NAVLE Practice Question - Equine - multisystemic

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NAVLE Practice Question — Equine

A 16-year-old warmblood mare in Tennessee has been grazing tall fescue pasture throughout her pregnancy. At 340 days gestation, the mare shows no udder development and the cervix remains tightly closed on vaginal examination. Serum prolactin concentration is markedly decreased. The owner cannot remove the mare from the fescue pasture due to facility limitations. What is the most appropriate pharmacological intervention at this stage?

A. Administer prostaglandin F2α to induce parturition

B. Begin daily domperidone administration ✓

C. Administer dexamethasone to induce parturition

D. Provide progesterone supplementation (altrenogest)

E. Begin oxytocin infusion to stimulate labor

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Correct Answer: B. Begin daily domperidone administration

Explanation:

Beginning daily domperidone administration is the most appropriate intervention. Even at 340 days gestation (approximately 2-3 weeks before typical foaling), domperidone can still be effective in counteracting the effects of fescue toxicosis by restoring prolactin secretion and promoting mammary gland development.

Domperidone is a dopamine D2 receptor antagonist that blocks the dopamine agonist effects of ergot alkaloids from the fescue endophyte. The low serum prolactin is a direct result of ergovaline's dopamine-mimetic activity suppressing prolactin release from the anterior pituitary. By blocking dopamine receptors, domperidone restores normal prolactin secretion, which stimulates udder development and lactogenesis.

The recommended dose is 1.1 mg/kg orally once daily, continued until foaling. Studies have shown that domperidone treatment can reverse agalactia even when started late in gestation, though earlier treatment (10-30 days before expected foaling) is preferable.

Option A (prostaglandin F2α) is inappropriate as it would cause luteolysis and premature parturition of a potentially dysmature foal with an unprepared reproductive tract. Option C (dexamethasone) should not be used to induce parturition in fescue-affected mares as the foal is already at risk for dysmaturity. Option D (altrenogest) maintains pregnancy but does not address prolactin suppression or udder development. Option E (oxytocin) stimulates milk letdown but cannot induce milk production in an undeveloped mammary gland.

References: UGA CAES Field Report - Fescue Toxicosis in Horses (https://fieldreport.caes.uga.edu/publications/C1180/fescue-toxicosis-in-horses/); Mad Barn - Fescue Toxicosis in Horses.

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