DEFINITION OF OBESITY

Obesity is defined as an expanded mass of adipose tissue in the body. In human medicine, body mass index (BMI; weight/height², kg/m²) is used to define people as normal weight (BMI 20 to 25), overweight (BMI 25 to 29.9), obese (BMI 30 to 34.9), or severely obese (BMI greater than 35). Changes in human dietary habits and an increasingly sedentary lifestyle and interaction of those factors with genetic susceptibility have been linked with the rise in obesity over the past few decades. More recently, a normal-weight obese syndrome has been described in humans. This syndrome is characterized by a normal body weight and BMI but a high fat mass (greater than 30%) and evidence of a proinflammatory state that, like other forms of obesity, is a risk factor for diabetes and cardiovascular disease.

ASSESSMENT OF ADIPOSITY IN THE HORSE

In horses, the most widely applied method for assessment of adiposity is evaluation of body condition according to a 9-point scale developed by Henneke. This system of body-condition scoring (BCS) assesses subcutaneous fat deposition in six areas: crest of the neck, withers, behind the shoulder, over the ribs, along the back, and around the tailhead. There can be considerable interhorse variation in patterns of fat deposition; for example, some horses deposit little fat over the ribs even when other areas of the body are well covered. In addition, fat deposits are sometimes asymmetric in distribution. It is therefore important to evaluate all six areas of the body on both sides. A score between 1 and 9 is assigned, where 1 indicates severe emaciation and 9 indicates extreme fatness (Table 15-1). Our preference is to assign a score to each of the six areas, with the overall BCS taken as an average of these individual scores. It has been estimated that each BCS unit (on the 1 to 9 scale) in a typical Thoroughbred, quarter horse, or Arabian weighing 450 to 500 kg represents approximately 20 to 25 kg of body weight, although data in this area are scant.

Table 15-1 Description of Body Condition Scores in Horses

The Henneke BCS system, originally developed for use in Quarter Horse broodmares, is most appropriate for use in light breeds such as Thoroughbreds, Arabians, and Standardbreds. The suitability of this system for ponies and larger-breed horses (e.g., draft breeds) has not been reported. Recently, a nine-point BCS system for Warmblood horses was developed to account for differences in conformation and patterns of fat deposition in this breed compared with quarter horses. A BMI has been applied to horses and found to be moderately correlated (r_s = 0.60) with BCS.

It should be noted that the BCS system is insensitive to differences in regional adiposity that may signify a higher risk of disease. In humans, visceral (abdominal) adiposity is more closely linked to risk for diabetes and cardiovascular disease than generalized obesity, and measurement of waist circumference is a better indicator of abdominal fat accumulation than BMI. In horses and ponies, there may be a similar association between regional adiposity and disease risk. In studies of horses and ponies with predisposition to pasture-associated laminitis, some affected animals are not obese on the basis of BCS (i.e., BCS less than 7) and have no external evidence of regional adiposity, but others have enlarged fat deposits on the neck (so-called cresty neck), thoracic, or tailhead regions; these fat deposits are sometimes asymmetric in distribution.

The ratio of mean neck circumference to height at the withers (NCHR) has been used as a measure of neck adiposity. In preliminary studies, the NCHR was more closely correlated with measures of insulin resistance than with BCS. Although cutoff values for NCHR in relation to obesity and disease risk have not been defined, repeated measurement of mean neck circumference is useful for monitoring the effectiveness of a weight loss program. In this context, regular assessment of BCS and body weight is recommended. Ideally, body weight is measured by use of a calibrated large-animal scale. As an alternative, weight can be assessed by formulae that include measurements of girth circumference, such as those used in weight tapes. Such formulae tend to be more accurate when used on mature horses with a BCS in the range of 5 to 7. It should also be recognized that weight estimates based on heart-girth measurements do not account for variation caused by hydration state, pregnancy, and the weight of intestinal contents.

DEFINITION, PREVALENCE, AND UNDERLYING CAUSES OF OBESITY

There is no universal definition of obesity in horses and ponies. According to the Henneke system, horses with a BCS of 8 (fat) or 9 (extremely fat) can be defined as obese, whereas animals with a BCS of 7 might be considered overweight if not obese. Few studies have been undertaken to examine the prevalence of obesity in horse and pony populations. In the 1998 National Animal Health Monitoring System (NAHMS) study, an estimated 4.5% of the horse population in the United States was overweight or obese. However, the accuracy of this estimate may be questioned because it was made on the basis of owner reporting, not the results of physical examination. Anecdotal observations by equine veterinarians suggest that the prevalence of obesity is far higher than the NAHMS estimate. In support of this contention, results of a recent cross-sectional, prospective study of 300 mature horses (Thatcher and associates) in southwestern Virginia revealed obesity (BCS of 8 or 9) in 57 horses, for a prevalence of 19%.

The cause of obesity in horses and ponies is likely to be multifactorial. However, overfeeding, lack of physical activity, or both are likely contributing factors. Many horses are kept in the confinement of a stall or small pen for much of the day, and even if used for riding activities 2 to 3 days per week, may not require more than maintenance energy intake. However, many of these horses are fed much more than the maintenance energy requirement, a problem that is compounded by the provision of grains, sweet feeds, and other feeds with high caloric density. Obesity also can be a problem in some horses and ponies given unrestricted access to pasture, particularly during the spring and fall, when pasture forage is actively growing, plentiful, and rich in energy. In the study by Thatcher and associates mentioned previously, more than 60% of horses in the study were maintained at pasture year round and were not used for any type of riding activity, yet many were provided supplementary feed.

Genetics may be another factor in predisposition to obesity. Horse owners and veterinarians often use the term easy keeper to describe a horse or pony that has a tendency to be overweight and appears to require fewer calories than most horses to maintain condition. Ponies and certain horse breeds (e.g., Morgans, Arabs, and Paso Finos) appear to fit this description. One hypothesis is that certain lines of horses and ponies have inherited genetic traits that facilitate survival on poor-quality forages or in the face of limited feed availability—the so-called thrifty genotype. This strategy may fail when these horses are supplied with abundant feed, particularly grains or pasture forage rich in nonstructural carbohydrates, and weight gain and obesity result.

DISEASE ASSOCIATION

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