Knowledge about genetic heritability of particular equine traits is an important tool for equine veterinarians. Where such knowledge exists, it allows veterinarians to more accurately advise clients as to the likelihood their horse will develop a certain trait when breeding two specific horses. Heritability is a statistical measurement used to help determine whether genetics contribute to a particular trait.
For some traits, such as horse height at maturity, heritability is quite strong. Even if an owner provides the best feed, in ample amounts, he or she will not be able to grow the horse taller than its genetically determined limitations (heritability of height is more than 80 percent). On the other hand, genetic factors do not appear to play a large role in temperament. The heritability of temperament is less than 20 percent. Other factors, such as environment, handling, and training, are much more important than genetics when it comes to shaping a mature horse’s disposition.
Equine metabolic syndrome
Equine metabolic syndrome (EMS) is the current term used to describe a group of metabolic abnormalities that include increased insulin, resistance to the effects of insulin (IR), and abnormal fat deposition. EMS appears to be a result of the horse’s body being unable to properly metabolize dietary carbohydrates. Previously, EMS horses may have been diagnosed as being hypothyroid with “peripheral” Cushing’s disease or as “prelaminitic.” Horses affected with EMS are typically overweight, with a body condition score (BCS) of “6” or higher on the Henneke scale, as well as regional adiposity in areas such as the neck (“cresty” neck) and tailhead. More concerning than the typical appearance, affected animals may have one or more of a variety of clinical conditions, including acute or chronic laminitis, infertility, altered ovarian activity, and an increase in appetite.
Laboratory findings include hyperinsulinemia with normal glucose concentration (insulin resistance), hypertriglyceridemia, increased serum concentrations of leptin (one of the hormones directly connected to body fat and obesity, released from fat cells in adipose tissue, which sends signals to the hypothalamus in the brain), and arterial hypertension.
There is no recognized sex predilection for EMS, although certain breeds appear to be overrepresented, including ponies, Morgans, Arabians, Pervian Pasos, Paso Finos, Andalusians, and Tennessee walking horses. EMS can be seen as early as five years of age in some individuals.
Dietary management and exercise are the usual treatments for equine metabolic syndrome. In fact, dietary adjustments—which are primarily related to carbohydrate restriction—may be all that is needed to treat affected horses. In addition, caloric restriction may be needed to help reduce the horse’s weight. Pasture access is a key trigger for EMS and treatment may involve restriction (e.g. through use of a grazing muzzle) or even elimination of pasture. Reducing the nonstructural carbohydrate (NSC) content of forage by soaking is frequently advocated; however, it has been shown this is unreliable. Many low-NSC feeds are commercially available. While many dietary supplements have been recommended for EMS, including chromium, magnesium, and cinnamon, none have been shown to improve insulin sensitivity when tested. After dietary restrictions are implemented, additional calories can be added in the form of fats (e.g. canola oil) if required.
Medical recommendations often include the use of thyroid hormone in the form of levothyroxine sodium. While EMS horses are not hypothyroid, the addition of thyroid hormone to the diet speeds up weight loss and improves insulin sensitivity. Metformin, a medication commonly given to human diabetic patients, lowers hepatic glucose production, decreases intestinal absorption of glucose, and improves insulin sensitivity by increasing peripheral glucose uptake and utilization. However, it is absorbed poorly in horses, and the efficacy and safety in horses has not been established. If used, it should be given at least 30 minutes before a meal.
EMS and genetics
In humans, metabolic syndrome has been shown to have a genetic component. Thus, there is good reason to consider genetics may be an important factor in the development of the condition in horses. In 2018, researchers at the University of Minnesota published a study that provided heritability estimates for EMS in a study of 286 Morgan horses and 264 Welsh ponies.*
In performing the study, the authors estimated the heritability of a number of biochemical measurements. These measurements included insulin and glucose, both pre- and postoral sugar test, nonesterified fatty acids (NEFAs), triglycerides, adipokines (cell signaling proteins secreted by adipose tissue), and adrenocorticotropic hormone (ACTH), which regulates levels of cortisol. Overall, eight biochemical traits were estimated. Confounding variables, such as age, sex, and season, also were considered.
The study concluded eight measured biochemical traits associated with EMS were “moderately to highly heritable” in both Morgan and Welsh ponies. The results were not identical. For some traits, such as for triglycerides and leptin, the correlation was not very high. However, in others, such as with insulin or with insulin after an oral sugar test, the correlation was quite small. Differences were attributed to genetic risk factors between the breeds.
When a trait is associated with a specific gene, such as hyperkalemic periodic paralysis (HYPP), breeding recommendations are generally rather straightforward (i.e. avoid the gene). However, when there are many genetic risk factors—as seems to be the case with EMS—breeding decisions are much more complicated. Attempting to breed out EMS would likely be impossible, and certainly not recommended. Nevertheless, the information from this study suggests it may be possible to develop genetic tests for EMS, which might allow identification of individuals before the development of clinical signs.
While this study confirmed there is a genetic component to EMS, its authors also indicated more work needs to be done to determine what other factors may be causing the condition.
David W. Ramey, DVM, is a Southern California equine practitioner who limits his practice to the care of performance and pleasure horses. Visit his website at doctorramey.com. Columnists’ opinions do not necessarily reflect those of Veterinary Practice News.
* Norton, EM, Shultz, ME, Rendahl, AK, et al. Heritability of metabolic traits associated with equine metabolic syndrome in Welsh ponies and Morgan horses. Equine Vet J. 2019; 51(4): 475-480.