General Concepts in Intestinal Disease

Small Intestine vs Large Intestine

• The small intestine neutralizes acid and digests and absorbs nutrients using pancreatic and brush border enzymes
• Absorption happens through the brush border on villi
• Water is absorbed, following electrolytes like Na+
• The small intestine also secretes water, following electrolytes like Cl-
• It mixes and propels ingesta
• The large intestine absorbs water and stores and expels feces
• It plays a major role in microbiome function in horses, rodents, and rabbits (caecotrophy/pseudorumination)
• Disruptions to the microbiome can have severe consequences
• The duodenum is responsible for IRON absorption
• Vitamins are absorbed in the upper SI, except for B12 which is absorbed in the ileum
• By the time ingesta reaches the large intestine, most nutrients are removed
• Water absorption is mediated by a sodium gradient
• Sodium pulls in glucose, and water follows
• Secretion is driven by chloride movement
• Sodium ions enter, and chloride ions typically follow
• Chloride ions are secreted out, and sodium ions often follow
• Large intestine mainly absorbs more water, concentrating fecal material
• Duodenal perforations show low bacterial numbers
• Perforations of the jejunum have a healthy amount of organisms entering the abdominal cavity
• Large intestine perforations have a huge number of bacteria and protozoa
• A huge microbiome exists by the time you get to the large intestine

Small Intestine Histology

• Villi are mature enterocytes with microvilli, and goblet cells
• Villi absorb, driven by Na
• Crypts are immature enterocytes/stem cells that secrete, driven by Cl transport
• Water moves passively into and out of the gastrointestinal lumen
• This movement is driven by electrochemical gradients established by the active transport of ions and other solutes
• Fluid reuptake is driven by the coupled transport of nutrients, such as glucose, with sodium ions
• Secretory fluxes of fluid are largely driven by the active transport of chloride ions into the lumen
• Stem cells reside in the base of the crypts
• The number of goblet cells increase as you move into the large intestine
• Villi are moving up away from that top surface
• Goblet cells keep things protected and coat the ingesta
• Villi go up, and crypts dive down
• Absorption with sodium occurs in the villi
• Secretion with chloride occurs in the crypts

Large Intestine Histology

• Large intestine has no villi, but has abundant goblet cells and tightly packed glands
• Colonic "crypts" are present in the LI, along with Mature cells at the surface and Immature cells at the base
• Some K+ is secreted into intestinal lumen as part of mucus
  • This is why hypokalemia should be considered with chronic ileal or colonic diarrhea
• The surface is relatively flat, with crypts diving down
• Stem cells are in the base of the crypts

Intestinal Epithelial cells

• Cell types include stem cells, paneth cells, neuroendocrine cells, goblet cells, enterocytes and M cells
• Epithelial cells differentiate during homeostasis as they function in villi and crypts
• In addition to enterocytes and goblet cells are neuroendocrine cells, Paneth cells and M cells

Stem Cells of Crypts

• Crypt epithelial cells are multipotential stem cells that divide every 24 hours which can be seen histologically as frequent mitotic figures differentiating and renewing the surface epithelium
• Complete turnover is rapid, ~2-6 days depending on the species and location
• They give rise to all cell types
• Stem cells are also thought to secrete Cl- into the intestinal lumen, and defective chloride secretion can lead to secretory diarrhea
• They replicate all other cell types and divide, and their progeny moves up the crypts to the tip of the villi
• They turn over rapidly, from every two days to every six days, depending on the location
• Targeted by viruses and chemotherapy

Goblet Cells

• Goblet cells increase in number from the duodenum to the rectum
• Mucus can trap pathogens and facilitate antigen-specific and non-specific interactions such as antimicrobial proteins
• More mucus (and diarrhea with mucus and blood) indicate large intestinal (LI) disease
• Protective layer of mucus- a Moderately thick layer on the surface of the small intestine that gets thicker in the large intestine
• Mucus facilitates interactions between antigens in the gut and produced antibodies
• There are antimicrobial proteins produced by several cell types, including epithelial cells
• They have a direct antibacterial effect
• There is a thinner layer of mucus in the small intestine to allow nutrients to reach the cells
• Thicker layer in the intestine as a protective barrier
• More watery layer of secretion sits on top of cells, then a thick mucinous layer on top

