20a: Intestines & Diarrhea

Where is most fluid from ingesta absorbed

Enterocytes in the small intestine

Why is it a problem if excess fluid makes it past the small intestines

The colon has a finite ability to absorb water, and small changes in fluids and electrolytes can have severe consequences

Feature of the mucosa specific to the esophagus

Folds

Feature of the mucosa specific to the stomach

Gastric pits

Feature of the mucosa specific to the small intestine

Villi

Feature of the mucosa specific to the colon

Gland opening

Two general mechanisms of diarrhea

Inflammatory and non-inflammatory

Four categories of diarrhea

• Osmotic

• Secretory

• Malabsorptive

• Increased motility

Osmotic diarrhea

Increased insoluble solutes in the intestinal lumen pulls fluid from circulation

Secretory diarrhea

Fluid secretion is greater than luminal absorption; enterocytes secrete excess Cl^- into lumen → changes dynamics of Na/K pumps → water pulled into the lumen

Malabsorptive diarrhea

Damage to the enterocytes or tights junctions results in a decrease in function and decreased absorption

Increased motility diarrhea

Not enough time for fluid absorption

In small bowel diarrhea, what is the problem

Too much fluid is entering from the ileum

In large bowel diarrhea, what is the problem

Reduced capacity to handle normal fluid and electrolyte volumes

Frequency of small v large bowel diarrhea

Small bowel diarrhea will have normal to increased frequency while large bowel has increased frequency (storage problem!)

Volume of small v large bowel diarrhea

Small bowel diarrhea has increased volume while large bowel diarrhea has decreased volume

Which anatomic location can result in mucus in the diarrhea

Large bowel

Compare blood in small v large bowel diarrhea

Small bowel diarrhea has melena while large bowel diarrhea has frank blood

Straining and urgency of small v large bowel diarrhea

Small bowel diarrhea typically has no tenesmus or urgency while large bowel typically has both

Which anatomic location can result in weight loss due to diarrhea

Small intestine

If a small animal patient has diarrhea, when is it time to see the vet

• No resolution after 24-48 hours

• >2 episodes

• Melena/hematochezia present

• Vomiting or decreased appetite

• Anemia

• Weakness

• Straining

Primary consequences of concern with diarrhea

• Electrolyte abnormalities

• Acid base imbalance (met acidosis >>>> met alk)

• Dehydration

Electrolyte abnormalities seen with diarrhea

• Hypokalemia

• Hyper or hyponatremia

• Hyperchloremia

Salmonella specie of most importance when considering diarrhea

Salmonella enterica

CS of salmonellosis

• Systemic septicemia

• Febrile enterotyphlocolitis

• Other signs depending on translocation

Primary route of Salmonella transmission

Ingestion, often via fomites

Epithelial cells affected by Salmonella

M cells (no protective mucus!) and enterocytes (protective mucus)

How does Salmonella get internalized from the intestinal lumen

Colonizes surface of enterocytes → endocytosis (or crosses tight junctions) → replicates within mucosa → invades lamina propria and GALT → infects dendritic cells → migrates to Peyer’s patches

Once Salmonella is inside macrophages and dendritic cells, what happens

Inhibits phagolysosome fusion → lyses macrophages → release of salmonella toxins

What part of the Salmonella infection results in secretory diarrhea and how

Bacterial replication in enterocytes disrupts pump activity → secretory diarrhea

Types of salmonellosis

Peracute, acute, and chronic

Peracute salmonellosis pathogenesis

From M cells and enterocytes in small intestine → gets to microvasculature → toxins cause necrosis → bacteremia → vasculitis and thrombosis → DIC

Acute salmonellosis pathogenesis

From M cells and enterocytes in the SI/LI/cecum → toxins and necrosis at mucosa → submucosal and laminal propria vasculitis → fibronecrotic inflammation

Chronic salmonellosis pathogenesis

From M cell sand enterocytes in the SI/LI/cecum → submucosal toxins and acute necrosis → submucosal/lamina propria vasculitis and thrombosis → mucosal infarction → mucosal necrosis and ulceration

What determines a serovar or serotype

Similar surface antigens

O antigen

Surface polysaccharide of G- bacteria

H antigen

Part of flagella

Why do we care about serotypes of Salmonella

Different serotypes have different pathogenicity

Salmonella seroar that is important in basically every species

Typhimurium serovar of Salmonella enterica