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

This study investigates the impact of manipulating intrauterine growth on the development of the adrenal cortex in newborn horses, finding that restricted growth conditions can impair post natal adaptive responses and heighten adrenal activity.

Study Overview

• The goal of this research was to assess the effects of a restricted or enhanced intrauterine environment on neonatal adaptation and adrenocortical function in horses.
• Embryos from large Thoroughbred mares were transferred to smaller Pony mares and vice versa. This created a situation where growth was restricted (being from a larger species but developing in a smaller one) or enhanced (being from a smaller species but developing in a larger one).
• Control groups where embryos remained within their own breed were also included for comparison.

Results

• Of the 30 foals born live at full term, four were stillborn. The Thoroughbred foals that grew in Pony mares were significantly lighter than Thoroughbred foals that stayed in Thoroughbred mares, but they were still heavier than the Pony foals.
• These Thoroughbred-in-Pony (TB-in-P) foals took longer to first stand up and begin to suck. Other side effects such as fetlock hyperextension (a condition affecting the joints in the legs) were observed as well as low plasma immunoglobulin G concentrations, which can affect the immune system.
• No abnormal behaviours were observed in the other horse groups. The blood parameters in all four sets of foals were normal.
• At birth, ACTH hormone levels were high in all foals and cortisol, a hormone associated with stress response, levels increased after birth, returning to baseline within 6 hours post partum, with the exception of the TB-in-P foals who had elevated levels until 48 hours post partum.

Conclusions

• Thoroughbred foals that developed in the smaller Pony mares experienced intrauterine growth retardation (IUGR) and demonstrated impaired post-natal adaptive responses and abnormal levels of cortisol, a hormone that plays a crucial role in stress response.
• Despite these negative impacts, these foals did appear to have mature adrenocortical function, the part of the brain that regulates stress response.
• The study concludes that foals born following IUGR may require clinical assistance in the early post-natal period, due to these observed challenges.