Weight Loss in Horses

Take Home Message  

The failure of an adult horse to gain or maintain weight is a common clinical complaint. A practical approach to determining a diagnosis is essential when veterinarians examine horses with weight loss without accompanying clinical signs that suggest an obvious diagnosis.  

Introduction  

In many cases the explanation for weight loss in a horse is readily apparent after conversation with the owner and physical examination of the patient. However, in some horses identification of the underlying cause of weight loss can be an exceptionally challenging clinical puzzle. As  with most such puzzles, an understanding of the relevant pathophysiologic mechanisms coupled with a systematic, problem-oriented approach will ultimately yield a diagnosis for most affected  horses.^1-3 A complete discussion of all the disorders that may result in weight loss in horses is  beyond the scope of this presentation. Instead, the goal is to review the pathogenesis of weight loss with an emphasis on the ways that various types of disorders might interrupt normal nutrient  acquisition or utilization. From this understanding, a practical clinical approach to the problem of weight loss in adult horses can be developed and illustrated through specific case examples.  

Pathogenesis of Weight Loss  

In essence, weight loss or a failure to gain weight must result 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. These critical processes are summarized in Box 1.  

Lack of access to appropriate feed. It is obvious that a horse must have access to adequate  quantities of nutritious and appetizing feedstuffs in order to maintain or gain weight and yet this  remains one of the most common causes of weight loss in horses. A complete discussion of equine nutrition is well beyond the scope of this manuscript. The clinician is referred to  numerous excellent texts that discuss detailed nutrient requirements.^4,5 When indicated, analysis  of available feedstuffs is recommended to confirm that available feed is of sufficient quality and  quantity to support weight gain. In select situations, it may be beneficial to remove the horse from its regular environment, provide good quality feed in a controlled environment, and  monitor for weight gain.  

Lack of ingestion of available nutrients. There are many reasons why a horse may not be able to ingest nutrients. The failure to consume feed may be a result of inappetence secondary to a systemic disorder or of a physical limitation in the ability to prehend, masticate, or swallow food.  Although the process of nutrient digestion begins with mastication and mixing with salivary secretions, absorption of nutrients cannot begin until feed reaches the stomach and intestines. Therefore, esophageal disorders can also result in weight loss.  

Abnormal digestion or absorption of nutrients. 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 is critical for nutrient absorption. While pancreatic dysfunction is rarely implicated as a cause of weight loss in horses, hepatic disorders are relatively common.  

Inadequate delivery of nutrients to peripheral tissues. Tissue blood supply must be adequate to  deliver nutrients to the peripheral tissues. Severe chronic cardiovascular or respiratory disease can result in weight loss. This may be observed, for example, in horses with end stage heart failure or chronic obstructive pulmonary disease. In most horses, the weight loss is compounded by a decrease in appetite and increased energy demands in an attempt to maintain tissue  oxygenation.  

Increased rate of protein or energy use or loss. This is one of the most common causes of  weight loss in horses. Nutrient loss may occur because of protein-losing gastrointestinal or renal diseases. However, chronic exudative skin disorders or chronic external blood loss may also  result in significant protein losses. Basal metabolic rate and demand for nutrients increases in a wide variety of disorders including infectious or inflammatory diseases, neoplasia, chronic pain  states, and immune-mediated disorders. These 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. An increase in basal metabolic rate with consequent weight loss  may occur with hyperthyroidism, but this is extremely rare in horses.  

Primary muscle wasting disorders. Post-streptococcal myopathy, equine motor neuron disease and polysaccharide storage myopathy are disorders that can present with a primary complaint of  weight loss in the face of a normal appetite and feed consumption. The problem is a progressive  loss of muscle mass rather than disturbances in nutrient acquisition or utilization.

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