Equine Digestive Physiology

Equus caballus
1. Trickle feeding plains animal
  1. Feeds for around 16 hours per day
  2. 8 hours spent sleeping, moving and playing
2. Herbivore
  1. Hind gut fermenter with a large caecum
    1. Ferments large quantities of fibre
Mouth
1. Teeth
  1. Hipsodonts
    1. Teeth are continually growing
      1. Diet is high in silica
        Hard substance which wears away the teeth
      2. If a tooth is lost, the one above or below continues to grow into the space and gum, stopping the horse from chewing
  2. Large molars
    1. Large grinding surface
  3. Maxilla (top jaw)
    1. Stays still during chewing
  4. Mandible (bottom jaw)
    1. Moves side to side as well as back to front
      1. Allows the horse to chew in different directions
2. 3 pairs of salivary glands that buffer acid in the stomach
  1. Also lubricating to prevent damage to the oesophagus when swallowing dry food
3. Split into quarters
  1. Top left, top right, bottom left, bottom right
    1. Each quarter has 3 molars, 3 premolars and 3 incisors
      1. Vestidual canines mainly found in males and are often removed
Stomach
1. Amylase
  1. In the stomach
  2. Buffers acid (as a bicarbonate) and acts as a lubricant to aid movement
2. Four sections
  1. Oesophageal
    1. Bottom of the oesophagus and the top of the stomach
    2. Squamous tissue
      1. No secretions
    3. Lots of bacteria
      1. Lactobacilli
      2. Streptococcus
      3. Small amount of bacterial fermentation of food
  2. Cardiac
    1. Glandular tissue
      1. Secretes mucous
        Coats and protects the stomach, preventing acidic digestion of the stomach wall
  3. Fundic
    1. Glandular tissue
      1. Secretes HCl
        Digestion
      2. Secretes Pepsinogen
        Tail cleaved off in acidic conditions, giving pepsin
        This is an enzyme which breaks down proteins into peptides
  4. Pyloric
    1. Glandular tissue
      1. Secretes Gastrin
        Upon gut fill, enzyme is released into the blood, inducing the satiety response
3. Contents undergo mechanical and chemical (acidic) digestion
Equine Gastric Ulcer Syndrome (EGUS)
1. Focal (or multi focal) ulceration of the stomach epithelium
  1. Estimated 74% of horses in training are affected (Habershon-Butcher et al., 2012)
  2. Likely due to lack of fibre (no grass or haylage), lots of unprocessed grain and lots of fast exercise
    1. During fast exercise, the horse experiences transient deceleration as the legs move, which can cause the contents of the acidic regions of the stomach to move into the poorly protected oesophageal region
2. Symptoms
  1. Loss in appetite
    1. Weight loss
  2. Lethargy
    1. Poor performance
  3. Colic
  4. Dull coat
3. Diagnosis
  1. Endoscope
4. Treatment
  1. Tendency to treat symptoms as it is cheaper
  2. Reducing acid production
    1. Not effective as an acidic environment in its stomach
    2. Antacids bring up the pH, but buffering it denatures enzymes and affects the microbial population
    3. Omeprazole
      1. Proton-pump inhibitor which prevents HCl secretion
      2. Metabolised in the liver and well excreted
      3. Well absorbed in acidic conditions, that it gets rid of as it goes along
      4. 77% healing after 28 days
Small Intestine
1. Highly specialised for digestion and absorption
2. Duodenum
  1. Low pH in the stomach and duodenum
    1. Causes secretin to be released from the crypts of Leiberkuhn
      1. Stimulates the pancreas and inhibits gastrin release
        Cholecystokinin (CKK) is released when food reaches the small intestine, stimulating the release of pancreatic fluid and bile, inducing a satiety effect
  2. Pancreas is found in the first loop of the duodenum
    1. Linked via the pancreatic duct
    2. Secretions
      1. Both endocrine and exocrine, and secretes digestive enzymes into the duodenum
      2. Insulin, glucagon and pancreatic fluid into the lumen of the small intestine
      3. Trypsin and Chymotrypsin to continue protein digestion
      4. Pancreatic amylase converts starch to maltose
      5. Pancreatic lipase hydrolyses fat
3. Jejunum
4. Ileum
5. Site of enzymatic digestion and absorption, an the pancreatic and bile ducts empty into it
6. The wall of the intestine is comprised of goblet cells with villi and microvilli, which increase the surface area for absorption
7. Liver
  1. Site of conversion for glucose to glycogen, and also a site of storage
  2. Secretes bile, detoxifies toxins and poisons and breaks down uric acid to form urea
    1. Hepatocytes secrete the bile
      1. 94% is reabsorbed into the liver and reused
    2. Horses do not have a gall bladder, so bile is secreted straight into the small intestine
    3. Bile acids emulsify lipids so that they can be absorbed easily
8. Carbs, fats and proteins are efficiently digested and products absorbed. The extent of digestion depends on transit time, which depends on diet and exercise. It has a neutral pH, and secretions from the pancreas and liver are involved
Large Intestine
1. Compartmentalised, with sections separated by valves such as the ileocaecal valve and caeco-ventral colonic valve
  1. Huge amount of mixing
  2. Tight areas are susceptible to colic, especially the pelvic flexure
    1. Pelvic flexure is a valve which prevents large particle from continuing, ensuring that they are fermented before continuing
  3. Malfermentation
    1. Due to lack of forage
    2. Worms
      1. Red worms migrate through the intestinal wall, causing blood clots and killing the tissue
      2. Tapeworms cause localised infection
2. Entrance to the caecum occurs when the stomach distends and food enters
  1. Neural gastro-ileal reflex
3. Retention time
  1. Longer fibre spends longer in the large intestine
  2. As feeding frequency increases, time spent in the large intestine decreases
  3. Markers such as titanium dioxide can be used to measure retention time, as they are indigestible and so pass straight through
    1. Have to be associated with food
4. Microflora
  1. Secrete cellulases, and synthesise vitamins and essential amino acids
    1. Cellulases break down cellulose into glucose, as well as breaking down hemicellulose

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