Paneth Cells

• Paneth cells reside at the base of crypts in primates, horses and rodents
• They have bright pink granules, perform secretory and phagocytic functions, and secrete antimicrobial proteins such as lysozyme and defensins (cationic proteins)
• They are involved in modulating the inflammatory response by secreting interleukins and cytokines
• Defensins disrupt bacterial cell membranes, inhibit bacterial cell wall synthesis, and neutralize toxins secreted by bacteria
• Against viruses: target viral envelopes, glycoproteins, and capsids, inhibiting viral fusion and post-entry neutralization, Bind to and modulate host cell surface receptors, and disrupt intracellular signaling
• Paneth cells produce interleukins, cytokines, and chemokines to induce inflammation and suppress inflammatory responses
• Specialized cells found in primates, horses, and rodents- columnar and sit at the base of crypts
• Common in the duodenum
• Part of the innate immune system and make antimicrobial proteins
• Dog and cat epithelial cells make antimicrobial proteins, but Paneth cells are another layer of defense
• Lysozyme targets bacterial cell walls

Enterochromaffin cells

• Enterochromaffin cells are neuroendocrine cells that inhabit crypts
• They produce and secrete gastrointestinal hormones and peptides
• These include gastrin, gastric inhibitory peptide (GIP), cholecystokinin, glucagon, somatostatin, vasoactive intestinal peptide (VIP), motilin, substance P, neurotensin, secretin and serotonin
• These hormones are secreted directly into the lamina propria and some enter bloodstream
• Make hormones that act locally (paracrine effect) and some have systemic effects
• Serotonin and Substance P are involved in vomiting
• Serotonin's function is to Modulate mood, appetite, and sleep
• Serotonin can promote vomiting

Hormone Function

• Serotonin: Mood, appetite, sleep
• Cholecystokinin: Gallbladder emptying, pancreatic secretion, satiety
• Bombesin: Negative feedback for eating
• Secretin: Regulates secretions of stomach, pancreas and water balance
• Enteroglucagon: Delays gastric emptying
• Enterogastrone-Brunner's gland: HCI from stomach
• Gastrin: Stimulates parietal cells to release HCl, ↑ motility
• Fibroblast growth factor 19: has effects on liver (bile acid production, glucose, glycogen)
• Substance P: Stimulates emetic center
• Vasoactive intestinal polypeptide: Relaxes smooth muscle of stomach, esophageal and gastric sphincters, and gallbladder while also inducing contraction of enteric smooth muscle
• Gastric inhibitory peptide = glucose-dependent inhibitory peptide (Gastrin,↑ insulin)
• Motolin: Stimulates peristalsis, promotes motility
• Peptide YY: Motility

Interstitial cells of Cajal

• Interstitial cells of Cajal are mesenchymal cells and pacemaker cells that regulate gut motility
• They are located between muscle layers & myenteric plexus
• These cells are hypothesized to be the cell type from which GIST (Gastrointestinal stromal tumors) arise
• Pacemaker cells that regulate gut motility
• Located between muscle layers and myenteric plexus
• They can be found in both of the, my enteric plexi
• They connect nervous system plexi
• Hypothesized to be the cell type from which Gastrointestinal stromal tumors (GIST) arise
• Require immunohistochemistry to tell apart
• Excision is the way to go for these tumors

