Post-weaning diarrhea and use of feedstuffs in pigs

The role of living microbes on swine health is undeniable. However, antimicrobial properties should not be solely based on selecting essential oils in swine production. Instead of higher doses required for their killing pathogens properties, using lower doses of essential oils could prevent inflammation and “leaky gut” induced by lipopolysaccharide. It is critical to look at the total blend of raw materials available for feed formulation, their inclusion levels, costs, feed preference as well as the targeted feed acid-binding capacity value and then make the final decision based on science and field experience. Minimizing mycotoxin contamination in feeds is an important component to control post-weaning diarrhea. Chemical approaches, such, as the use of sodium metabisulfite and biological approaches, such as the use of microorganisms for detoxification, have shown promise in reducing vomitoxin. Weaned piglets are highly susceptible to many stressors including bacterial pathogens, oxidative stress, and inflammation, which predisposes pigs to post-weaning diarrhea, eventually leading to reduced growth performance, high mortality and morbidity rates, and compromised animal welfare (Yang et al., 2015a, 2015b; Hassan et al., 2018). Since post-weaning diarrhea commonly results from the proliferation of pathogenic Escherichia coli, antibiotic growth promoters have been widely used in piglet diets, especially in nursery diets, to control incidences of diarrhea during the transition. Total consumption of antimicrobials in animal food production worldwide was estimated at 63,151 tons in 2010, with an increasing trend; the annual consumption of antimicrobials per kilogram body weight was 148 mg/kg for pigs (Van Boeckel et al., 2015). This practice may lead to the spread of antimicrobial-resistant bacterial pathogens in pigs and humans, challenging the sustainability of the pork industry (Yang et al., 2015a, 2015b). With environmental, health, and safety concerns, the public demands antibiotic-free pork (e.g., raised without antibiotics). However, the withdrawal of antibiotics from feeds can result in several challenges including compromised gut health and increased gut diseases. So far, we do not have a single “magic bullet” that can replace in-feed antibiotics. Although different types of alternatives to antibiotics (e.g., essential oils and probiotics) have been widely recognized as promising alternatives to antibiotics in feeds, an integrated approach to control post-weaning diarrhea should be taken, including supplementation of antibiotic alternatives, and measures related to nutrition, biosecurity, and management. In this review article, we reviewed the use of essential oils as antibiotic alternatives, the use of ingredients to lower dietary acid-binding capacity (ABC), and the use of innovative chemical and biological approaches to detoxify vomitoxin, which may be considered important parts of an integrated approach to control post-weaning diarrhea in piglets. Essential oils have antioxidative, anti-inflammatory, and antimicrobial properties. Some essential oils (e.g., thymol, eugenol, and cinnamaldehyde) have been widely used to replace antibiotics in swine production mainly because of these essential oils’ antimicrobial properties (Yang et al., 2015a, 2015b; Hassan et al., 2018; Omonijo et al., 2018a). The adverse effects of pathogenic microbes on swine health are undeniable. However, antimicrobial properties should not be solely based on selecting alternatives to antibiotics in swine production (Gresse et al., 2017; Fouhse et al., 2016; Figure 1). Lipopolysaccharides, also called endotoxins, are cell wall components of Gram-negative bacteria (e.g., Salmonella and Escherichia) that are present everywhere in the environment including in the intestine, ground, air, and water, and have received much attention due to their ability to stimulate a low-grade inflammation in pigs (Huang et al., 2016; Nordgreen et al., 2018). One of the negative consequences of inflammation at the intestine level is increased intestinal permeability, or “leaky gut,” associated with impaired nutrient absorption and increases in diarrhea incidence (Gresse et al., 2017). Interestingly, in addition to antimicrobial properties, thymol (50 µM, 7.5 mg/kg), eugenol (100 µM, 16.4 mg/kg), and citral (10 to 20 µM, 1.52 to 3.04 mg/kg) can reduce inflammation associated with lipopolysaccharide or peptidoglycan in porcine intestinal epithelial cells (Omonijo et al., 2019; Hui et al., 2020; Li et al., 2022a, 2022b) and cinnamaldehyde (25 µM, 3.3 mg/kg) can improve intestinal mucosal barrier function (Sun et al., 2017). These levels are much lower than the minimum inhibitory concentration of thymol and eugenol against major pathogens (Yang et al., 2015a, 2015b; Omonijo et al., 2018a). Moreover, the minimum inhibitory concentration of most essential oils is much higher than the acceptable levels in the animal industry in terms of cost-effectiveness and feed palatability (Omonijo et al., 2018a). Since the industry’s acceptance of essential oils also depends on inclusion cost, those results are encouraging producers to use lower doses of essential oils to prevent inflammation and “leaky gut” induced by lipopolysaccharide instead of using higher doses required for their killing pathogens properties. Moreover, using higher doses has drawbacks