Weaning is a critical period in pig husbandry. In the wild, pigs naturally wean at 10–12 weeks of age, which coincides with the almost complete development and maturation of the gastrointestinal tract (GIT); in contrast, commercial weaning occurs at 2–4 weeks of age. Commercial weaning induces transient alternations to the gastrointestinal tract (GIT). These morphological and physiological changes are most likely driven by the post-weaning reduction in feed intake. As feed intake resumes, the GIT undergoes a period of intestinal maturation [9]. The villi and crypts that line the epithelium of the small intestine are essential for the digestive and absorptive processes [10]. Dietary composition has marginal effects on the small intestinal morphology of weaned pigs, with the level of feed intake found to be the most important determinant of mucosal function and integrity [11]. Food deprivation leads to a lack of luminal stimulation. This results in a rapid decrease in villous height [10]. Villous height is at its lowest after 2–5 days post-weaning, resulting in a reduced ability to absorb nutrients [12]. Villous height starts to recover in feed deprived piglets 4 days after feeding is restarted and can take more than 10 days to completely recover [13]. The villus surface area is also altered in the post-weaning period. Pre-weaning, villi are dense and finger-like, while the weaning transition changes the villi into predominantly smooth, compacted, and tongue-shaped villi [14]. As well as the intestinal morphology being affected by weaning, gastrointestinal functionality is also impaired as indicated by the reduction in brush border enzymes such as lactase, sucrase, and peptidases, and the disturbances in nutrient absorption and electrolyte secretion with the latter also contributing to the weaning-associated diarrhoea [12,13,15]. The resulting maldigestion and malabsorption leads to the weight loss observed during the first 4–5 days post-weaning [16,17].

A compromised intestinal barrier characterised by increased paracellular permeability, reduced transepithelial resistance, and reduced gene expression of tight junction proteins is additionally observed at the immediate post-weaning period and may lead to overstimulation of the immune system due to the increased presence of dietary and microbial antigens [5,16,18]. The activation of the immune system further contributes to the reduced intestinal barrier function and diarrhoea in newly weaned pigs. Several studies have reported infiltration of immune cells such as lymphocytes, macrophages, and mast cells in the lamina propria [2,5], increased expression of genes encoding for inflammatory cytokines such as tumour necrosis factor (TNF), interferon gamma (INFG), and interleukins IL1B and IL6 [18,19], and activation of several pathways associated with immune responses [17] in the small and large intestine of pigs in the immediate post-weaning period.

The composition of the GIT microbiota is also altered in response to the weaning stress, diet alteration, reduced feed intake, and gastrointestinal dysfunction. Several studies have investigated the weaning-induced compositional and functional changes in the GIT microbiota of pigs [20,21,22,23,24]. Lactobacillus spp. are amongst the intestinal bacterial populations that are frequently monitored during the post-weaning period due to their high abundance in pigs and known beneficial effects. A significant reduction of this population, as well as shifts of the dominant strains, has been observed in the ileum of pigs post-weaning [25,26]. The decrease in the Lactobacillus spp. is transient, as seen in the ileum and faeces of weaned pigs and is followed by restoration or even an increase in its numbers and dominance of strains that utilise complex carbohydrates [20,21,24,26,27]. Enterobacteriaceae is an important indicator of dysbiosis in the faeces of newly weaned pigs, as an increase in the counts of this bacterial family was associated with higher incidence of diarrhoea [28]. Nevertheless, the increase in Enterobacteriaceae relative abundance is transient under normal circumstances, as this bacterial population and its members (Escherichia/Shigella) are minor constituents of the maturing GIT microbiota [21,22,26,27]. The reduction in Bacteroides spp. and increase in Prevotella spp. is another common change in the faecal microbiota of weaned pigs that is probably associated with the transition from milk mono- and oligo-saccharides to plant-derived polysaccharides [20,21,23]. Weaning-induced gastrointestinal dysbiosis is considered a key contributor to the development of diarrhoea and predisposes pigs to PWD [29]. The most common causative agent of PWD is the α-haemolytic Gram-negative enterotoxigenic E. coli (ETEC) that colonises the epithelium of the small intestine via F4 (ab, ac, ad) and F18 (ab, ac) fimbriae and non-fimbrial AIDA (adhesin involved in diffuse adhesion) [30,31]. 

