BVetMed1: Diarrhoea - Lecture 152

Diarrhoea - Lecture 152

1. Knowledge of normal structure and function and key features of various organs/tissues and how abnormal structure/function (pathology) can lead to the specific signs of clinical disease relating to that organ.

Structure/function of intestinal villi:

Villus tip

cells die and are shed in lumen after 3-5d
cells migrate to the villous tip and develop absorptive and surface digestive capabilities (brush boarders and carrier proteins)
Villi become shorter, but more numerous in ileum (compared to duodenum) - surfaces areas are equivalent (see picture below)

Crypt

cells are formed by division
stem cells to regenerate villous tip are made here

Any disease that increases the space between epithelium and vessels will result in problems with digestion/absorption

Absorption:

Carbs and Proteins:

brush border enzymes on mature enterocytes (epithelium) are involved in final digestion or monosaccharides, amino acids and peptides
anything that destroys enterocytes hampers this

Fats:

lymph lacteals take up monoglycerides and free fatty acids via diffusion
increase width of villi, fats cannot make it through to lacteals

Water:

~ 98% of fluid reabsorbed in intestines therefore faecal fluid content is very low

2. To appreciate that the intestinal tract is well adapted to its function of digestion and absorption of the products of digestion – and water, that the intestine has evolved a specialised anatomy to serve that function and that diarrhoea is a sign of loss of that structure/function

Fluid dynamic in a normal small intestine:

Villous tip:

mature enterocytes absorb sodium, glucose and amino acids across brush border on apical surface
Sodium pumped out of cell at basolateral aspect
results in net absorption of fluid (water follows sodium)

Crypt base:

stem cells ( with leaky intercellular junctions allow sodium to leak back into lumen
results in net secretion of fluid

Glucose and amino acid transport - major forces for absorption in duodenum

Sodium pump - more important in ileum and colon

Net absorption at villous tip > net secretion in crypts

Diarrhoea when decreased absorption and/or increased secretion

Diarrhoea

increased volume/fluidity of faeces
sign of disease affecting intestinal tract
not a disease, a sign of disease

3. To be able to name, describe and give examples for each of the four underlying mechanisms to diarrhoea, some cases of diarrhoea have more than one mechanism acting at once

Pathogens of diarrhoea:

1) altered epithelial cell transport (secretory diarrhoea)

2) altered structure of permeability of mucosa (largest group)

a) Cytotoxins (eg salmonellosis)
b) Infiltration of cells into villi (eg, lymphoma, inflammation, Johnes disease)
c) Lytic infection of villous epithelial cells by viruses (coronavirus and rotavirus)
d) Lytic infection of stem cells in crypts by viruses (parvoviruses)

3) osmotic effects

4) altered motility

1) Altered epithelial cell transport (secretory diarrhoea)

some types of bacteria attach to villus epithelial cell and inject a toxin into cell
toxin prevents sodium pump at apical surface

promotes excretion of sodium from mature villus epithelial cells

Water follows sodium and massive fluid loss into lumen
produces profuse, watery diarrhoea
no destruction of cells; histology and gross appearance of intestinal tissue are normal
self-limiting as cells shed every 3-5d so new cells replaced 'blocked' cells

E Coli in calf:

2) Altered structure of permeability of mucosa

a) Cytotoxins: (eg salmonellosis)

Bacterial infection of enterocytes
bacteria (eg Salmonellae) produce toxins that damage and kill cells
death of enterocytes - allow blood, fluid and other contents to leak into the lumen
villus isn't absorptive as enterocytes have been lost
altered structure and permeability ---> loss of function
blood and shreds of intestine tissue in diarrhoea = dysentery

Haemorrhagic enteritis - calf

b) Infiltration of cells into villi: (eg, lymphoma, inflammation, Johnes disease)

Massive increase of cells of villus core will cause width of villus to increase and height to decrease. Therefore surface area decreases
epithelial have to stretch to cover - flatten and lose brush border. Loss of SA and enzymes for digestion
cannot absorb nutrients. Nutrients stay in lumen, bacteria grow to eat it - can make condition even worse

Intestinal lymphoma - dog

Lymphocytic/plasmacytic enteropathy - dog
- Inflammatory bowel disease
- infiltrate is mixed with population of lymphocytes and plasma cells (cellular infiltrators)