The pre-digested feed from the stomach passes into the small intestine which is a highly specialised part of both the horse’s digestive and the immune system. It enters via muscular one way valve into the first section of the intestine namely the duodenum also entering at this juncture via a common duct is liquid bile directly from the liver (the horse has no separate store of bile) along with digestive juices and bicarbonates from the pancreas. The bicarbonate nearly neutralises the liquid feed rendered acid in the stomach so that pancreatic enzymes can work more efficiently. Bile salts serve to emulsify fats (triglycerides) into very small droplets so as to increase their surface area to assist the lipase (fat) enzymes to release free fatty acids. Other digestive enzymes convert proteins into polypeptides and complete carbohydrate digestion to glucose.
By the Ilium, the middle part of the small intestine digestible carbohydrates are well into being converted to disaccharides and glucose, fats into free fatty acids digestible proteins are converted into peptides (small chains of amino acids) and by the time the Jejunum, the last part of the small intestinal tract, is reached the digestible proteins will have been broken down into amino acids.
The entire length of the intestine has a very specialised single cell thick lining, the interior wall being tightly wrinkled into projections called circular folds that greatly increase the surface area and on these wrinkles are finger-like villi which further increase the surface area and on the tops of these villi are billions of micro-villi upon which are anchored suitable enzymes for this final stage of both carbohydrate and protein digestion. These projections massively increase contact between the lining and the liquefied feed to maximise nutrient absorption such micro-villi are called brush borders as they resemble the bristles on a paintbrush.
The single-cell lining is also covered with special lymphoid immune cells along with a mucous coat to protect against the infiltration of pathogenic bacteria and other micro-organisms. Once absorbed the nutrients including water-soluble vitamins and minerals pass into the adjacent capillaries and are transported directly to the liver for processing. Long-chain fatty acids are converted back to fats (triglycerides) and enter the lacteal (lymph system). Fat-soluble vitamins are also absorbed here by the lacteal. The small intestine absorbs medium-chain fat directly the bloodstream and theses are rapidly converted into energy by the liver.
The lymph system plays a very important role in the defence against invading pathogenic microorganisms, lymph nodes and other lymph organs filter the lymph fluid to remove microorganisms and other foreign particles. Lymphatic organs contain lymphocytes and macrophages that play a central role in the immune system by killing invading microorganisms.
It can be quickly seen from this brief overview how worms can cause much physical damage to the highly specialised mucosal lining of this part of the alimentary tract and interfere with nutritional absorption allowing pathogenic bacteria, virus and deleterious yeasts to enter the bloodstream and the lymph system destroying important intestinal immune defence cells.