Effects of manipulating intrauterine growth on post natal adrenocortical development and other parameters of maturity in neonatal foals.
Authors: Ousey J C, Rossdale P D, Fowden A L, Palmer L, Turnbull C, Allen W R
Journal: Equine veterinary journal
Summary
# Editorial Summary: Intrauterine Growth and Neonatal Adrenocortical Development in Foals By manipulating the intrauterine environment through embryo transfer between Thoroughbred and pony mares of markedly different sizes, Ousey and colleagues investigated whether restricted or enhanced fetal growth affects neonatal maturity and stress hormone physiology in foals. Thoroughbred embryos carried by smaller pony mares (creating growth restriction) produced significantly lighter foals with delayed standing and suckling responses, occasional fetlock hyperextension, and reduced passive transfer in some cases, whilst Thoroughbred foals in Thoroughbred mares and pony foals in pony mares developed normally. The restricted-growth foals demonstrated notably abnormal cortisol kinetics, maintaining elevated plasma cortisol for 48 hours post-partum rather than returning to baseline within 6 hours like control groups, though all groups showed appropriate adrenocortical responsiveness to exogenous ACTH challenge by days 1 and 5. These findings suggest that intrauterine growth retardation substantially impairs the foal's immediate post-natal adaptation and stress hormone clearance despite apparently normal endocrine maturity, indicating that growth-restricted neonates warrant close monitoring and potential clinical support during the critical first 24–48 hours of life.
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Practical Takeaways
- •Foals born with IUGR (lighter birth weight, delayed motor responses) may require early clinical intervention including assistance with standing, sucking, and colostrum intake; monitor for low immunoglobulin G and provide supplemental care if necessary
- •IUGR foals can exhibit fetlock hyperextension and behavioral immaturity despite appropriate adrenocortical maturity, so clinical support should focus on musculoskeletal and behavioral support rather than assuming hormonal immaturity
- •Mares with significant size mismatch to recipient uterus (large embryos in small mares) carry increased risk of IUGR and stillbirth; embryo transfer programs should consider mare size compatibility
Key Findings
- •TB-in-P foals (restricted intrauterine environment) were significantly lighter and showed impaired post-natal adaptive responses including delayed standing and sucking
- •TB-in-P foals had elevated plasma cortisol concentrations persisting until 48 hours post-partum, compared to baseline by 6 hours in control groups
- •IUGR foals demonstrated basal hypercortisolaemia but normal haematological parameters and appropriate adrenocortical response to exogenous ACTH
- •3 of 4 stillbirths occurred in the restricted intrauterine environment group (TB-in-P), indicating increased perinatal mortality risk