standards are based on healthy pigs. However, when the disease is challenged, the nutrient distribution and metabolism of the pig completely change, and the nutrients needed for the immunity are completely different from those needed for growth. According to the study of Sandberg et al. (2007), a large number of acute phase proteins and immunoglobulins are synthesized under the challenge of diseases, and their amino acid composition is different from that of body tissues. The proportions of phenylalanine, tyrosine, tryptophan, threonine, and proline are much higher than those in pig tissues. If pigs rely on mobilized body tissues to meet the need for immune amino acids under disease stress, it is clear that these amino acids may become limiting amino acids.
Bhargava (1970) challenged broiler chickens with Newcastle disease and showed that threonine increased to more than 0.6%, the weight gain of chickens did not increase, but threonine levels continued to increase to 1.0%, and anti-NDV antibody drops. The degree rises linearly (from 212 to 223). The situation of valine is also very similar. It is clear that threonine and valine are the limiting amino acids in the immune response. Studies on piglets showed that the lack of tryptophan caused the most negative nitrogen balance during immune stress, suggesting that the epidemic challenge caused a significant increase in tryptophan in pigs. The increase in tryptophan requirement is used to synthesize acute-phase proteins on the one hand, and tryptophan degradation on the other hand.
3 Nutrition and immunity
Nutrition is the material basis for immunization of pigs and other animals. In fact, all the nutrients are necessary for immunity. Once it is limited, it will suppress immune function. When animals are challenged by diseases, the most restrictive nutrient is the focus of our attention.
3.1 Energy
A serious lack of energy affects the animal's immunity. Adequate protein was given to 8-week-old mice, but the energy intake was reduced by 37%. After 52 weeks, the weight of the energy-deficient mice was reduced by 83% and the number of white blood cells was decreased by 8,000 To less than 1000/mm3, the spleen dendritic cells decreased by 80%, and the lack of energy significantly reduced the immune organization. Compared with energy-rich mice, energy-deficient mice were reduced in antibody titer by 94% and cellular immunity by 70% after vaccination with hepatitis (Niiya, et al. 2007). It appears that in animals with severely deficient energy, their immune organs and immunity are significantly reduced. Once a disease challenge is encountered, the prognosis of the animal is poor. For modern lean-type pigs, sufficient energy is provided during the growth phase to ensure that the pig maintains high immunity. If there is a low tide in raising pigs, or if the pig farm has an economic hardship, reducing the pig's feed supply, once the epidemic strikes, the consequences are likely to be disastrous.
3.2 Proteins and Amino Acids
Proteins and amino acids are very important for the immunity of pigs. Amino acids are the basic raw materials for synthesis of antibodies, lymphocytes, cytokines, acute phase proteins, etc. Some amino acids are an important part of the body's antioxidant system. Modern pork has a very high growth potential. Highly-selected pigs have the metabolic characteristics of being able to use more amino acids to distribute to the muscles and less to the immune organs and tissues. So once protein and amino acids are lacking, the pig's immunity will be sacrificed, not just sacrifice the production performance of the pig. In the recently introduced pig breeds, the growth potential of lean meat is very high. These high growth potential pigs require a higher dietary protein and amino acid supply. These pigs were raised in poorly-conditioned pig farms in China. The pressure of the disease was very high and the health level was significantly reduced. The challenge of the disease leads to a significant reduction in growth performance of pigs with high growth potential, which is a common phenomenon in our country. However, the growth potential of pigs with high lean growth potential due to epidemic challenges is lower than that of local pig breeds with low growth potential. High lean growth potential Pigs should not reduce dietary protein and amino acid levels even if growth is reduced due to disease challenges. If the growth rate is thought to be low, the protein and amino acid levels of the diet will be reduced. As a result, the pig's health status and immunity will be worsened and deaths will increase.
Studies have shown that tryptophan is very important for the viability of lymphocytes and macrophages in pigs (Kim et al. 2006). Arginine promotes the integrity of the small intestine mucosa of pigs and improves pig immunity. According to the study of Kim et al., 1% arginine was added to the diet of pregnant sows. All of them recovered after an outbreak of blood stasis, and all gestation sows without arginine died. This fully explains the effect of arginine on blood stasis immunity. .
Immunization requires higher levels of threonine and tryptophan, and threonine increases animal immunoglobulin levels.
Cystine is a component of important antioxidant systems in the body, and the antioxidant-oxidation balance is important for maintaining animal immunity while protecting their tissues from free radical damage. The body's antioxidant system is also related to VE, VC, selenium and so on.
The lack of protein in the sow during pregnancy affects the development of the immune organs of the fetus, reduces the proliferation of lymphocytes in the thymus and spleen of piglets, and decreases the viability of NK cells in the spleen. Protein deficiency during pregnancy also alters the hypothalamus-pituitary-adrenal axis of piglets, increases piglet's baseline corticosteroid levels, poor immunity, and increased piglet mortality. Especially in the late gestation of sows, it is crucial to ensure the supply of protein and amino acids.