Immune Components

• Diffuse lymphoid tissue & intraepithelial lymphocytes
• Lymphoid follicles = Peyer's patches are most numerous in the jejunum and proximal ileumsecondary
• Microfold (M) cells are epithelial cells only found over (MALT) or (GALT), especially in Peyer's patches
• They are broad domed cells with few short microvilli & invaginations
• They uptake antigens from the intestinal lumen transferring them to underlying lymphoid tissue via transocytosis
• These cells are portals of entry for some pathogens like Salmonella, Shigella, Rhodococcus, Yersinia and Bovine Viral Diarrhea Virus (BVDV)
• Lamina propria is loose connective tissue between villi with lymphocytes and dendritic cells
• Lymphocytes crawl up through the epithelium, heading towards the lumen
• There is a type of lymphoma that will have a lot more intraepithelial lymphocytes than normal
• Peyer's patches have a lymphoid follicle extending up into the villus with a T-cell area on either side and a B-cell area
• Microfold cells (M-cells) are found in the dome
• M-cells are sitting amongst the enterocytes and goblet cells
• M-cells have inward folds instead of microvilli
• Facilitate interactions between lymphoid cells and organisms
• Salmonella, Shigella, Rhodococcus, and Yersinia take advantage of this route to enter and cause disease

Microbiome

• Normal commensal microbes are vital to homeostasis, and produce nutrients necessary for epithelial cell health/metabolism
• This also limits pathogens' ability to colonize
• They also promote immunological tolerance to luminal contents
• Microbiome disruption can lead to intestinal dysbiosis, diarrhea, abdominal pain/colitis, and systemic disease
• Treatments include fecal bacteria transplantation, and probiotic supplementation
• Antibiotic administration → Clostridial overgrowth (likely C. diff) leads to “dysbiosis”→ toxin production → severe ulcerative and necrohemorrhagic colitis
• The GIT, especially large intestine, contains trillions of commensal bacteria, 10x more bacteria in gut than host cells
• Normal commensal microbes are protective and may bind to receptors that pathogens want to access
• Produce nutrients and promote immunologic tolerance
• Intestinal dysbiosis- disruption of normal microflora where the animal becomes vulnerable to disease
• Clostridium difficile overgrowth is a bad example of intestinal dysbiosis
• C. diff produces a toxin that causes necrotizing hemorrhagic colitis
• This is a particular problem in horses and hindgut fermenters
• Diagnosing C. difficile involves looking for the toxin

Intestinal Defense/Barriers

• Innate Defense
• Resident microbiota to prevent pathogen colonization
• Motility propels organisms & toxins
• Mucus captures IgA and antimicrobials
• Tight junctions and rapid turnover by the Epithelium
• Innate immune cells like Macrophages, dendritic cells and M-cells
• Aquired Defense
  • Lymphocytes (T & B cells), cell-mediated immunity, antibodies
• Peristalsis is protective, and it moves things out and gets rid of toxins
• IGA production by lymphocytes and plasma cells
• Antimicrobial proteins are able to destroy organisms without an inflammatory response
• Rapid turnover of epithelial cells sloughs off anything bound to the epithelium

Mechanisms of Intestinal Diseases

• Inflammation
• Necrotizing Processes
• Lymphangiectasia—disorders of fluid drainage
• Disorders of innervation
• Diarrhea
• Malabsorption
• Chloride hypersecretion
• Exudation (increased permeability, protein-losing enteropathy, PLE)
• Hypermotility

Small Intestine targets

• Infectious agents target Absorptive Enterocytes (villi) specifically, causing rotavirus, enteric coronaviruses, intracellular bacteria (Brachyspira), coccidia, cryptosporidium an resulting in loss of absorption
• Crypt Cells (undifferentiated) can also be targets like parvoviruses, BVDV, rinderpest and mycotoxins, which result in a loss of crypt cells and villi
• Abnormalities of Microvilli and Glycocalyx can be affected by Attaching and effacing E. coli (AEEC) on Neomycin therapy
• Some pathogens target the villi, and some target the crypts
• Rotavirus, enteric coronaviruses, coccidia, and cryptosporidium target the villi, causing a Loss of absorption
• Parvovirus, BVDV, rinderpest, and mycotoxins target crypts
• When crypts are targeted, villi start to collapse
• Attaching and effacing E. coli (AEEC) on Neomycin therapy and can affect microvilli and glycocalyx

