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1、Overview of Malassimilation Syndromes in Large AnimalsMalassimilation is a defect in the ability of the GI tract to incorporate nutrients into the body either due to malabsorption or maldigestion. Malabsorption is the failure of passage of nutrients from the lumen of the bowel into the bloodstream,
2、while maldigestion is the failure of intraluminal degradation of dietary constituents due to a defect in pancreatic exocrine function, bile acid content, or brush border enzymes. Maldigestion alone is an infrequent cause of malassimilation in large animals. Maldigestion syndromes are uncommon in hor
3、ses compared with other domestic species. The equine pancreas secretes only low concentrations of digestive enzymes and probably plays a small role in nutrient digestion. Some disease processes involve both maldigestion and malabsorption, such as is seen in young animals with lactase deficiency. Dis
4、eases of malabsorption are much more common in horses than are diseases of maldigestion.Etiology and PathogenesisMany diseases, by altering the normal absorptive mechanisms of the small intestine, induce a malabsorption syndrome. In horses, these include the following: 1) inflammatory or infiltrativ
5、e disordersdiffuse lymphosarcoma of the small intestine (alimentary lymphoma); enteritis due to eosinophilic, lymphocytic-plasmacytic, or basophilic infiltrate; multisystemic eosinophilic epitheliotropic enterocolitis; granulomatous enteritis (inflammatory bowel disease); Lawsonia intracellularis (w
6、eanling foals, yearlings); intestinal ischemia and damage due to migration of Strongylus vulgaris larvae, small strongyles, or Strongyloides westeri (foals) infection; cryptosporidia; postinfarction inflammation; amyloid Aassociated gastroenteropathy; multiple abscessation in the bowel; tuberculosis
7、; histoplasmosis; intestinal Rhodococcus equi infection; invasive enterocolitis (Salmonella spp); 2) biochemical or genetic abnormalitiescongenital or acquired lactase deficiency (lactose intolerance), dietary-induced enteropathy, monosaccharide transport defect, pancreatic exocrine insufficiencies;
8、 3) diseases causing inadequate absorptive areavillous damage or atrophy due to viral infection (rotavirus, coronavirus) or bacterial enteritides in foals, cryptosporidia, intestinal resection; 4) cardiovascular disorderscongestive heart failure, intestinal ischemia; 5) lymphatic obstructionlymphosa
9、rcoma, mesenteric lymphadenopathy, intestinal lymphangiectasia, abscessation, thoracic duct obstruction; and 6) miscellaneousdrug-induced, heavy metal toxicosis, zinc deficiency.Malabsorption syndromes in cattle are poorly documented but likely are seen most frequently in calves with diarrhea. Disea
10、ses that cause malabsorption syndromes in ruminants and swine include viruses (rotavirus, coronavirus), cryptosporidia, local or generalized ischemia, protein malnutrition, small-intestinal resection (short-bowel syndrome), congestive heart failure, lymphatic obstruction, parasitism (trichostrongylo
11、sis of sheep and cattle), tuberculosis, and Johnes disease in ruminants and proliferative enteropathy (Lawsonia intracellularis) in swine. Oral antibiotics may alter absorptive epithelial cells and cause an imbalance in GI tract flora. Treatment with high doses of ampicillin, neomycin, or tetracycli
12、ne significantly decreases and delays glucose absorption during oral glucose tolerance tests in calves.Camelids are affected by many of the same conditions that cause malabsorption syndrome in ruminants. Coronavirus is particularly a problem in young crias. Eimeria macusaniensis, during either the p
13、repatent or patent phase of infection, may result in weight loss, hypoproteinemia, and severe debilitation in affected young and adult camelids.Maldigestion syndromes are uncommon and poorly understood in large animals. They may be due to alterations in gastric function or activity of rumen microflo
14、ra, abnormal bacterial proliferation in the small intestine, or a decrease or lack of small-intestinal brush border enzyme activity (lactase deficiency). Less likely causes include drug-induced alteration in secretion or excretion of bile salts (induced by drugs or by hepatic or intestinal disease),
15、 or deficiency or inactivation of pancreatic lipase. Changes in bile salt concentration may not impair digestion in the adult herbivore but may exacerbate diarrheal states in milk-fed neonates. Surgical resection or bypass of the distal small intestine may facilitate bacterial overgrowth with associ
16、ated bile salt abnormalities.Lactose is a disaccharide composed of glucose and galactose. The small-intestinal brush border enzymes of foals and calves include lactase, which catalyzes the degradation of lactose into its component monosaccharides that are then absorbed. Primary lactase deficiency is inherited as an autosomal recessive trait in humans, but its occurrence and the mode of inheritance in large animals is poorly documented. Acquired or secondary lactase deficienc
