ISSN 2398-2977      

Therapeutics: foal

pequis

Introduction

  • Specific data relating to therapeutics in the foal is sparse.
  • Compared to many other species, the foal has a realtively short neonatal period in relation to drug pharmacologoy: estimated at 1-2 weeks.
  • It is a common misconception that neonates should be given lower doses of drugs (on a mg/kg basis) than adults:
    • The total body water (TBW) of a foal represents 75-80% of bodyweight cf 60% for a 500 kg adult, therefore, extracellular fluid (ECF = 20% TBW) and intracellular fluid (ICF = 40% TBW) are proportionally higher. This means that for drugs which distribute to ECF or TBW, any given drug dose will result in plasma concentrations that are generally lower in the neonate than in the adult, and higher therapeutic doses may be required.
    • As the foal matures ICF percentage of TBW remains the same whereas ECF percentage decreases.
    • Dose rates of some drugs have to be increased to compensate for the higher neonatal ECF, and decreased within weeks as the foal matures and TBW percentage decreases, eg amikacin Amikacin 30 mg/kg IV SID for a foal <2 weeks old or 25 mg/kg IV SID for a foal >2 weeks; ceftiofur Ceftiofur >5 mg/kg IV TID, in the treatment of neonatal septicemia Foal: neonatal septicemia syndrome (cf 2 mg/kg SID or BID in the adult).
    • Higher doses can mean greater risk of side effects/toxicity.
    • When considering the risks of toxicity, the risk of under-dosing and ineffective therapy should also be considered.
  • Immature organs can → reduced renal clearance rates or slower hepatic metabolism (<2 weeks old) of some drugs → longer duration of action and maintenance of therapeutic levels. Longer dosing intervals are therefore required with many drugs and should be implemented to avoid toxicity.
  • Oral bioavailability is higher for some drugs in foals than adults, eg trimethoprim sulfonamides Therapeutics: sulfonamides, doxycycline Therapeutics: tetracyclines, ampicillin Ampicillin, amoxicillin Therapeutics: beta-lactam antibacterials.
  • The diet of the neonatal foal (mainly milk) can reduce the efficiency of absorption of some orally administered drugs, eg tetracyclines Therapeutics: tetracyclines are chelated by the high calcium concentrations in milk resulting in poor absorption.
  • Neonatal animals have an increased susceptibility to the side effects of some drugs, eg fluoroquinolones Therapeutics: nitrofurans / nitroimidazoles / quinolones, nitroimidazoles, quinolones have been shown to cause cartilage erosion; erythromycin Erythromycin can cause hyperthermia in foals; aminoglycosides can be toxic Toxicity: aminoglycoside.
  • In certain cases foals appear to have a reduced susceptibility to the side effects of some drugs, eg macrolides Therapeutics: macrolides lincosamides: diarrhea is rarely a problem, cf adults.
  • There is an increased permeability of the blood-brain barrier in the neonate.
  • The neonate has a lower body fat content than the adult, affecting lipophillic drugs pharmacokinetics: 
    • This results in a smaller volume of distribution and may therefore result in higher plasma concentrations of the drug.
    • It should be noted that the variation in body fat in neonate (3%) versus adult (5%) is less in the horse compared to other species.
  • Sepsis, renal dysfunction, hypovolemia and reduced hepatic function are often present in sick foals, thereby increasing the risk of susceptibility to side effects or drug toxicity.

Analgesia

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Anesthesia

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Fluid therapy

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Antimicrobials

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Specific neonatal diseases

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Further Reading

Publications

Refereed papers

Other sources of information

  • Bertone J & Horspool L (2004) Eds. Equine Clinical Pharmacology. Saunders. ISBN: 0702024848.
  • Prescoot J F, Baggot J D & Walker R D (2000) Eds. Antimicrobial Therapy in Veterinary Medicine. 3rd edn. Iowa State University Press.
  • Koterba A M, Willa H D & Kosch P C (1999) Eds. Equine Clinical Neonatology. Lea & Febiger, USA. ISBN: 0812111842.

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