Opsonic capacity of foal serum for the two neonatal pathogens Escherichia coli and Actinobacillus equuli.
Authors: Gröndahl G, Sternberg S, Jensen-Waern M, Johannisson A
Journal: Equine veterinary journal
Summary
# Editorial Summary: Opsonic Capacity in Neonatal Foals Neonatal septicaemia in foals caused by *Escherichia coli* and *Actinobacillus equuli* remains a significant clinical challenge, yet the immunological basis of foal susceptibility to these pathogens has been poorly characterised. Gröndahl and colleagues employed flow cytometric analysis of neutrophil phagocytosis to measure serum opsonic capacity (the ability of serum antibodies and complement to enhance bacterial uptake) in healthy foals from birth through six weeks of age, correlating findings with IgGa and IgGb concentrations. Colostrum ingestion produced a marked improvement in opsonic capacity within the first hours of life, but contrary to the authors' hypothesis, foal serum subsequently demonstrated equivalent or superior opsonisation of both pathogens compared to adult horses—suggesting that antibody quantity alone does not explain neonatal infection risk. Importantly, IgGb levels below 1.9 mg/ml correlated with reduced opsonisation of *E. coli* and yeast (but not *A. equuli*), whilst complement activation proved essential for effective phagocytosis of all tested organisms. These findings indicate that failure of passive transfer and complement insufficiency may represent more significant risk factors for neonatal septicaemia than previously recognised, with implications for colostral management protocols and targeted immunisation strategies in pregnant mares.
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Practical Takeaways
- •Foals that receive adequate colostrum have improved immune capacity against common neonatal pathogens, reinforcing the critical importance of early colostrum feeding in disease prevention
- •Serum opsonic capacity alone does not fully explain neonatal foal susceptibility to E. coli and A. equuli septicaemia; other immunological or management factors likely contribute to disease risk
- •Monitoring IgGb levels may help identify at-risk foals with compromised opsonisation capacity, though this parameter alone is not predictive for all pathogens
Key Findings
- •Foal serum showed similar or higher opsonisation of E. coli and A. equuli compared to mature horses, contrary to the initial hypothesis
- •Colostrum ingestion significantly increased serum opsonic capacity in foals
- •IgGb levels below 1.9 mg/ml correlated with lower opsonisation of E. coli and yeast but not A. equuli
- •Complement activation was essential for opsonisation of all tested microorganisms