Clinical Manifestations

• Vomiting
• Diarrhea
• Acute Abdomen/Colic
• Protein-losing Enteropathy (PLE)

Vomiting

• Vomiting (emesis) involves complex coordinated movements that forcefully remove upper GIT and gastric contents, protecting the airway
• Stimuli can be irritating to the upper GIT (including small intestine) through infection, inflammation or toxins, distension/stretch
• Chemoreceptors CRTZ, Cerebral cortex/limbic system and Vestibular/inner ear can also be stimuli vomiting has three phases
  • Pre-ejection: nausea, salivation
  • Retching phase: deep inspiration and closure of epiglottis
  • Expulsive phase: Relaxation of esophageal sphincter, contraction of diaphragm and abdominal muscles
• The vomiting center in the medulla oblongata, and the chemoreceptor trigger zone also in the medulla oblongata. Control vomiting
• It is important to differentiate vomiting from regurgitation and dysphagia
• Dysphagia is oropharyngeal disease results in defective swallowing (often worse with liquids), is usually immediate and Non-forceful
• Regurgitation = esophageal disease involves expulsion from the esophagus, is passive and initiated by gravity and contains Undigested food
• Coughing = respiratory disease can be confused by pet owners as vomiting
• Vomiting (emesis) involves coordinated movements to remove upper GIT and gastric contents
• Three phases of vomiting- Pre-ejection (nausea, salivation), Retching, Expulsive
• Vomiting center in the medulla oblongata controls the three phases, Chemoreceptor trigger zone is also in the medulla oblongata

Diarrhea

• Diarrhea can be classified by the site of disease within the small and large bowels
• Feces volume is normal to greatly increased in the SI but normal to decreased in the LI
• Consistence is very watery to semi-formed in the SI
• Colour is normal in the SI but frequent in the LI
• The frequency of defecation is Normal to slightly increased in the SI but Markedly increased in the LI
• SI diarrhoea causes vomiting, while Large Bowel Diarrhoea includes tenesmus (straining)
• It is important to figure out whether its small or large bowel because the etiologies are different
• Small bowel has a lot of fluid, so you tend to get large volumes of feces
• Large bowel does not have much water, so you tend to get small volumes of feces
• Large bowel has a lot of mucus and straining
• Small bowel diarrhea is more likely to cause weight loss
• Blood digested in the upper GIT (stomach or small intestine) shows MELENA (black & tarry)
• Lower GIT bleed (colon, rectum) shows blood is RED

Diarrhoea Continued

• Upper GIT bleed (stomach or small intestine) shows blood is digested➔ MELENA (black & tarry)
• Lower GIT bleed (colon, rectum) shows blood is RED
• Color of diarrhea can be helpful when you have hemorrhage
• if I see evidence of hemorrhage, whether it's melanoma or frank blood, I'm going to think about ulcers, things like that
• Viruses can cause bleeding in the GI tract

Fluid Flow

• The small intestine: Moves a LOT of water
• Water moves in or out of the small intestinal lumen based on osmotic gradients where the osmolarity of the chyme equals osmolarity of the plasma
• Absorption of water (and many solutes) follows Na. A gradient generated by Na/K ATPase
• Secretion of water follows Cl. Normally absorptive and secretory processes are tightly regulated so that absorption predominates
• In the small intestine, water moves in and out through leaky tight junctions
• Water moves based on osmolarity
• Tight junctions tighten up as you move down into the gut, into the large intestine
• In a normal animal, water movement is controlled by the sodium-potassium ATPase

