Weight Loss in Horses

Introduction

The failure of an adult horse to gain or maintain weight is a common clinical complaint. Often the explanation for weight loss is readily apparent from the history and physical examination. However, in some cases identification of the underlying cause of weight loss can be a challenging clinical puzzle. An understanding of the relevant pathophysiologic mechanisms coupled with a systematic, problem-oriented approach will ultimately yield a diagnosis for most horses.

Pathogenesis of Weight Loss

Weight loss or a failure to gain weight results from an imbalance between acquisition and utilization of nutrients. A stepwise consideration of the critical processes involved provides a conceptual background on which to develop a practical clinical approach to the problem of weight loss. Horses must have access to appropriate feed and the ability to ingest that feed. The failure to consume feed may be a result of inappetance or of a physical limitation in the ability to prehend, masticate or swallow food. The failure to adequately digest and absorb nutrients may result from gastrointestinal, hepatic, pancreatic, metabolic or toxic disorders. Changes in gastrointestinal motility and transit time, infiltrative or ulcerative disorders of the intestine, disorders of the intestinal villi, and parasitism can affect nutrient absorption. Normal hepatic and pancreatic function are critical. Tissue blood supply must be adequate to deliver nutrients to the peripheral tissues. Some horses lose weight because of an increased rate of protein or energy use or loss. Nutrient loss may occur because of protein losing gastrointestinal or renal diseases, chronic exudative skin disorders, or chronic external blood loss. Basal metabolic rate increases in a wide variety of disorders including infectious or inflammatory diseases, neoplasia, chronic pain states, and immune-mediated disorders. Affected horses may consume quantities of nutrients that would be considered appropriate for a healthy horse, yet will lose weight because of the increased demand for calories. Occasionally, horses may present for weight loss because of primary muscle wasting disorders.

Approach to investigation of Weight Loss

Investigation of the horse with weight loss should begin with a complete history and thorough physical examination. The most common causes of weight loss in the horse are poor nutrition, dental abnormalities, and internal parasitism. In most cases, these problems can be ruled out largely on the basis of the history and physical examination. If these problems do not appear to be responsible for the observed weight loss, and the horse is able to prehend, masticate and swallow food normally, some clinical pathological investigation is indicated. For most horses, this will include a complete blood count (CBC), serum biochemical profile, and urinalysis. [...]