shows the cross section of the small intestine of the weaned piglet to consist of many thousands of finger like projections called villi, which increase the absorptive capacity of the small intestine. During suckling they are continuously bathed by sows milk which contains the immunoglobulin IgA. This becomes absorbed into the mucous covering the villi surfaces and prevents E. coli and other organisms attaching to the fingers. If they are unable to attach they are unable to cause disease. The secretory IgA also helps to destroy bacteria. After weaning time however no more IgA is available, the levels rapidly decline and bacteria damage the villi causing them to shrink. This atrophy reduces the absorptive capacity of the gut and the ability of the pig to use its food. The enzymes produced by the cells of the villi are likewise reduced. The changes result in malabsorption of food and poor digestion with or without the development of scour. The villi normally regenerate within 5 to 7 days after weaning from cells at their base called enterocytes, which multiply and migrate upwards causing the villi to return to their normal length. The rate of multiplication and regeneration is in part an environmental temperature and energy dependent phenomena. If the pig is weaned in an environment below its lower critical temperature (LCT), the rate of regeneration of the villi is reduced and in some cases ceases. (This results in the hairy pig that doesn't grow). Feed intake is a crucial part of the equation.
Before weaning the piglet receives milk as a liquid feed at regular intervals. As a result the bacterial flora of the gut, although relatively simple compared with that of a mature pig, is stabilised.
At weaning cessation of milk removes secretory IgA and there is a period of starvation, followed by irregular attempts to eat solid feed. This results in a dynamic disruption of the bacterial flora of the gut which may last for 7 to 10 days before stabilising. This bacterial disruption may also contribute to poor digestion and possibly scour, particularly when high levels of pathogenic strains of E. coli are involved.
Before weaning the piglets led an ordered life, being "called" with their litter mates to suckle and obtain small amounts of milk at regular intervals, sleeping between meals in a warm creep. All this suddenly changes at weaning, the pigs finding themselves in strange surroundings with strange piglets, and only solid feed. Psychological trauma is inevitable and is likely to affect some pigs more than others, resulting in impaired digestibility and lowered resistance to disease. The more this psychological stress can be minimised the better