It would be interesting to see what a survey of equine horse owners and caregivers might think is the biggest threat to equine health. Colic might be high on the list, so would respiratory disease and arthritis. However, perhaps overlooked—at least in horse owner circles—is a very important and sometimes-even-thought-to-be desirable condition: obesity.
Over the years, numerous studies on obesity in horses have been conducted in the U.S. and elsewhere. The problem, like the horses, appears to be growing. According to a recent report by the British Equine Veterinary Association (BEVA), obesity is the biggest threat to equine health. In the U.K., hundreds of horses are put down every year as a result of obesity-related health problems.
The roots of the problem
The causes of obesity are well understood. Feral horses thrive under conditions where they move continuously—25 to 50 mi. per day—while eating frequent, small meals of relatively poor-quality forage. (Wild horses are observed to eat virtually every hour, around the clock.) Domestication has changed almost everything about how horses live. For example:
- Most domesticated horses are confined, at least to some extent, limiting the distances they can travel. When they are exercised, often it is for short intervals that are measured in minutes. Horse owners may not have the time to give horses adequate exercise, or may not understand horses do very well with many hours of low-intensity exercise.
- Many horses are isolated from seasonal changes. Feral horses typically get thinner in the winter and gain weight in the spring and summer.
- In lieu of coarse, sparse grass, most domesticated horses are fed either calorically dense feeds, such as high-quality hay, grains, and fats. Horses have no inherent need for grain, and overfeeding of concentrates is associated with obesity, as well as other problems, such as gastric ulcers and colic. Nevertheless, grain concentrate feed products are heavily and persuasively advertised, and owners may be persuaded by paid testimonials about the latest and greatest product.
- Alternatively, horses may be left to stand in pastures comprised of high-quality forage.
- Young horses may be pushed to develop quickly for certain competitions (e.g. halter horses). Many of these young horses are obese.
While legally considered livestock, horses may be thought of by their owners as animals for pleasure and sport. Further, they may have little experience in knowing what a healthy horse of normal weight looks like. Their “knowledge” about equine nutrition often comes from tack and feed stores, rather than trained nutritionist, researchers, or veterinarians. As a result, horse owners may be presented with a variety of misinformation from untrained, but perhaps well-meaning individuals, whose primary goal is to sell feed.
Horses that carry an unhealthy amount of weight may even be considered desirable in some circles, and even rewarded in a number of show horse disciplines. As such, horse owners may be reluctant to get their horses to lose weight, even when it is in the animal’s best health interest.
According to the results of at least one study, even when their horses are obese, many owners do not believe it. When confronted with the fact their horses are overweight, they may become defensive. Overweight horses can present challenges from a health and client communication perspective.
The following are equine health problems associated with obesity:
Although the underlying mechanism(s) of action have not yet been fully elucidated, it is well-known obesity is associated with the development of laminitis. In fact, in the U.K., it estimated that approximately 600 horses are euthanized yearly due to the disease. It is increasingly clear insulin plays a pivotal role in the development of laminitis, and overweight horses may also be hyperinsulinemic. Regional adiposity, which may manifest as “cresty” necks or fat deposits in the abdomen or hindquarters, is also associated with laminitis.
Horses have limited mechanisms to dissipate body heat—sweating is their primary mechanism of heat loss. Animals with a large body size have a smaller surface area relative to body mass than animals with smaller bodies. Thus, in horses, compared to smaller animals, there is less relative surface area from which to radiate body heat. Add a layer of insulating fat, and heavy horses have a more difficult time cooling their bodies than do horses of normal weight. In especially hot weather conditions, this inability to radiate body heat can become dangerous.
Insulin resistance occurs when insulin does not effectively regulate blood glucose. While something of a chicken and egg question, an association between obesity and insulin resistance has been demonstrated in several studies. (It is not known if obesity causes insulin resistance or if insulin resistance develops and then contributes to obesity.) Resting insulin concentrations correlate well with body score; horses with higher body condition scores (greater than “7” on the Henneke body condition scale) are routinely less sensitive to insulin.
Once thought to be simply a storehouse for excess calories, adipose tissue is now known to be an active metabolic organ. In addition to its metabolic functions, adipose tissue also produces inflammatory cytokines; cytokine concentrations have been shown to be associated with body condition score. In humans, obesity-related inflammation is associated with heart disease, diabetes, and metabolic alterations.
Lipomas are fatty tumors that typically occur in the mesentery of obese and older horses. The pedunculated stalk of the tumors may wrap around the intestines and result in strangulation obstructions requiring emergency colic surgery.
The effects of obesity on reproductive function are unclear. While older work indicated mares with higher body condition scores had higher reproductive efficiency, obesity has also been shown to cause disturbances to the estrous cycle that were resolved by caloric restriction.
Veterinarians should assist horse owners in learning how to recognize good body condition and to understand the adverse health consequences of obesity. In addition, they should educate owners about healthy weight in horses, monitor the weight of their patients (e.g. with weight tapes and/or body condition scoring), and assist clients with good dietary recommendations to help prevent numerous obesity-related health problems in their horses.
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.