Daily treatment of growing foals with equine somatotropin: pathologic and endocrinologic assessments at necropsy and residual effects in live animals.

This research experiment evaluated the effects of a year-long daily treatment of growing horses with recombinant equine somatotropin (eST). It investigated the impacts on internal organ traits and any additional effects on live horses for 60 days after the treatment ended.

Study Design and Procedure

• Fourteen horses were involved in the study. Seven were given daily eST treatment at a ratio of 20 micrograms per kilogram of body weight, while the remaining seven received a control substance.
• Four horses from each group were euthanized at the end of the treatment period for necropsy and organ assessment.

Effects of eST Treatment

• There were minimal impacts of eST treatment on hematologic (relating to the blood) tests or histopathologic evaluations (microscopic examination of tissue in order to study the manifestations of disease) of internal organs.
• However, treatment with eST increased the weights of several internal organs, including adrenal gland, kidneys, liver, pancreas, spleen, and heart. The somatotropin (ST) content in the adenohypophysis (a part of the pituitary gland) decreased.
• The loin-eye area at the 10th rib (a measure of body condition or musculature) was larger in horses that had received eST treatment compared to the control group.

Residual Effects in Live Animals

• There were no significant differences between the treated and control horses in terms of weights of the left adrenal, brain, parathyroid glands, or thyroid gland, or thickness of the 10th-rib fat.
• In the remaining live horses, plasma IGF-I concentrations (a hormone that plays an important role in childhood growth and continues to have anabolic effects in adults) were elevated in the treated animals six days post-treatment. This then dropped quickly.
• Insulin levels in treated animals appeared to rise after treatment, but only on day 0. A treatment-related increase in plasma urea nitrogen levels was observed, which went up in the treated horses.
• There was a weight decrease in the treated horses observed by 21 days after treatment.
• There were no differences in insulin or glucose responses to glucose tolerance tests given on days 0 to 60 post-treatment. The overall ST response to secretagogue (a substance that promotes secretion) was reduced in the eST-treated horses compared to the controls.

Conclusion

• Long-term treatment of growing horses with eST reduced the endogenous ST response to secretagogue, increased plasma IGF-I concentrations and many internal organ weights but did not greatly affect hematologic or histopathologic characteristics at necropsy.
• Effects on IGF-I concentrations disappeared within six days, and body weight in treated horses decreased within three weeks after cessation of treatment.