Equine GI Flashcards
Broadly describe differences in ascending, transverse and descending colon between species
- Ascending smallest section = modified
2. T and D = largely similar
Describe the functional anatomy of the equine GI
- Ascending colon vastly modified into a fermentation chamber
- 3 compartments which are separated by physiological valves
- different types of fermentation occurs in each compartment
- compartments: Caecum, ventral colon, dorsal colon
- relatively short transverse colon narrows to meet descending colon
- Longitudinal muscle not continuous strand but arranged into taeial bands - diff number in each part
Name the valves and where they are in the equine large intestine
Ileum 1. ileo-caecal valve Caecum 2. caeco-colic valve Ventral colon 3. narrow pelvic flexure dorsal colon 4. Narrowing transverse colon
In horse what is unique about the caecum?
- ileum f SI goes INTO caecum in other species caecum is plopped on the side
Fermentation of non hydrolysable CHO in horse
- What does it depend on?
- How different to cow
- microbial products
- positioning of LI means what?
- degree/ type depends on material reaching large intestine (like cow)
- But SI before fermentation in H, cow = other way around
- vast majority/all of non-hydrolysable CHO reaches large intestine so fermentation extensive
- Microbial products of fermentation are VFAs / CH4 / CO2
• Large intestinal mucosa can absorb these VFAs
• Gases passed to rectum by peristalsis and expelled (flatulence) - Microbial balance easily upset, just as in ruminants – diarrhea and acidosis
- As large intestine is terminal part of GI tract some VFAs AND ALL microbial protein egested / wasted
• Rabbits / rats practice coprophagy to avoid this waste (some sick horses can do)
What about fermentation in carnivores?
- Most digestion/ absorption has already occurred prior to large intestine therefore fermentation is minimal
Equine colic
- Colic is not a disease but a clinical sign of abdominal pain
- True colic relates to gastro-intestinal pain
- False colic relates to other abdominal organ pain (e.g. bladder, kidney, uterus, foal etc)
- Colic results in the highest levels of equine morbidity & mortality therefore an accurate diagnosis is essential in order to treat successfully
- The most useful procedure in diagnosis of colic is rectal examination
Metabolism of VFA in equine GI
- similar to ruminants = used as an energy source
What are the main VFAs
Improtance of one?
acetate, propionate and Butyrate. Can only generate glucose from propionate!
How is Acetate used?
- used in liver
- oxidised in most other cells to generate ATP
- Major source of acetly CoA for lipid synthesis
How is Propionate used?
- Substrate for gluconeogenesis
How is Butyrate used?
- Energy production
2. Cellular homeostasis
Absorption of VFAs
- (acetate, propioante, butyrate) absorbed by short chain fatty acids/ in exchange for HCO3- - this helps control pH
- VFAs absorbed intact rather than metabolised as they’re absorbed like with R
- propionate = gluconeogenesis to produce glucose
What is another name for VFA
short chain fatty acid
Absorption of Na
- Absorbed by sodium channels & Na+/H+ exchanger (NHE transporter)
- Sodium absorption enhanced by aldosterone
Absorption of Cl-
• Absorbed by bicarbonate / hydroxyl exchange
Absorption of water
- Most of rest of water passing from SI absorbed in large intestine
- Osmotic pressure
- Hydrostatic pressure
- Solvent drag
- Extensive re-absorption in horses – 100s of litres
Water absorption in horse vs most animals
- SI is where main water re absorption occurs (70-80%)
- Horse LI still absorbs some but most water is secreted into SI so most occurs here
- by same process
COMPARATIVE PHYSIOLOGY
- Large intestine physiology similar in most domestic species EXCEPT horse
- In feral horse on diet of grass 75% energy derived from VFAs produced by microbial fermentation in the hind-gut
- Fermentation similar process to that occurring in ruminant fore-stomach
- EXCEPT small intestine occurs before fermentation vessel
- In ruminants any hCHO immediately fermented so none passes into SI
- However in horse potentially lots of hCHO can pass into LI if overload of SI capacity to digest / absorb it can upset microbial populations
CARBOHYDRATE FERMENTATION
- Fibre (containing β-glycosidic bonds) requires microbial fermentation
- Products of fermentation are principally VFAs
- VFAs cause a decline in pH of luminal contents - acidic
• Neutralised by copious pancreatic secretion containing HCO3-
• Goblet cells in large intestine secrete mucous & HCO3-
• Ileum also secretes HCO3- - If production of VFAs excessive H+ ions too numerous to be neutralised by HCO3- secretions
• pH of luminal contents declines
• Favours multiplication of acid resistant microbes (amylolytic bacteria / lactate producers)
• Lactic acid poorly absorbed cf VFAs causing further pH decline
PROTEIN FERMENTATION
- Probably similar to ruminants
- HOWEVER
