Research on metabolic issues in mules is limited. Researchers collaborated to look at equine metabolic syndrome (EMS) and pituitary pars intermedia dysfunction (PPID) in mules. They said veterinarians often rely on horse reference values, which may not reflect mule metabolism.
“This study aimed to assess EMS risk factors in mules and establish species-specific metabolic reference intervals for summer months,” noted the researchers. Researcher Elaine Norton presented the findings from this study at the 2025 AAEP Convention.1
The Research
The researchers collected data in June and July from 228 healthy mules and 42 horses. The information on each animal included body condition score (BCS), cresty neck score (CNS), baseline insulin, triglycerides, non-esterified fatty acids (NEFA), and ACTH and insulin concentrations 60- and 90-minutes post oral administration of Karo syrup (INS-OST).
“The comparisons identified significant differences between horses and mules,” noted the researchers. The study worked to minimize the inclusion of mules with EMS or PPID through testing. Among mules, 29% had a body condition score ≥ 7. Thirty-two percent of mules had a cresty neck score ≥ 3.
Cresty neck score, sex, month, and farm were the strongest predictors of metabolic issues, explaining 17.9% to 45.6% of phenotypic variation.
The researchers also looked at 134 mules (age 2–15 years, BCS 3.5–7) to generate baselines.
Triglycerides were 1.6 times higher in mules, while ACTH was 1.23 times higher in horses. Reference intervals were established for insulin (2.6–18.0 mIU/mL), triglycerides (11.6– 98.6mg/dL), NEFA (≤0.44mmol/L), INS-OST (2.5– 27.9 mIU/mL), and ACTH for the month of June (6.9–20.8pg/mL).
“Significant species differences in triglycerides and ACTH confirm distinct metabolic profiles and lipid metabolism in mules compared to horses,” noted the researchers.
What This Means
For mule owners, this first means that if you or your veterinarian are worried about metabolic issues in an animal, you can’t rely on horse parameters. The research shows that mules have higher triglycerides than the horse “norms” used in veterinary medicine. On the other hand, ACTH measures were higher in horses than mules.
The good news is that researchers established new mule reference values for insulin, triglycerides, ACTH (for June), non-esterified fatty acids (NEFA), and insulin concentrations 60- and 90-minutes post oral administration of 0.15mL/kg light Karo syrup that now can be used when determining metabolic levels for mules.
Reference
- Establishing Metabolic Parameters in Mules. Elaine Norton, Audrey Johnston, Cynthia Xue, Sarah Jacob, and Amy K. McLean. AAEP Convention 2025 Proceedings.
Further Content
- Mule Heart Rates and Variabilities Studied. Kimberly S. Brown. MySeniorHorse.com
- Mules and Hinnies: A Scientific Point of View. Dr. Amy McLean. MySeniorHorse.com
- How Long Can a Horse, Donkey, or Mule Live? Kimberly S. Brown. MySeniorHorse.com
- Hematological and serum biochemical parameters in healthy working horses, donkeys, mules and hinnies in Portugal and Spain. 2014. A.K. McLean; W. Wang; F.J. Navas-Gonzalez; J.B. Rodrigues. 2014. Proceedings 7th International Colloquium on Working Equid, Royal Holloay, University of London, London, U.K. pg 201
- Donkey and Mule Behavior. 2019. Amy K. McLean, Francisco Javier Navas Gonzalez, and Igor Fredericko Canisso. In “Diseases of Donkeys and Mules in Vet Clinics of North America.” Elsevier, 35 (2019). Pp 575-588.
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