(2003)reported that 3 d after oral ETEC challenge, piglets had severe diarrhea with scour score of 2

(2003)reported that 3 d after oral ETEC challenge, piglets had severe diarrhea with scour score of 2.8 that lasted for more than 7 d.Nyachoti et al. of the animals to them are also discussed in this review. Keywords:Challenge Models, Feed Additives, Gut Health, Pigs, Response Criteria == INTRODUCTION == Gut health is a term increasingly used in the medical literature and by the food industry. It covers multiple positive aspects of the gastrointestinal tract (GIT), such as the effective digestion and absorption of CZC-25146 hydrochloride food, the absence of gastrointestinal illness, normal and stable intestinal microbiota, effective immune status and a state of well-being (Bischoff, 2011). The GIT of a pig is a complex environment. Particularly in newborns and around the time of weaning, the pigs gut rapidly changes in size, has high protein turnover rates, undergoes rapid changes in microbiota, and quickly alters its digestive and immune functions (Pluske et al., 1997). The correct functional development of the GIT is of special importance during the weaning phase of reared piglets (Domeneghini et al., 2006). The mucosal epithelium of the small intestine is reputed anatomically and functionally immature in neonatal pigs, a feature that appears to be exacerbated at weaning, when a colonization of the gut occurs by new microorganisms entering the alimentary canal with solid feed. This frequently exposes piglets to diarrhoeic syndromes and other intestinal disturbances (Domeneghini et al., 2006). In the lifespan of the pig, the neonatal and weaning phases represent critical periods for both the correct development of the gut and the growth of the young animal (Pluske et al., 1997; Lay et al., 2001). The correct and timely functional development of the GIT, in which it should be able to sustain growing digestive, absorptive and immune functions, is of particular importance for these adaptable processes correctly occurring (Domeneghini et al., 2006). The GIT serves a key functional role in the growth of the young piglet, even though it represents a relatively small fraction of its body weight (BW), in that it is approximately 2% of BW at birth and increases nearly threefold, to more than 6%, two weeks after weaning (Shields et al., 1983;Mitchell et al., 2001). To improve productivity of pigs, researchers have evaluated a wide range of feed additives (such as immunoglobulin, omega 3 fatty acids, yeast derived B glucans, organic and inorganic acids, high levels of zinc oxide, essential oils, herbs and spices, some types of prebiotics, bacteriophages and anti-microbial peptides, probiotics, glutamine, threonine, cysteine, and nucleotides) as substitute products for antibiotics. The role of these additives and their potential use in managing gut health and function in newly weaned pigs has been reviewed extensively elsewhere (de Lange et al., 2010;Heo et al., 2013;Thacker, 2013;Zeng et al., 2015). An approach to evaluate the efficacy of various additives as replacements for antibioticsin vivois to use an appropriate disease challenge model. Over the years, researchers have used a number ofin vivochallenge models some of which were discussed in this review. == DISEASE CHALLENGE MODELS == == Post-weaning diarrhea using specific pathogenic strains of enterotoxigenicEscherichia coli == EnterotoxigenicEscherichia coli(ETEC) is the main causative agent of diarrhea in weaned pigs. It attaches to and effaces the intestinal mucosa, thus leading to malabsorption of large molecules as a result of a compromised barrier function. The ETEC diarrhea is the most common enteric disease in piglets, accounting for 50% of the piglets that die annually worldwide (Gyles, 1994). Investigations have indicated that colonization of the small intestine of the piglet by ETEC adhering to the epithelium accounts for CZC-25146 hydrochloride most gastrointestinal disorders (Yolken et al., 1988) (Table 1). The fimbrial K88, CZC-25146 hydrochloride K99, and 987P antigens of porcine ETEC that are associated with intestinal colonization have been extensively investigated with respect to their genetic background, protein chemistry, and immunological properties (Fusco et al., 1978). == Table 1. == Challenge models and responses in weaned pigs BW, body weight; PWD, post-weaning diarrhea; ETEC, enterotoxigenicEscherichia coli; ADG, average daily gain; ADFI, average daily feed intake; LPS, lipopolysaccharide; IGF-1, insulin-like growth factor 1; TNF-, tumor necrosis factor-; G:F, feed conversion efficiency. EnterotoxigenicEscherichia coli(E. coli) K88 is the pathogen most frequently isolated in piglets and associated with colibacillosis. Studies ERK2 (Owusu-Asiedu et al., 2002;Owusu-Asiedu et al., 2003;Bhandari et al., 2008) have demonstrated the suitability of ETEC K88 challenge model in evaluating the role of feed additives in nursery pig diets..