3.3 Vitamins
Vitamins are important for the immunity of pigs. Water-soluble vitamins are mostly components of metabolic enzymes and are consumed and increased in demand when animals have strong metabolism. As mentioned earlier, when animals face metabolic challenges, their metabolism is significantly increased, which inevitably increases the need for water-soluble vitamins.
Fat-soluble vitamin VA is mainly related to maintaining the integrity of cell membrane structure and function. The integrity of the membrane structure is the basis for the immune function. Therefore, VA is very important for immunity. VE is a very powerful free radical scavenger and an important part of the body's antioxidant system. VE is mainly present in mitochondria because the mitochondria's oxidative reactions continue to generate free radicals. In the event of a disease challenge, the production of free radicals suddenly increases. Free radicals have a strong damaging effect on the active components of the animal's membrane structure, and there are sufficient VEs in the mitochondria. The free radicals produced can be removed at any time. VE, VC, and β-carotene enhance cellular and humoral immunity. When animals lack VE, VC, and selenium at the same time, the animal's immunity is significantly reduced. In this case, some normal and harmless viruses can also cause diseases. Some pathogens with very weak pathogenicity can also cause death.
The body's full antioxidant reserve is a guarantee of immunity. Antioxidant reserves can be consumed in large quantities due to high levels of iron, copper, polyunsaturated fatty acids, gossypol, rancid oils, and disease challenges in feeds, and in this case it is necessary to increase VE levels.
3.4 Trace elements
Many trace elements are also components of metabolic enzymes that require increased amounts of metabolism. Selenium is an important component of the animal's antioxidant system, and the immunity of selenium-deficient animals is significantly reduced. In the absence of selenium, the virulence of the virus increases and the virulence increases. It is also believed to be related to the antioxidant mechanism of selenium. Zinc is very important for animal immunity. Zinc is involved in mucosal immunity and cellular immunity in animals. Zinc content in leukocytes is very high. Zinc is required to increase in disease challenge. Zinc deficiency causes animal immunosuppression (T-cell and B-cell immunosuppression). The lack of other trace elements, iron, manganese, copper, and iodine also reduces the animal's immunity.
Special attention should be given to the status of iron. Iron plays a key role in the animal's biological oxidative energy supply. The iron deficiency caused by the animal's immune system has been confirmed by numerous studies. However, we must also pay attention to the other side of iron. Excessive iron causes adverse consequences. The reason why overdose of iron must be paid attention is because there is a tendency for pig feed to generally overdosing iron. Iron is a very active transition element. Iron is highly oxidative. Excessive iron can cause oxidative stress in animals, reduce the level of antioxidant enzymes, reduce antioxidant reserves, and reduce animal immunity. Since iron is very reactive, normal iron in the body binds to proteins (transferrin, for example). Once iron is not completely bound by protein, iron ions attack cells and active molecules and cause disease. Excessive amounts of iron added to the diet can cause iron accumulation in the body, which exceeds the binding capacity of iron-binding proteins and can lead to serious adverse consequences. On the other hand, the value-added of iron to germs is an important limiting factor. After an animal becomes infected with a bacterial disease, the more iron in the body, the faster the bacterial growth. In particular, free iron greatly accelerates the growth of bacteria and has the opportunity to overcome the animal's immune system. Therefore, the addition of excess iron may lead to counterproductive adverse consequences. For example, after guinea pigs were infected with E.Coli, the iron-free mortality rate was 10%, iron ions were added, and the mortality rate was increased to 90%; iron supplementation to patients with infections resulted in a poor prognosis; increased dietary iron levels, intestinal bacteria Increased virulence; when iron is excessive, it causes post-infection toxemia; excess iron accelerates the development of the disease; excess iron increases malaria-related disease, and so on.
The addition of other trace elements, such as zinc and copper, also causes oxidative stress in animals, reduces animal immunity, and also pollutes the environment.
When the pig is in the challenge of disease, proper increase of trace elements will help improve the pig's immunity and reduce the pig's death. However, trace elements are not always beneficial. The long-term adverse consequences of high-iron, high-copper and high-zinc diets should not be underestimated. Piggers should not only look at the short-term effects but ignore the long-term consequences!
4 Summary
In short, the high growth potential of modern lean-type pigs sacrifices the disease resistance of the pigs. In the case of the current epidemic pressure in our country, we must pay attention to the role of nutrition in maintaining the high disease resistance of pigs; after the pigs become infected with the disease, Metabolism has changed, and certain nutrients have become limiting factors. Supplementing these nutrients to diseased pigs helps to reduce death losses; in order to have short-term effects, trace elements and drugs are needed in excess, and long-term results are counterproductive.
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