Serum cortisol concentrations in newborn foals are increased by transfusion with anti-rhodococcal hyperimmune plasma and inversely associated with immunoglobulin activity.

Overview

• This study investigated how transfusions of Rhodococcus equi hyperimmune plasma (REHIP) affect cortisol levels in newborn foals and whether these cortisol levels relate to immunoglobulin activity and the risk of developing rhodococcal pneumonia.

Background

• Rhodococcus equi is a bacterial pathogen causing pneumonia in foals, which can be severe or fatal.
• To prevent rhodococcal pneumonia, neonatal foals are often transfused with REHIP, plasma rich in antibodies against R. equi.
• Cortisol is a hormone associated with stress and immune modulation, so understanding how REHIP transfusion affects cortisol could reveal insights into foals’ immune and stress responses.

Objectives

• To determine how transfusion of REHIP affects serum cortisol concentrations in newborn foals.
• To examine the relationship between cortisol levels and immunoglobulin activity (specifically antibodies targeting the VapA protein of R. equi).
• To assess if variations in cortisol concentrations influence the likelihood that foals develop rhodococcal pneumonia.

Methods

• Subjects: 356 newborn foals from 13 different farms in New York State.
• Intervention: Foals were transfused with either 1 liter or 2 liters of REHIP shortly after birth.
• Sample collection: Serum samples were taken immediately before and after transfusion to measure cortisol, total globulins, and specific immunoglobulin activities against VapA.
• Laboratory analyses: Measured serum cortisol concentration, total globulin levels, and antibody activities (immunoglobulin G, IgG) using appropriate assays.
• Statistical analysis: Linear mixed-effects regression to analyze changes in cortisol, chi-square testing for categorical variables, and Spearman rank correlation to assess relationships between variables.

Key Findings

• Transfusion with REHIP significantly increased serum cortisol levels across the foal population from an average of 21.9 ng/mL before transfusion to 44.1 ng/mL after transfusion (P < 0.05).
• Post-transfusion cortisol was higher in foals given 2 liters of plasma compared to those given 1 liter, indicating a dose-dependent effect.
• Before transfusion, cortisol levels were negatively correlated with total globulin concentrations (r = -0.19), meaning foals with higher immunoglobulin levels had lower cortisol.
• Similarly, pre-transfusion cortisol had a negative association with the activity of anti-rhodococcal IgG antibodies (r = -0.22 and r = -0.16 for different IgG assays), suggesting that better immune preparedness correlates with lower cortisol.
• However, neither pre- nor post-transfusion cortisol concentrations predicted whether a foal would develop rhodococcal pneumonia.

Conclusions

• REHIP transfusion acutely increases serum cortisol levels in newborn foals, with greater volumes leading to higher cortisol rises.
• There is an inverse relationship between cortisol and immunoglobulin levels/activity, indicating that foals with stronger immune antibody profiles tend to have lower cortisol concentrations.
• Despite cortisol changes and their association with immune factors, cortisol concentrations were not linked to an increased risk of clinical rhodococcal pneumonia in this study group.
• This suggests that while REHIP transfusion influences stress hormone levels, cortisol alone is not a reliable biomarker for disease risk in this context.

Implications

• Understanding the hormonal and immune responses to REHIP transfusion can help veterinarians optimize strategies to protect foals from infection.
• The rise in cortisol might reflect a physiological stress or immune-modulating response to plasma transfusion rather than direct disease risk.
• Further research is warranted to explore other mechanisms by which REHIP confers protection and how hormonal responses interplay with immunoglobulin-mediated immunity.