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Pharmaceutical Care of Equine Patients: Student Summary

Equine Anatomy

  • Average horse weighs 500 kg or 2000 lbs
  • Skeleton – average of 205 bones and no collarbone
  • GI tract – very long and complex
    • Long and strong esophagus – vomiting impossible for horses
    • Monogastric with functioning cecum
    • Proper dental care is essential for digestion
    • Horses will overeat due to poorly functioning satiety center

Pharmacokinetic Considerations

  • Absorption – differs between different species
    • IV is preferred due to 100% absorption; IM and SC 80-90% absorbed; oral can be very erratic
    • pH of stomach: large range (1.5-7.4)
    • Stomach is never empty – can cause encapsulation, protein binding, etc. which can affect absorption
    • Transient time – more water in horses = increased transient time
  • Distribution – “flip-flop” pharmacokinetic phenomenon can occur
  • Metabolism –
    • IV has most consistent patterns; IM/SubQ has less consistency; oral is variable
    • CYP3A – oxidation process with many drugs (macrolides, diazepam, naproxen, etc.)
  • Elimination/excretion –
    • Dosage adjustments for each drug may be reported on the package insert
    • Hepatic function is very important!
    • Low protein = must adjust the dose (higher free drug concentrations can cause toxicity)

Common Disease States

 Colic (Abdominal Pain)
  • Etiology: gas, obstruction or impaction (fecal material gets hard and becomes difficult to pass), dehydration, parasites, ingestion of sand, tangled intestines, poor supply of blood to intestines, ulcers, stress, medication
  • Diagnosis: monitor vital signs, temp, HR, RR
  • Treatment: medical or surgical
    • Medical: analgesics (flunixin meglumine, detomidine, or xylazine), placement of nasogastric tube to relieve pressure, IV fluids, laxative
    • Anti-diarrheal drugs: activated charcoal (absorbs endotoxins), probiotics (very short acting)
    • Cathartics/laxatives for large colon impaction: magnesium hydroxide, magnesium sulfate (be wary of giving too much because it can cause toxicity and death), psyllium
    • Metoclopramide (must be diluted)
    • Prokinetic drugs
    • Anti-endotoxin therapy and continuous therapy- aftercare is critical

 

 Laminitis
  • Disease that affects the feet of hooved animals; causes tenderness that progresses into inability to walk, increased digital pulses and increased temperature in the hooves
  • Severe cases = founders; can lead to inability to stand and need for euthanasia
  • Treatment:
    • Early stage: NSAIDs
      • Phenylbutazone, flunixin meglumine, ketoprofen, aspirin
      • Equioxx injectable – COX-II selective in horses and leads to less gastric ulceration
      • 97-99% bound to albumin
      • Can accumulate – hepatotoxic

 

 Gastric Ulcer (EGUS)
  • Etiology: prolonged exposure of squamous mucosa to gastric acid, not enough sodium bicarbonate secretion, NSAIDs, improper feeding, stress, exercise, digesting food
  • Diagnosis: GI scope to examine the lower region of the stomach
  • Treatment:
    • PPIs – omeprazole paste is the only approved FDA product for horses
    • Famotidine IV and oral – be careful with dosing, can cause colic
    • Sucralfate oral tablets or paste – works by coating injured mucosa and/or lining of stomach and small intestine

 

 Infections
  • Sepsis Treatment: antimicrobials
    • Want drugs with low Vd, cover gram negatives (e.g. gentamicin, amikacin)
  • Secondary anaerobic infection: clostridial infections seem to be common
    • Penicillins – avoid contact with aminoglycosides in catheter
    • Beta-lactam antibiotics – must be above MIC at all times (using pulse dosing or continuous infusion)
    • Metronidazole
  • Pneumonia Treatment:
    • Macrolides – covers gram positives; avoid exposure to sun
    • Azithromycin
    • May add rifampin for difficult cases with abscesses to break down the cell wall
  • Wound infections – once daily bandage changes, cefotaxime or ticarcillin, bactroban