Classifying Diarrhoea

• Diarrhea classification can be Osmotic /malabsorption /maldigestion, Secretory, Inflammatory/exudative or due to Abnormal motility
• OSMOTIC= “Malabsorption" often has a Stool volume which is Moderately increased
• In secretory diarrohoea the absorptive capacity is exceeded producing Very large volumes and the diarrhea continues
• The Toxin "usually" causes this often with bacterial
• Increased capillary or epithelial permeability may cause exudation of serum and/or blood into the intestinal lumen leading to "inflammatory exudate diarrhoea" often linked to "Protein-Losing Enteropathy"
• Increased longitudinal contractions → insufficient contact time lead to diarrhea so there is "Abnormal motility"
• Diarrhea can be osmotic, secretory, inflammatory/exudative, or due to abnormal motility
• Osmotic- often due to malabsorption where food particles stay in the intestine and that increases osmolarity
• When they target the villi, those villi, now you've lost absorptive surface
• Lactose intolerance can pull water into the lumen
• Puppy eating a bag of dog food can create an osmotic diuresis
• Stop feeding the animal, and the diarrhea generally resolves with osmotic diarrhea
• Secretory diarrhea occurs through chloride secretion
• Secretory diarrhea is Very super watery diarrheas and the Taking away food is not going to fix this problem.
• Sodium-potassium ATPase generates a sodium gradient, and that gradient makes the potassium chloride sodium co-transporter
• The co-transporter is activated by cyclic AMP and cyclic GMP, which activate protein kinase A and produce chloride channels
• Secretin can also stimulate this process

Diarrhoea From Hypomotility

• Hypomotility causes Increased circular contractions → resist flow which causes Bacterial overgrowth to produce Secretagogues
• Hypomotility can cause an osmotic component from bacterial overgrowth

Intestinal syndromes

• Clinical syndromes are not specific deseases
• Colic/Acute Abdomen can be caused by a Painful abdomen where Clinical presentation is Not a specific disease therefore stabilization is key to identify the inciting cause

Abdominal Dz sings

– Restlessness, Kicking/biting at abdomen (horses), Elevated HR/RR, Abnormal postures (“prayer position," head low), Pain/tense on palpation, Abdominal distension if effusion, Hypovolemia and Shock

• Colic- painful abdomen, Clinical presentation is not a specific disease, therefore stabilization is key to identify the inciting cause
• Rolling in pain may be seen in horses
• Small animals may have prayer position, hunched over, and pain on abdominal palpation

Gastrointestinal dz:

• GDV
• Obstruction
• Severe enteritis
• Ulcers
• Intussusception
• Mesenteric volvulus
• Neoplasia
• **Extra-abdominal dz can ALSO be a sign of acute abdomen →Pleuropneumonia

Other Dz

• Biliary obstruction
• Urinary obstruction
• Pyelonephritis
• Uterine/testicular torsion
• Peritonitis

Protein-losing enteropathy (PLE)

• Protein Losing Enteropathy is asyndrome characterized by excessive loss of serum albumin across the intestinal wall. This is different toProtein losing nephropathy
• Small Animal diseases can cause PLE though any thing disrupting intestinal wall function
• Lymphangiectasia, Idiopathic inflammatory bowel disease, Neoplasia particularly lymphoma
• Infectious enterocolitis Parvovirus, Salmonella, Histoplasma also Blood sucking parasites hookworms, +/-GI hemorrhage is an important
• Large Animal diseases that can cause PLE Parasitism Horse Strongylus, Ruminant Haemonchus, camelids Elmira Johne's disease Infections enterocolitis Salmonella, Lodges, Lawsonia, lymphoma and Gl ulcers all contribute
• Epithelium has enormous regenerative capacity Intact basement membrane → best outcome while, Damage to basement membrane/lamina propria may be scarring, Serve elaciation → may be loss of function and limited healing
• A syndrome characterized by excessive loss of serum albumin across the intestinal wall
• May see low oncotic pressure that causes ascites or edema in the limbs
• Hypocalcemia may result
• Animals will often have malabsorption and may see a lymphopenia
• Animals are losing antithrombin, so you may see a hypercoagulable condition and thromboemboli
• Disruption often inflammatory disruption, of probably large swaths of your small intestine
• Protein losing enteropathy is not a specific diagnosis and is generally related to inflammatory conditions