Doctor of Philosophy (PhD)


Animal Science (Animal, Dairy, and Poultry Sciences)

Document Type



Plasma leptin concentrations in obese adult horses have been shown to vary widely, and horses tend to fit into two groups: low leptin (<10 ng/mL) and hyperleptinemic (10 to 50 ng/mL). Observations over time revealed that the hyperleptinemic condition was consistent, possibly indicating a relatively permanent underlying cause. Based on these observations, three experimental approaches were used to further study equine obesity-related hyperleptinemia. The first experiment determined the prevalence of hyperleptinemia among postpartum, lactating mares, evaluated its consequence on their re-breeding success, and investigated correlations between leptin levels in lactating and non-lactating mares. Postpartum mares (n = 198) and non-foaling mares (n = 31) were categorized based on their leptin status: normoleptinemic or hyperleptinemic. Leptin in the lactating mares averaged 4.8 ng/mL, and 11 of the 198 (13%) displayed hyperleptinemia. Leptin in the non-lactating mares averaged 7.5 ng/mL, with 9 mares (29%) displaying hyperleptinemia. Of the 198 lactating mares bred, 81% became pregnant; there was no effect of leptin status on re-breeding success. To study one possible cause for hyperleptinemia in well-fed horses, a second experiment explored polymorphism(s) within exon 2 of the equine leptin gene. The DNA from five hyperleptinemic and five normal mares of high body condition was used to analyze exon 2 of the leptin gene for polymorphisms. Based on the 10 mares tested, there was no polymorphism in exon 2 of the equine leptin gene; therefore, polymorphism is not a likely explanation for the high vs. low leptin difference. The third experiment explored the possible effects of hyperleptinemia on the endocrine and immune systems. Endotoxin was given to mares and geldings to investigate the role and/or regulation of leptin in the pro-inflammatory cytokine response. Of the endpoints measured, only platelet count differed between normal and hyperleptinemic horses. Endotoxin infusion caused the expected pro-inflammatory cytokine and endocrine responses, but leptin status was not a significant factor for any endpoint. It is concluded that hyperleptinemia in mares is not associated with polymorphism in exon 2 of the leptin gene, does not affect re-breeding rates of foaling mares, and does not alter the endotoxin-induced responses of the endocrine and immune systems.



Document Availability at the Time of Submission

Release the entire work immediately for access worldwide.

Committee Chair

Donald L. Thompson, Jr.