Assessment of Nutritional Status

Before initiation of dietary therapy, the large animal patient must be examined to determine its nutritional needs. Animals may be anorectic owing to systemic disease, or they may be dysphagic owing to a mechanical (foreign body, abscess, poor dentition) or neurologic (botulism, tetanus, viral encephalitis) disease. Assessment of the nutritional status of the patient should include a measurement of the body weight (BW) and body condition score (BCS) of the animal. BW measurements should be taken at the time that the large animal patient is presented to the clinician and as frequently as possible during hospitalization. If a scale is not available, estimations of BW can be made with a weight tape or by using length and girth measurements (see Chapter 9). The BCS also helps in the evaluation of the nutritional status of the patient. The BCS system enables the clinician to subjectively assess the endogenous protein and lipid stores in a large animal patient. A list of BCS descriptions for different species is provided in Chapter 9. Changes in weight or BCS are often easily overlooked in day-to-day observations of the patient if a concerted effort is not made to detect them. Palpation of the animal (ribs, dorsal vertebral processes) is necessary in sheep with a heavy fleece, camelids with long fiber, and horses with a thick winter hair coat. Animals with a low BCS (1 to 3 of 9; 1 to 1.5 of 5) have minimal protein and lipid stores and are at greater risk for developing protein calorie malnutrition after a period of anorexia. Large animal patients with a high BCS (7 to 9 of 9; 3.5 to 5 of 5) that are anorectic may have an increased risk for developing complications (hyperlipemia, hepatic lipidosis) from abnormal lipid metabolism. A clinician should initiate dietary therapy in a large animal patient that loses 3% to 5% of its initial BW or whose BCS diminishes by 1 grade or more.

Biochemical tests provide another method to evaluate the nutritional status of the large animal patient. Endogenous protein catabolism is a normal physiologic response to anorexia in animals. To date, few biochemical tests are available to assess protein malnutrition in large animals. Although anemia and hypoproteinemia (hypoalbuminemia) are occasionally seen in cases of malnutrition, they are not specific and frequently are associated with another primary disease process such as parasitism or protein-losing enteropathy. Severe protein malnutrition can result in abnormally low serum urea nitrogen (SUN) concentrations in horses and ruminants. Liver disease also decreases the formation of urea nitrogen and must be ruled out when evaluating the animal. Protein malnutrition can result in an increase in the urinary excretion of 3-methylhistidine, a myofibril amino acid that is not metabolized. Measurement of this metabolite in the future may be a useful tool to monitor protein catabolism in large animal patients.⁶

Underfed horses develop a mild hyperbilirubinemia (unconjugated) that, although not directly related to nutritional status, is readily reversed when food intake resumes.^7,8 Animals that are in negative energy balance rely on endogenous stores of lipid as an energy source. Serum non-esterified fatty acids (NEFAs) and/or triglycerides are expected to rise in these animals. Once nutritional support or refeeding is initiated, these lipid metabolites usually decrease. Pathogenic elevations in serum triglyceride (greater than 500 mg/dL) develop in some equine patients that are anorectic and that have a high daily energy requirement (lactation, pregnancy), but hypertriglyceridemia can also occur in patients with insulin resistance, renal failure, obesity, and in predisposed breeds (miniature horses, ponies). Ketone bodies have a glucose-sparing effect in the body and are normally produced from fatty acids and amino acids in animals that are in negative energy balance. Ketonuria develops after the excess production of ketones and can be used to indirectly monitor the severity of caloric malnutrition in ruminants. Hypoglycemia is a common complication in large animal neonates that are malnourished but is an uncommon finding in adults. Both adult and neonatal large animals may develop glucose intolerance and hyperglycemia during periods of systemic illness and may require treatment with a diet that will not exacerbate the hyperglycemia. Electrolyte and mineral derangements including hypocalcemia, hypomagnesemia, hyponatremia, hypochloremia, hypophosphatemia, and hypokalemia may develop in inappetent, systemically ill large animals or during the refeeding process.^9,10 Chronically malnourished animals must be closely monitored, and biochemical signs of the refeeding syndrome should be managed with appropriate electrolyte supplementation.

