Transfer of passive immunity in foals: Can the plasma transfusion efficacy be predicted?

Transfer of passive immunity in foals by plasma transfusion shows variable efficacy, and this study aimed to assess safety, effectiveness, and identify factors predicting transfusion success to better estimate plasma volume needs.

Background and Objective

• Foals rely on passive immunity transferred through colostrum, rich in immunoglobulins (IgG), to protect them from infections early in life.
• Failure of transfer of passive immunity (FTPI) occurs when foals do not receive adequate IgG, necessitating treatment with hyperimmune plasma transfusions.
• While plasma transfusion is standard treatment, its efficacy varies widely between cases.
• The study’s objective was to evaluate the safety and effectiveness of plasma transfusions and examine whether any factors can predict the success of transfusions, enabling estimation of the required plasma volume before treatment.

Methods

• A retrospective review was conducted at an equine hospital on foals treated for FTPI using plasma transfusions.
• 110 foals were included in the study, undergoing a total of 150 plasma transfusions.
• Data collected included:
  • Age
  • Body weight (BW)
  • Sex
  • Breed
  • Year of transfusion
  • Colostrum intake history
  • Clinical health status of the foal
  • Timing of post-transfusion IgG measurement
• The primary measure of transfusion efficacy was the increase in plasma IgG concentrations (∆IgG) after transfusion.
• Statistical correlations between these factors and transfusion outcomes were analyzed.

Results

• The median increase in plasma IgG after transfusion was 223.5 mg/dL, showing moderate improvement in IgG levels overall.
• Only three factors significantly correlated with transfusion efficacy, although with modest strength (correlation coefficient r < 0.5):
  • Pre-transfusion IgG concentration: Lower baseline IgG correlated with greater increases post-transfusion.
  • Body weight (BW): A heavier BW was linked with a smaller increase in IgG, particularly in healthy foals.
  • Delay in post-transfusion IgG measurement: Foals tested within 12 hours post-transfusion showed higher increases in IgG, while later testing revealed less difference.
• No strong predictive relationships were found based on other factors such as age, sex, breed, year, colostrum intake, or clinical status.
• Specifically, body weight’s impact was notable only in clinically healthy foals, with each additional kilogram reducing ∆IgG by approximately 9.17 mg/dL; in clinically ill foals this effect was negligible.

Conclusions

• The study demonstrated that multiple interacting factors influence plasma transfusion efficacy in foals, making it difficult to reliably predict IgG increases from pre-transfusion data alone.
• This complexity prevents accurate estimation of the precise volume of hyperimmune plasma needed before transfusion to achieve target IgG levels.
• Therefore, direct measurement of plasma IgG levels shortly after transfusion remains critical to determine treatment success.
• Timely post-transfusion IgG assessment is essential for guiding further clinical decisions and ensuring foals receive adequate immune protection.
• The findings highlight the need for individualized post-treatment monitoring rather than reliance on predictive models based on foal or transfusion parameters.