on optimal feed intake and cost. Schematic diagram illustrating the four different potential mechanisms by which essential oils improve the gut ecosystem and growth performance of piglets. The combined use of essential oils and other additives should lead to higher advantages. For two reasons, a combination of other alternatives with essential oils holds the most promise as a substitute for antibiotics in pig feeds. First, no single antibiotic alternative has been reported to be able to replace antibiotics completely. Second, a combination of products can have a synergistic effect that will reduce the effective dosages required to combat pathogens (e.g., organic acids and essential oils). For example, essential oil can change the structure and functions of bacterial cell membranes. This results in membrane swelling and thus increased membrane permeability, leading the bacteria toward an increased susceptibility to organic acids. Moreover, the hydrophobicity (being water-repellent) of essential oil is increased at low pH. Combining essential oils with organic acids will enable the essential oil to pass through the lipids of the bacterial cell membrane more easily. Essential oils exhibit great potential to prevent post-weaning diarrhea. However, their direct inclusion in pig diets has compromised efficacy because of such factors as low stability, poor palatability, and low availability in the lower gut. Therefore, an effective and practical delivery method is very important for the use of essential oils in swine production. It is documented that the formulation of microparticles could effectively deliver thymol and lauric acid to the pig intestinal tract. Lauric acid not only acts as a carrier for thymol but also has synergistic antibacterial effects with thymol (Omonijo et al., 2018b). The stability of thymol in commercial lipid matrix microparticles (encapsulated essential oils and organic acids) was investigated during feed pelleting and feed storage and determined the intestinal release of thymol (Choi et al., 2020a). The thymol concentration was not significantly different in the mash and pelleted feeds, suggesting that the pelleting of feed did not affect total thymol in those lipid matrix microparticles. Encapsulated thymol was also stable in simulated pig gastric fluid (26.0% thymol released). The rest of the thymol was progressively released in the simulated intestinal fluids until completion, which was achieved within 24 h. In a pig experiment, 15.5% of thymol was released in the stomach, and 41.9% of thymol was delivered in the mid-jejunum section, demonstrating a slow release, and 2.2% of thymol was recovered in feces (Choi et al., 2020a). The lipid matrix microparticles maintained the stability of thymol during the feed pelleting process and storage and allowed a slow and progressive intestinal release of thymol in weaned pigs (Choi et al., 2020a). Subsequent studies were conducted to investigate further the effects of these commercial lipid matrix microparticles on growth performance, immune system, gut barrier function, nutrient digestion, and absorption in disease-challenged weaned piglets and demonstrated that the supplementation of those lipid matrix microparticles showed anti-diarrhea effects in disease-challenged weaned piglets (Choi et al., 2020b; Xu et al., 2020). Therefore, microencapsulated essential oil and organic acid combination can be a useful method to control post-weaning diarrhea in swine production. Direct-fed probiotics refer to live microorganisms supplied to the host with adequate amounts benefiting the host (Pluske, 2013). The benefits of providing direct-fed probiotics into swine diets are categorized into several aspects:1) benefiting gut health by modifying the composition of enteric microflora (Sartor, 2004); 2) promoting immunity (Yan and Polk, 2011), 3) increasing efficiency of nutrient digestion and utilization (Yadav and Jha, 2019) and 4) enhancing gut function and improving growth performance (Vohra et al., 2016). Direct-fed probiotics contain three main categories: Bacillus, lactic acid-producing bacteria, and yeast (Kerr et al., 2013). Bacillus is a potent producer of extracellular fiber-degrading enzymes, which can increase nutrient digestibility and utilization (Kiarie and Mills, 2019). In addition, Bacillus synthesizes enzymes that degrade feed and produces short-chain fatty acids through fermentation (Merchant et al., 2011). Those short-chain fatty acids are considered a useful energy source utilized by pigs to develop the large intestine (den Besten et al., 2013). Recent studies have identified the protective effect of Bacillus strains on intestinal cells challenged with enteric pathogenic bacteria, since they found the pre-treatment of Bacillus could upregulate tight junction protein expression and decrease quorum sensing (Chen et al., 2021; Li et al., 2022a, 2022b). For weaned piglets, including lactic acid-producing bacteria in their dies could relieve weaning stress, reduce diarrhea, and enhance growth performance (Yang et al., 2015a, 2015b). Lactic acid-producing bacteria are the primary bacteria in the nursing pig gut, and the lactic acid produced by lactic acid bacteria fermentation can inhibit the growth of intestinal pathogenetic bacteria and helps to aid immunity (Guevarra et al., 2019). That makes the supplementation of lactic acid-producing bacteria beneficial for the weaned piglets (Guevarra et al., 2019). The common forms of yeast supplied to pig diets include whole live yeast cells, heat-treated yeast cells, ground yeast cells, purified yeast cell cultures, and yeast extracts. Yeast supplementation has been reported to boost intestinal development by providing fermentation by-products such as short chain fatty acids to pigs and reducing post-weaning scour by supplying weaned piglets with beneficial nutrients such as specific sugars and nucleotides (Broadway et al., 2015). Accordingly, introducing probiotics in the creep feed is increasingly being explored (Barba-Vidal et al., 2018). However, the results of these studies are not consistent and further studies are still needed to interpret the mechanisms of action of probiotics and their interaction in various gut health situations. Due to the decreased capacity of gastric acid secretion at weaning, weaned piglets have a higher pH value in the stomach than sow-reared piglets (Heo et al., 2013). Maintaining a lower gastric pH value is pivotal for the gut health of weaned piglets because this can positively affect the nutrition digestion and pathogenic bacteria inhibition. In contrast, the elevated gastric pH level makes weaned piglets more susceptible to enteric infections (Heo et al., 2013). Hence, not only the amino acid profile or the energy content of the diet but also other nutrients and key parameters should be considered (e.g., dietary ABC, Table 1). It refers to the ration’s resistance to a low pH in the pig’s stomach, is highly related to raw materials used in the feed and has a great impact on the pH of the stomach and feed digestibility. A high ABC can lead to lower digestibility of dry matter and crude protein and, therefore, adversely affect the growth performance of piglets. Moreover, a high ABC can increase the release of amine and ammonia that are toxic and could lead to diarrhea. The ABC value of feed ingredients and complete feeds can be calculated as the amount of acid in milliequivalents (meq) required to lower the pH of 1 kg of a sample to pH 4 and pH 3 based on the measurement value from a 0.5 g sample (Lawlor et al. 2005) and it can be used a nutrient constraint in feed formulation to select suitable ingredients. Therefore, lowering the ABC of the diets for new weanlings with the addition of feedstuffs with a low ABC in piglet feed formulations can be a good strategy for controlling diarrhea in weaned piglets (Huting et al., 2021). Acid-binding capacity for different ingredients (Scholten et al., 2001; Lawlor et al., 2005; Karvelis, 2014; Hajati, 2018) aAcid-binding capacity to pH 3.0; bAcid-binding capacity to pH 4.0. Acid-binding capacity for different ingredients (Scholten et al., 2001; Lawlor et al., 2005; Karvelis, 2014; Hajati, 2018) aAcid-binding capacity to pH 3.0; bAcid-binding capacity to pH 4.0. Some mineral sources are key contributors to a high ABC value of feed, especially limestone (calcium carbonate), dicalcium phosphate (DCP), mono-dicalcium phosphate (MDCP) and zinc oxide (ZnO). So, it is very important to avoid limestone, DCP, and MDCP in formulating a weaner diet. Lowering levels by the limestone content of the feed has a impact on the capacity of the feed and on improving growth performance et al., 2016). can benefits including the of limestone in For example, the and levels the should be reduced to to and to the a can replace limestone to which will lead to increased but will effectively reduce the feed ABC supplementation of high levels of to mg/kg) has been widely used as an effective approach to reducing the incidences of post-weaning diarrhea et al., 2021). However, using a high of in piglet feeds has been associated with several negative effects including the acid in the stomach because of a high ABC, being associated with post-weaning zinc in the with and antibiotic resistance et al., 2016; et al., 2019). The of zinc in the feed will be effective in and the use of a high of zinc will be in Although it to reduce the effective of to combat post-weaning diarrhea and at the the negative related to the high of in feeds with the availability of new such as organic and et al., 2019; et al., is still a to develop to replace higher doses of zinc in the feeds. acids have negative ABC (e.g., and So, the addition of organic acids can decrease the ABC value of feed and then lower stomach pH et al., et al. reported a in stomach pH from to with the addition of acid and a pH from to with acid in the diet. However, we should the palatability of organic acids. acid and acids feed a high inclusion of organic acids (e.g., may reduce feed intake et al., feed intake is very important for weaned piglets for promoting gut development and growth performance the other acids (e.g., or can also reduce stomach pH but may also decrease feed preference et al., for of pathogenic bacteria and of ABC, organic acids can also as an energy source in the gut of pigs as these are the products of acid and improve mineral utilization et al., 2020). Therefore, several including ABC palatability, bacteria and other should be considered in the of organic acids in feeds. and by-products have a lower ABC with the sources of and So, it is important to feed crude protein levels as low as a of amino acids by using high protein raw materials and amino acids. with and and have a lower ABC and are highly in weaner piglet feeds. 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