Obesity
Mechanisms of Obesity
Obesity is a common problem in domestic large animal species, especially in horses, and in ruminants raised as companion animals. In ruminants, obesity increases the risk of metabolic disease, reproductive failure, and health complications during late pregnancy and lactation.
The risk of obstructive urolithiasis increases in overweight bucks and rams, especially wethers. In horses, obesity is related to a variety of diseases including insulin resistance, equine metabolic syndrome, laminitis, and colic associated with strangulating lipomas. Obese horses and ponies that have increased metabolic energy demands (late pregnancy, lactation) that are rapidly losing weight or that are anorexic are particularly susceptible to hyperlipidemia and hyperlipemia.Obesity results from the consumption of more calories than are required by an animal based on its stage of life and level
of activity. Individual animals and certain breeds appear to be predisposed to obesity even if they are fed to meet their maintenance energy requirements. Obesity frequently occurs in horses fed high-energy feeds including grain and energy- dense commercial feeds and in horses grazing highly digestible pasture. Even horses that are only fed hay may gain weight if they consume an excess amount of energy. Purebred or pet sheep and goats (particularly wethers) tend to be overfed. In dairy cattle, obesity occurs when cattle are fed well above requirements for maintenance and milk production. Poor reproductive performance is often associated with the initiation of obesity in ruminants and is also a common sequela to obesity. Feeding for high milk production for lengthy periods in production groups predisposes infertile cows to become fat cows. Dairy cows dried off with a body condition above target are also predisposed to hepatic lipidosis, increasing the risk for periparturient diseases.
The systemic complications associated with hepatic lipidosis, as well as its diagnosis and treatment, are described in Chapter 33.Diagnosis of Obesity in Horses and Ruminants
Obesity is diagnosed by measuring the animal's body weight and assigning a subjective BCS number on the basis of muscle mass and external fat deposits. The clinical description of obesity provided by the BCS system is straightforward (see Figs. 9.2 and 9.3, Tables 9.1, 9.14, and 9.15). Overweight and obese animals have an elevated BCS (7 to 9/9 for horses; 8 to 9/9 for beef cattle; 4 to 5/5 for dairy cattle, sheep, and goats). Palpation of the back, gluteal area, and ribs should be included in the physical examination of sheep and camelids with long wool or fiber and of horses with a long winter hair coat so that a BCS can be determined. In some animals, external signs of fat deposition may be subtle. In horses, a BCS greater than 5 to 6/9 does not accurately predict the total body fat mass of the animal in a linear fashion as may be expected using the linear BCS system.23 Instead, body fat shows an exponential increase in horses with moderate to obese BCS.23 Other measurements of body fat deposition including abdominal girth and fat depth (subcutaneous and retroperitoneal) measured by ultrasound may provide a better assessment of obesity in horses when the BCS is greater than 5.23,24 Assessment of back fat thickness in dairy and beef cattle can also be evaluated with an ultrasound examination and has been shown to correspond well with total body fat.25
In some animals, obesity can be mistaken for the normal physiologic condition of pregnancy. In other cases, obesity could be mistaken for a distended abdomen from acute pathologic disease conditions including uroabdomen and peritonitis. A CBC and serum biochemical analysis can identify many subclinical diseases in an obese animal. A full physical examination should always be performed on a large animal patient before a weight loss ration is fed.
Treatment of Obesity
Overweight and obese horses and ruminants should lose weight to improve their health, extend life span, and improve production efficiency. In many cases a reduction in energy intake can be achieved by simply eliminating excess calories in the ration. Animals that fail to lose weight after 1 to 2 months after removing high-calorie feeds often require a more aggressive weight loss program with a therapeutic ration and a gradual implementation of an exercise program. Voluntary activity in animals that are turned out in pasture is rarely high enough to promote weight loss. Obese laminitic horses present a difficult challenge because exercise may not be practical, and weight loss must rely solely on dietary energy restriction.
Horses and companion ruminants that require a managed weight loss program should have the energy content of their diet evaluated by the veterinarian or nutritionist. The actual energy intake should be compared against the energy requirement for the animal's current life stage (see Tables 9.10, 9.11, 9.13, 9.16 to 9.20). The animal's current calorie intake should be reduced by 10% to 20% at the start of the weight loss program. This can typically be achieved in horses by decreasing forage intake to 1.5% of body weight and removing supplemental concentrates. Decreasing forage to 1% of body weight may be required under veterinarian care for some horses resistant to weight loss. Dietary protein should not be restricted, and adequate protein intake can be met by ensuring the forage available has appropriate protein content, or through the addition of soybean meal. Supplemental feeds and treats should be reduced or eliminated from the diet; however, a vitamin and mineral supplement should be fed with a weight loss ration to ensure all nutrient requirements other than energy are met. For horses, a concentrated vitamin-mineral supplement (typically offered in amounts EBCGM, Jacobs DO: Metabolic assessment. In Rombeau JLRRH, editor: Clinical nutrition: parenteral nutrition, ed 3, Philadelphia, PA, 2001, WB Saunders, pp 80-108.
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