Dietary interventions are one strategy with which to prevent or alleviate dysbiosis and its associated impact on the growth and health of pigs. A diverse range of feed additives have been studied as preventatives and prophylactics in pig diets. An array of natural compounds have been investigated as alternative strategies to AGPs and ZnO such as yeast β-glucans [33,34], mannan-oligosaccharides [35], prebiotics such as galacto-oligosaccharides [36], organic acids [37,38], probiotics [39], spray dried plasma proteins [40], exogenous feed enzymes [41], and essential oils [42]. These compounds can support the microbial composition, health, and growth performance of pigs. However, there is only a limited number of compounds that result in a similar improvement in growth performance and reduced the occurrence of diarrhoea compared to in-feed AGP or ZnO. Therefore, there is still a need to identify natural bio-actives with growth promoting and immunomodulatory properties as suitable substitutes to AGPs and ZnO. It is also critical to explore the underlying mechanisms when evaluating the functional properties of feed ingredients and feed additives [43]. Key components of GIT function that should be considered include absorptive capacity (villi architecture and nutrient transporters expression), digestive capacity (activity of pancreatic and brush-border enzymes), physical and chemical barriers, microbial load, microbial diversity, and immune function.

Marine macroalgae, broadly classified into brown, red, and green seaweeds, are a major source of novel bio-actives with potential benefits on animal health. While they consist of ≥94% water, they also contain varying concentrations of non-digestible polysaccharides, polyphenols, minerals, vitamins, proteins, and lipids [44]. Of particular interest are the non-digestible polysaccharides of brown seaweeds, namely alginate and fucoidan which, along with cellulose, are structural components of the algal cell wall, while laminarin and mannitol are located in the cytoplasm [44,45,46]. Feeding intact or whole macroalgae has attracted considerable interest in recent years as potential substitutes for AGP and ZnO to maintain performance and health in weaner pigs, due to their prebiotic, antibacterial, antioxidative, and immunomodulatory activities [47,48].

The supplementation with crude seaweed extracts containing both laminarin and fucoidan have been shown to be effective in post-weaned pig diets [49,50,51,52], however, the supplementation of intact seaweed has been less successful in the immediate post-weaned pig diet, as presented in Table 1. In a recent large commercial experiment in Denmark, Satessa et al. [53] could not obtain any positive effects of intact macroalgae on piglet health and performance. Previous studies with intact brown macroalgae also reported similar results in weaned pigs [54,55] or reduced performance when fed to finishing pigs [56]. The application of the intact macroalgae in a dry meal, means that the nutritional value of the final product is dependent on the seaweed variety, season of harvest, geographic location, and environmental and climatic conditions, all of which influence chemical composition [57,58,59,60]. The extraction methodologies and conditions used to extract polysaccharides (i.e., combination of parameters such as solvent, pH, temperature, time, solvent to seaweed ratio) are also an important contributing factor to the quantitative, structural, and functional variability of seaweed polysaccharides [58,59,61].

Chitin is a natural polysaccharide found in the exoskeletons of arthropods. Chitosan is formed by partial deacetylation of chitin under alkaline conditions or by enzymatic hydrolysis. Chitosan has exhibited antimicrobial activities against many bacteria, fungi, and yeasts, with a high killing rate for both gram-positive and gram-negative bacteria and low toxicity towards mammalian cells, indicating its suitability as an antimicrobial supplement [62]. The antimicrobial activities of chitosan are dependent on several factors including pH, the species of the microorganism, pKa, molecular weight, degree of deacetylation, and the presence or absence of metal cations [63]. This review will focus on the feeding of laminarin, fucoidan, chitosan, and chitosan derivatives and their ability to alter the composition of the GIT microbiota, inhibit intestinal pathogens, modulate the immune system, and enhance performance and health in the post-weaned pig.

Laminarins are low molecular weight β-glucans consisting of a linear backbone of (1,3)-β-linked glucopyranose residues with a varying level of β-(1,6)-branching [69] (Figure 1). Water solubility of laminarin depends on the level of branching [70]. Laminarin accumulates in the vacuoles of algal cells during summer and early autumn to support survival and growth during the winter and early spring when it reaches its lowest levels [69,71,72]. In terms of laminarin quantity, Laminaria hyperborea and L. digitata were reported to have the highest laminarin concentration among the different seaweed species, indicating that Laminaria spp. are an important source of this polysaccharide [70].