Effects of manipulating intrauterine growth on post natal adrenocortical development and other parameters of maturity in neonatal foals.

Reasons For Performing Study: Intrauterine growth retardation (IUGR) impairs post natal adaptive responses and is associated with increased adrenocortical activity in many species.

Objectives: To determine whether a restricted or enhanced intrauterine environment affects neonatal adaptation and adrenocortical function in horses.

Methods: Embryos from large (577 kg) Thoroughbred (TB) mares were transferred to smaller (343 kg) pony (P) mares and vice versa, to create a restricted (TB-in-P, n = 11) or enhanced (P-in-TB, n = 8) intrauterine environment. Control groups (TB-in-TB, n = 8; P-in-P, n = 7) were also included.

Results: Thirty foals were born live at full term (range 314-348 days) and 4 (3 TB-in-P, 1 P-in-TB) were stillborn between 275 and 335 days. TB-in-P foals were significantly (P<0.05) lighter than TB-in-TB, but heavier than P-in-P foals. TB-in-P foals took longer to first stand and suck and some had fetlock hyperextension and low (<4 g/l) plasma immunoglobulin G concentrations. Other foal groups showed normal behavioural responses. Haematological parameters were normal in all 4 groups of foals. Plasma ACTH levels were high at birth and plasma cortisol concentrations increased after delivery and returned to baseline within 6 h post partum in all but the TB-in-P foals, which had elevated levels until 48 h post partum. Plasma cortisol concentrations increased in all groups following exogenous ACTH administered on Days 1 and 5 postpartum.

Conclusions: The TB-in-P foals showed IUGR and impaired post natal adaptive responses with basal hypercortisolaemia.

Potential Relevance: Foals born following IUGR may require clinical assistance in the early post natal period, but appear mature with respect to adrenocortical function.