How often have you had a client ask to discuss breeding their favorite retired mare, only for you to discover that the mare is grossly overweight and in her mid-teens or older?
So began the presentation by Teresa Burns, DVM, PhD, DACVIM, of The Ohio State University, for a VetGirl continuing education webinar. Burns stressed that not only is an overweight body condition predictive of hyperinsulinemia and the potential for laminitis, but it also has other effects on reproductive efficiency.
One common hormonal condition develops in aged horses: pituitary pars intermedia dysfunction (PPID, also known as Cushing’s disease). Age and characteristics of a horse’s haircoat (hypertrichosis) are risk factors for prediction of PPID.
In one study, 30% of horse owners reported haircoat changes in horses over 20 years of age, and on postmortem,
up to 40% of those had PPID. A metastudy (2018) reviewed 29 publications and summarized that 21.2% of horses over 15 years of age have PPID, and of the general population, nearly 3% have PPID. Burns noted that, to date, the only documented risk factor for PPIID is advancing age.
Primary pituitary disease might not have a great effect on ultimate reproductive success, said Burns, but pregnant PPID mares with high levels of ACTH experience greater fetal loss or greater failure to get pregnant.
Many PPID mares are managed with pergolide, which is an ergot alkaloid that can prolong gestation, cause premature placental separation (red bag), and/or hypogalactia or agalactia through its dopaminergic agonist actions. She recommended temporarily discontinuing pergolide two to four weeks prior to the foaling date until 30 days post-foaling. Domperidone helps to support lactation but is ineffective if the mare remains on pergolide. Taking a PPID mare off pergolide becomes a calculated risk, she reported, and it might be safer for the mare to be an embryo donor rather than carry a pregnancy to term.
Another endocrinologic challenge is the mare with equine metabolic syndrome (EMS) that often experiences decreased fertility associated with insulin dysregulation. Burns described one hormone of particular interest in EMS horses: leptin, from white adipose tissue. It has a role in appetite suppression, enhances catabolic metabolism and is also a pro-inflammatory substance. The amount of circulating leptin concentration is proportional to the amount of fat tissue. She noted that measured levels of leptin in the blood are predictive of clinical laminitis. Leptin concentrations elevate with diet-induced obesity and correlate with the degree of a horse’s insulin dysfunction.
Burns advised that some increase in insulin resistance is normal and progressive during pregnancy as part of glucose translocation across the placenta for fetal uptake. Studies reviewing the effect of pregnancy on insulin sensitivity noted that the glycemic response to a high-starch diet is different in pregnant mares compared to those consuming a high-fat/high-fiber diet—this is despite comparable calorie content between the two diets. Hyperglycemia is the most predictive risk factor for laminitis.
Obesity during pregnancy doesn’t adversely affect post-partum fertility. There appears to be little effect on time to first and second post-foaling ovulations, conception rate or rate of early pregnancy loss. However, obesity during pregnancy might compromise colostral intake by the foal due to the amount of adipose tissue surrounding the mammary gland.
Other concerns of maternal obesity—with or without insulin dysregulation—have to do with effects on
the foal. In people, there’s a degree of metabolic programming of the fetus that is linked to metabolic disease in later life. There is evidence that this might be true of horses as well, said Burns. Pregnant mares fed grain have foals with lower body weights at birth and the mare produces less colostral IgG. A study of 20 mares on pasture looked at insulin sensitivity and glucose dynamics in foals and found that a mare’s high-starch diet results in higher baseline blood glucose levels in foals for up to 160 days.
“What a mare consumes while pregnant can have lasting effects on foals as they get older,” said Burns.
Mare body condition score also might significantly affect orthopedic outcomes in foals. A study noted that maternal obesity increases insulin resistance, elicits low-grade inflammation based on elevated SAA levels and predisposes foals to osteochondrosis (OCD) lesions until 18 months of age. At 1 year of age, foals born to mares with normal body condition scores developed OCD in only one individual whereas seven foals born to obese mares had OCD. By 18 months of age at recheck, many of the foals with cartilage lesions had reversible OCD of the femoral condyle.
When counseling clients who want to breed their older, fat mares, it is important to screen for PPID and consider EMS to help guide an appropriate nutrition program. The goal is to achieve body condition scores of 4-6 (on a scale of 1-9) for breeding and to control appropriate food intake throughout pregnancy to maintain a body condition score of 5-6, especially the last trimester. This helps achieve optimal reproductive success in the face of aging and body condition challenges.