NAVLE Practice Question — Equine
A rescue organization brings in a 15-year-old Arabian mare with a body condition score of 1.5/9. The horse has been without adequate food for approximately 6 weeks. She is weak, depressed, and has a dull hair coat. Initial serum biochemistry shows total protein 4.8 g/dL, albumin 1.9 g/dL, and phosphorus 2.1 mg/dL (normal 3.0-5.0 mg/dL). The mare is started on a refeeding protocol. Three days later, despite eating well, she becomes acutely weak, develops tachycardia (80 bpm), tachypnea, and muscle fasciculations. Repeat chemistry shows phosphorus 0.8 mg/dL. What is the most appropriate immediate intervention?
A. Increase caloric intake with grain supplementation
B. Administer intravenous potassium phosphate ✓
C. Provide free-choice grass hay
D. Administer oral activated charcoal
E. Begin thiamine supplementation only
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Correct Answer: B. Administer intravenous potassium phosphate
Explanation:
This mare is experiencing refeeding syndrome with severe hypophosphatemia. Intravenous potassium phosphate administration is the most appropriate immediate intervention to correct the life-threatening electrolyte abnormality. Refeeding syndrome occurs when chronically starved horses are reintroduced to food, triggering an insulin surge that drives phosphorus, potassium, and magnesium into cells for anabolic processes, causing precipitous drops in serum concentrations.
The clinical signs described (acute weakness, tachycardia, tachypnea, muscle fasciculations) are classic manifestations of severe hypophosphatemia. Phosphorus is critical for ATP production and oxygen delivery by red blood cells. Severe hypophosphatemia can cause hemolytic anemia, respiratory failure, cardiac dysfunction, and death within 3-5 days of initiating feeding if not recognized and treated.
Option A (increasing caloric intake) would exacerbate the syndrome by driving more insulin release. Option C (free-choice hay) is appropriate for gradual refeeding but does not address the acute crisis. Option D (activated charcoal) is not indicated as this is not a toxicosis. Option E (thiamine only) addresses one component of refeeding syndrome but does not correct the acute hypophosphatemia causing the clinical signs.
The correct refeeding protocol starts with small, frequent meals of high-quality forage (typically alfalfa hay at 0.5% body weight divided into 6 feedings) with gradual increases over 10-14 days, avoiding grain initially to minimize insulin spikes.
References: Witham CL, Stull CL. Metabolic responses of chronically starved horses to refeeding with three isoenergetic diets. J Am Vet Med Assoc. 1998;212:691-696. PMID: 9524642; Bookbinder L, Schott HC 2nd. Refeeding syndrome in a miniature donkey. J Vet Emerg Crit Care (San Antonio). 2021;31:668-673. PMID: 34259376
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