Nutrient Requirements of Large Animals during Clinical Illness

Although it is generally accepted that sick animals have increased nutritional requirements, these requirements have not been quantified for specific disease conditions. Therapeutic nutrition is designed to provide adequate caloric intake during a period of hypophagia. Both overfeeding and undernutrition should be avoided when nutritional support is offered.¹¹ Currently, the best estimation of energy and protein requirements for hypophagic horses and ruminants is calculated from the animal’s BW (see Chapter 9) or they can be determined from the National Research Council (NRC) tables.¹² For adult horses, the resting digestible energy (DE) requirement is calculated as DE_rest (Mcal) = 0.975 + 0.021 BW (kg), and maintenance crude protein (CP) requirement is calculated as CP (g) = 1.26 BW (kg).¹² For a 450-kg horse, these would equate to a 10.4-Mcal resting energy requirement and a 567-g crude protein requirement. The ideal BW should be used for calculating the energy requirement of an obese animal. Protein requirements can be calculated using an obese animal’s current BW as long as protein restriction is not required based on the patient’s clinical disease. Indirect calorimetry provides the best estimation of a patient’s resting energy requirements, but currently is only available for critically ill neonatal foals who have an estimated resting energy expenditure of 40 to 50 kcal/kg BW/day.^13,14 Older foals can be fed between 70 and 100 kcal/kg BW/day. Dietary protein for foals ranges between 2 and 5 g protein/kg BW/day. For calves, the resting energy requirement is approximated as DE (Mcal) = 0.07 BW (kg) and digestible protein as DP (g) = 3.5 BW (kg), which would equate to 3.50 Mcal (3500 kcal) and 175 g of protein for a 50-kg calf. These calculated values represent starting points for formulating dietary therapy, and adjustment based on clinical response or specific medical conditions may be necessary.

Vitamin and mineral requirements, also available from NRC tables, can usually be met if an enteral diet is formulated with commercial complete feed pellets or pelleted hay. Parenteral solutions can be supplemented with vitamins and minerals. Although B-vitamin deficiencies do not occur naturally in horses and cattle, supplementation may be beneficial in large animals with gastrointestinal diseases that result in the disruption of the normal tract flora that produce B vitamins.

Oral Supplementation

The animal’s feed intake should be monitored during treatment. If the animal is losing condition despite consuming all feed offered, more or higher-quality feed should be provided. If the animal’s appetite is poor, the patient should be offered a variety of highly palatable feeds, including fresh grass, dried forages, and complete commercial feed pellets. Although sweet feeds are palatable, they contain a high concentration of soluble carbohydrates (sugar, starch) and their use should be limited to a top dressing of other feeds. Ruminants in particular may consume small quantities of fresh feed if it is offered frequently, whereas if the same quantity is offered in one feeding, it may be ignored after a few bites. Many dairy cows can be coaxed into eating hay if it is placed in the back of the pharynx by the clinician, and oropharyngeal stimulation may result in increased voluntary feed consumption. Fresh silage and dried brewer’s grain frequently appeal to the hypophagic cow. Many sick horses and ruminants benefit from grazing if grass is available.

A hospital feeding chart should be created to facilitate monitoring of the feed consumed by a hospitalized patient. Use of an inexpensive farm scale is the most accurate way to measure the amount of feed offered to a patient. Feed that is not consumed within a few hours should be weighed and discarded to prevent the accumulation of stale, and possibly fermented, malodorous feed. If the animal does not voluntarily consume enough feed to meet 75% of its resting energy requirements or 75% of its maintenance protein requirements, or if the animal loses weight or condition, dietary therapy with a liquid diet by intragastric administration should be considered. Dysphagic animals can also be managed with a tube-fed liquid diet as the sole source of nutrition.