• Equine hind-gut (LI) more capable of absorbing amino acids / peptides
• Hence less amino acids / peptides taken up by microbes, more being absorbed by the host
• This is a good thing as (all) microbial protein is lost in faeces in hind-gut fermenters
• Microbial nitrogen requirements satisfied by urea secreted by ileum / large intestine
Broken down by microbial urease into NH3 to be used as NPN source for microbial protein synthesis
WATER ABSORPTION
- Compared to other species more water reabsorption occurs in large intestine
- At luminal pH (6.5) most VFAs in ionic form which are poorly absorbed
- Local mechanism of secretion of H+ in exchange for Na+ converts ions to their respective acids thus enhancing their absorption
- When VFAs absorbed there is also a net absorption of NaCl which enhance water reabsorption via osmotic pressure & solvent drag – inc efficiency
- HCO3- secreted in exchange for Cl- maintain pH at level suitable for fermentation
FOREGUT VS HINDGUT FERMENTATION efficiency
- Hind-gut fermentation 70% as efficient as fore-gut fermentation. Ruminant = 90%
- Microbes less efficient lower prop of amyloytic fermenters
- Hind-gut fermenters lose microbial protein in faeces
- Coprophagy - rabbits / rodents eat certain component of their faeces high in microbial protein
Gut transit time fore gut vs hind
- gut-transit time in fore-gut fermenters dependent on rate of fermentation and can’t be reduced whatever the nutrient quality of the fibre
• Horses on poor quality forage can decrease gut transit time and therefore consume more
• Ruminants more efficient on good quality forage
• Horses more efficient on poor quality forage as long as it’s in abundance
What is the main neutralised of VFAs decreasing pH in foregut vs hind gut
- foregut = mainly saliva
2. hind = copious pancreatic secretion containing HCO3-
Acidosis in horses
- Too high level of hydrolysable CHO
- Amylolytic poliferate at expense of cellylolytic
- A = rapid fermentors = inc VFA
- also favours lactate producers
- both lactate p and amylolytic bacteria are acid resistant
- LActic acid poorly absorbed = further pH decline
Where does most water reabsorption occur?
- Caecum
- ventral colon
in particular
Microbiology:
Bacteria
Similar categories to ruminants
1. horses tend to have less amylolytic (rapid fermenter) and more celluloytic (slow fermenter) = overall F = slwoer process
Microbiology: Protozoa
- contribution unknown as if removed v little difference
2. Protozoa lost in faeces therefore can’t be digested as in ruminant
Microbiology: Fungi
- Probably similar to ruminants
* Split apart lignin thereby rendering it susceptible to digestion by celluloytic bacteria
Motility - why slow?
Different in hind and fore gut time
- Large intestinal transit time must be slow enough to allow fermentation to occur
- Hind-gut fermenters - several days
- Fore-gut fermenters - 24 hours
Caecum adn colon, most species vs horse
- In most species caecum & colon form continuous compartment – pelvic flexures slows
- In horses caecum separated from colon by caeco-colic valve
- Separate fermentation compartment with no retrograde flow of material from colon to caecum
Point of caecum
Reserve capacity to allow food to pass back into it
- main reason to slow down rate of fermentation, must have peristalsis occurring
- food moving backwards = slows down
- In most animals it can’t go back into illeum due to physiological valve
- If caecum didn’t exist it would cause distension of gut = pain = not contribute to normal digestive process of animal
Horse caecum contractions
What is the point
MAss movement occur ev 3-5 mins extra as well as peristalsis, antiperistalsis, segmental contractions
1. empty all contents into ventral colon
How to tell if healthy
should hear toilet flushing sound every 3-5 mins on RHS = caecal contraction
Colonic contractions in most species:
- Main = segmental, peristaltic and anti p
- Anti p contractions are more prominant in proximal colon
- Retrograde movement of chyme from colon into caecum NOT into ileum
Colonic contractions in horse
- NO retrograde movement from colon into caecum - once emptied caeco-colic valve SHUT
- reverse P in distal ventral colon not proximal like in other species
- Slow movement of passage fo chyme from ventral to dorsal colon via pelvic flexure
- Only small particles/ well fermented material can pass via pelvic flexure into dorsal colon
- Ensures slow enough transit time to allow efficient fermentation
How to diagnose Equine colic
- REctal examination
2. Abdomincal Auscultation
How many ribs in horse
18 pairs
Normal sound of horse gut on LHS
- Left dorsal = SI and small colon (descending)
Fluidy in nature, higher pitch - Left ventral = pelvic felxure
Normal gut sounds on RHS
1. R dorsal/ ventral = caecum Loud sound, like toilet flushing = emptying via caeco-colic valve 3-5 per 5 min
Abnormal sounds
- Inc frequency = enteritis, spasmodic colon
- Tympanitic = gut distension with gas, obstruction
- Decreased frequency = ileus, obstruction
