Determining Cause of Weight Loss in Horses

Weight loss in horses is a common, frustrating, and expensive struggle horse owners and practitioners face. It occurs when the body uses more energy than it’s taking in, which can happen for a variety of reasons.

In her presentation at the 2019 Annual American Association of Equine Practitioners Convention, held Dec. 7-11 in Denver, Yvette Nout-Lomas, DVM, PhD, Dipl. ACVIM, ACVECC, assistant professor of equine internal medicine at Colorado State University’s Veterinary Teaching Hospital, provided veterinary attendees with an outline for pinpointing and addressing causes of weight loss in horses.

Causes of Weight Loss

Weight loss typically occurs for one of six reasons:

  1. Lack of access to appropriate food;
  2. Lack of ingestion of available nutrients;
  3. Abnormal digestion, absorption, or metabolism of nutrients;
  4. Inadequate delivery of nutrients to peripheral tissues;
  5. Increased rate of protein and energy use or loss; or
  6. Primary muscle wasting disorders.

Horses might not ingest available nutrients if they lack appetite, have inadequate prehension (ability to grasp food), or have abnormal mastication (chewing), swallowing, or esophageal transit. Heart failure, asthma, and liver disease can all result in decreased nutrient delivery to peripheral tissues. Gastrointestinal dysfunction (gastric ulcers, inflammatory bowel disease, parasitism), liver disease, and toxicities can lead to abnormal digestion, absorption, and metabolism of nutrients. Even conditions that don’t directly affect nutrient availability can result in weight loss. Horses that are in pain and suffering various disease states, for instance, might have an increased rate of protein and energy use and loss, resulting in greater-than-expected calorie needs.

Finding the Cause

First, said Nout-Lomas, determine the horse’s use and age, as these factors are important when assessing his diet. Certain age groups are at greater risk than others for certain conditions, such as dental and musculoskeletal disease in aged horses. Evaluate the horse’s diet to determine whether the owner is providing an appropriate feed in a suitable amount.

The key is to determine whether the horse is receiving adequate calories, said Nout-Lomas. She uses the equation to calculate energy requirements for adult horses at maintenance and multiplies by a factor of 1 to 3 to estimate the calories needed for those in work.

A 500-kilogram (1,100-pound) horse at maintenance requires about 16.4 Mcals of digestible energy a day. To meet these needs, his should consume about 20 pounds of grass hay (at about 0.8 Mcals per pound) per day. Horses should ideally consume 2-3% of their body weight per day as forage, which in this case would equal 10 to 15 kilograms, or 22 to 33 pounds. Therefore, a horse fed 2-3% of his body weight per day as quality forage shouldn’t be suffering weight loss as a result of inadequate calorie provision.

However, said Nout-Lomas, it is important to verify that the horse is, indeed, consuming this amount of feed. For example, in a group situation is it possible he’s being chased off feed or has soundness issues that make it more difficult to access the feed being provided? Can he consume the feed, or is chewing or swallowing an issue?

The veterinarian should then conduct a thorough physical examination, paying particular attention to temperature, pulse, and respiration and noting any abnormal heart or lung sounds, said Nout-Lomas. He or she should assess the horse’s muscling as well as overall body condition score and look for the presence of dependent edema (lower limb swelling). The vet should also note whether the horse has diarrhea and consider performing bloodwork.

The results of these initial investigations will guide what ancillary diagnostics the veterinarian performs. For example, a horse with an elevated respiratory rate and abnormal lung sounds should warrant further respiratory tract investigation. The veterinarian might collect fecal samples to assess fiber length and conduct a fecal egg count to determine worm burden. He or she can pass a nasogastric tube to easily assess swallowing and whether an esophageal stricture might be limiting food from reaching the stomach.

Many additional diagnostics exist at varying levels of invasiveness and cost. Nout-Lomas encouraged clinicians to consider all possible mechanisms of weight loss and to base ancillary testing on signalment (age, breed, gender), history, and examination findings. She shared that in a retrospective study of horses presenting for weight loss, 93% did receive a diagnosis. Most were suffering from parasitism (30%) and dental disorders (20%). Digestive causes and those resulting from kidney and liver diseases were far less common.

Take-Home Message

  • Weight loss is a frequently encountered problem in equine veterinary practice.
  • There are multiple causes of weight loss in horses.
  • Veterinarians must take a systematic approach when diagnosing horses displaying weight loss.
  • When assessing a horse for weight loss, the vet should start with a dietary evaluation, a good physical examination, transrectal abdominal palpation, and bloodwork.