Equine gastro-intestinal system

Functional anatomy

Ascending colon (large) vastly modified into fermentation chamber

3 compartments separated by physiological valves

Ventral colon from dorsal colon - Narrow pelvic flexure

Dorsal colon from small colon - Narrowing transverse colon

Caecum separated from ventral - Caeco-colic valve

Transverse colon relatively short and norrows down to meet descending

Longitudinal muscle not continuous strand, arranged in taenial bands

Different number of taeniae in different parts

Shorter than LI itself resulting in sacculations

Enable mixing and delays transit time to allow fermentation

Fermentation

Vast majority of non-hydolysable CHO reaches LI, so extensive fermentation (cf moderate in ruminants)

Microbial products are VFAs, CH4, CO2

Some VFAs and ALL microbial protein egested (rabbits/rats - coprophagy)

Metabolism of VFAs

Energy source, especially horse

Homestasis of colonic epithelium by regulating genes controlling proliferation/apoptosis/differentiation

  • Acetate - Used in liver, oxidised in other cells to generate ATP, major source of acetyl CoA for lipid synth
  • Propionate - Substrate for gluconeogenesis
  • Butryrate - Energy production + cellular homestasis

Absorption

  • VFAs Absorbed by SCFA/bicarb exchanger
  • Na+ - Sodium channels + Na+/H+ exchanger, enhanced by aldosterone
  • Cl- - Bicarb/hydroxyl exchange
  • Water - Extensive reabsorption in horses as lots of water reaches LI, osmotic pressure, hydrostatic pressure and solvent drag

Comparative physiology

In ruminants and hCHO immediately fermented so none passes into SI

In horse potentially lots of hCHO can pass into LI if overload of SI capacity to digest/absorb

SI occurs before fermentation vessel

Carbohydrate fermentation

VFAs cause decline in pH of luminal content - Can alter microflora

Can alter microflora

Protein fermentation

Equine hind-gut more capable of absorbing amino acids/peptides

Hence less taken up by microbes ("stolen")

Microbial nitrogen requirements met by urea secreted from ileum/LI

Water absorption

At luminal pH most VFAs in ionic form - poorly absorbed

Local mechanism of converting ions to respective acids to enhance absorption

Bicarb secreted in exchange for Cl- maintain pH at suitable level

When VFas absorbed net absorption of NaCl - Solvent drag so water reabsorbed

VFA absorption

Cf to ruminants VFAs absorbed intact rather than metabolised as absorbed

As in ruminants proportion of propionic acid increased with more hCHO passing into LI (If fed in excess, amylolytic bacteria with proliferate too much)

Microbiology

Bacteria - Lower levels of hCHO so amylolytic population consequently lower so fermentation slower

Protozoa - Ciliates as in ruminant but different species, small number but bacteria larger, if removed very little difference to fermentation efficiency

Fungi - Probably similar to ruminants, split apart lignin

Fore-gut vs hind-gut fermentation (hind gut 70% efficient as foregut)

Microbes less efficient, lower degree of amylolytic fermentation

Hind-gut fermenters lose microbial protein

However gut transit time can be altered to consume more if on low quality forage

Motility

  • Large intestinal transit time (fermentation takes several days in hind gut fermenters)
  • In horses caecum separated from colon by caeco-colic valve. No retrograde flow from colon to caecum

Caecal contractions: Segmental contractions principal type, mass contractions occur every 3-5 mins

Colonic contractions: Main are segmental, peristaltic and anti-peristaltic. Anti-peristaltic contractions more prominent in proximal colon (no retrograde flow)

Anti-peristaltic contractions occur principally in distal part of ventral colon (HORSES), slow movement of chyme from ventral to dorsal colon. Only small particles can pass via pelvic flexure ensuring slow transit time

Equine colic

Rectal exam [use pictures from powerpoint]

Clinical sign of abdominal pain

  • True colic = Gastro-intestinal pain
  • False colic = Other abdominal organ pain (e.g. bladder, kidney, uterus)

Colic results in highest levels of equine morbitity & mortality and therefore accurate diagnosis is essential to treat successfully

Most useful diagnostic procedure is rectal examination (can only reach caudal 1/3rd of abdomen)