Chronic weight loss in the horse presents a diagnostic challenge that requires careful attention to detail. While there are numerous causes for weight loss, a methodical evaluation of these horses is more important than the lengthy list of possible diagnoses.

The primary drivers of weight loss are decreased caloric intake, decreased nutrient utilization, and increased metabolic demands. Decreased caloric intake is caused by either a poor appetite or inadequate feeding. If the horse has a good appetite, then feeding management should be investigated. Assess feed for both appropriate quantity and quality. Steps should be taken to guarantee all horses, particularly those on the low end of the social order, have access to feed in group situations.

After examining feeding management, physical abnormalities should be examined. A poor appetite might be related to an inability to eat (due to swallowing and chewing problems or discomfort with lowering head/neck). Particularly in older horses, poor dental care can cause weight loss. Dental malocclusions (misalignments), tooth root abscesses, and periodontal disease will hamper the horse's ability to process his feed. In geriatric horses, loss of multiple teeth also will make their efforts to chew forage far less efficient.

Decreased nutrient utilization might result from conditions that alter absorption or digestion of nutrients. Examples of this include chronic parasitism, chronic diarrhea, and infiltrative disease-altering absorption in the gastrointestinal tract.

Increased metabolic demand results from normal conditions such as growth, increased exercise, pregnancy, and lactation, as well as illnesses such as sepsis (blood infection), organ failure, cancer, burns, or other severe trauma.

Diagnostic Approach

Your veterinarian can best evaluate the horse's environment and management as part of a chronic weight loss work-up. First-line diagnostic tests for all weight loss cases include conducting a case history, physical examination, routine labwork, and fecal parasite analysis. A detailed history of the horse includes information about diet, supplements and medications, feeding arrangements, medical history, and deworming. Evaluation of body condition score and weight, an oral exam, observation of appetite/ingestion of feed, auscultation (listening with a stethoscope) of the patient's heart and lungs, examination of hair/skin quality, hoof growth, and observation for any lameness are all part of the physical exam. Rectal palpation is also likely indicated for chronic weight loss cases.

Blood tests can identify causes of weight loss and problems secondary to poor body condition. A complete blood count (CBC) can help identify infectious or inflammatory processes, and significant anemia might be present in chronic cases. Biochemistry values are useful to assess liver and kidney dysfunction, altered protein concentrations, muscle disorders, and electrolyte abnormalities.

Additional diagnostic tools are available if initial weight loss evaluation does not reveal a cause. Intestinal absorption capacity can be evaluated via oral carbohydrate absorption tests. A veterinarian administers a specific quantity of glucose orally and tests blood samples to identify the absorptive peak. Decreased absorption is consistent with malabsorptive or infiltrative intestinal disease. Abdominocentesis (sampling of abdominal fluid) and cytologic evaluation of the sample can identify peritonitis (infection/inflammation of the abdominal lining) or abdominal cancer. Rectal biopsy can also characterize infiltrative disease of the GI tract. Additional blood testing can evaluate liver function, diagnose metabolic disease, and assess kidney function. Finally, your veterinarian might also recommend a trial larvacidal deworming treatment (moxidectin or five treatments of double-dose fenbendazole).

If diagnostic tests and evaluations in the field fail to yield a diagnosis, then more extensive testing is recommended. Referral to a larger veterinary practice is typically required for gastroscopy, abdominal radiography, abdominal/thoracic ultrasonography, echocardiography, and organ biopsies. A careful diagnostic approach to most cases of chronic weight loss will reveal a cause, though some cases will remain undiagnosed even after extensive testing.

About the Author

Martha Mallicote, DVM, Dipl. ACVIM

Martha Mallicote, DVM, Dipl. ACVIM, is a large animal clinical assistant professor at the University of Florida. Her interests include neonatology and endocrine disease.

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