The hypothalamic-pituitary-adrenal axis response to vasopressin stimulation test in healthy and critically ill foals

Introduction: Sepsis remains the leading cause of death in foals. The hypothalamic-pituitary-adrenal gland axis (HPAA) dysfunction is a common complication of sepsis, resulting in decreased survival. HPAA dysfunction can be diagnosed with arginine vasopressin (AVP) stimulation test in other species. The goal of this study was to evaluate HPAA response to AVP stimulation in healthy and hospitalized foals. We hypothesized that AVP would stimulate a rise in ACTH and cortisol in healthy foals. We also proposed that cortisol and ACTH response would be decreased in critically ill foals compared to healthy foals, and that the diminished response would be associated with disease severity and outcome.

Materials and Methods: HPAA function was assessed in 12 healthy and 18 hospitalized foals <7-days old on admission to a referral hospital in USA, utilizing 2 doses of AVP (2.5, 5 IU). Cortisol and ACTH were measured at 0 (baseline), 15, 30, 60, and 90 minutes after AVP administration with immunoassays. A fold increase 15 and 30 minutes from baseline was calculated for cortisol and ACTH concentrations.

Results: All doses of AVP resulted in a significant increase in cortisol concentration and a dose-dependent increase in ACTH concentration over time in both groups. ACTH and cortisol concentration increased 15 and 30 minutes after both doses of AVP compared to baseline in healthy and hospitalized foals (P<0.01). Cortisol and ACTH response to AVP administration (2.5 and 5 IU) at 30 and 15 minutes was lower in critically ill foals compared to healthy foals suggesting HPAA dysfunction (P<0.05).

Discussion: Administration of AVP is safe and results in a significant rise in ACTH and cortisol in both healthy and hospitalized foals. A stimulation test with 2.5 and 5 IU of AVP can be considered for HPAA assessment in critically ill foals.

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