A horse can lose weight for many reasons, from something as simple as underfeeding to a very serious condition such as cancer.
Your veterinarian will first look for three major causes: anorexia, increased nutrient demands, or protein-calorie malnutrition. Horses with anorexia experience a loss of appetite, which can be due to a primary underlying disease such as cancer, or functional problems such as the inability to swallow, chew, or get hold of food. Anorexia can lead to rapid weight loss because the horse is unable to take in adequate nutrients through feed, hay, and supplements. The condition can go unnoticed, especially in winter if blankets are not removed regularly so the animal can be seen.
The second cause of weight loss is increased demand for nutrients that can be caused by physiologic conditions such as exercise, growth, pregnancy, lactation, and even cold weather. Increased need for nutrients also can be based on pathologic processes, such as sepsis (severe systemic infection), trauma, parasites, burns, and neoplasia (cancer). Nutritional requirements for horses are based on weight, growth, pregnancy, lactation, and exercise. Nutritional requirements for pathologic processes are derived from the human experience. For example, a horse that is undergoing elective surgery may need a 10 percent increase in nutrients; a horse that has experienced a fracture, a 20 percent increase; a horse with severe infection or sepsis, a 30 to 60 percent increase; a horse with peritonitis (infection of the abdominal cavity), a 40 percent increase; and a horse with major burns, a 50 to 110 percent increase. The stress that the body is experiencing stimulates the nervous system, resulting in a release of adrenaline, which affects the animal’s ability to process nutrients efficiently.
The third major weight-loss mechanism is protein-calorie malnutrition. This can be caused by underfeeding, competition with other animals, dental disease, inadequate feed quality, or problems with digestion.
Micronutrient (vitamins and minerals) deficiencies also can play a role in weight loss, contributing to inefficient digestion in horses. Although rare, copper, cobalt (Vitamin B12), and Vitamin A deficiencies do happen. Parasites can contribute to all the aforementioned causes of weight loss, resulting in increased nutrient requirements, loss of bodily fluids, inflammation, organ or vascular damage, protein-calorie malnutrition, competition for nutrients, inefficient digestion, anorexia, and micronutrient deficiencies.
Veterinarians typically will take a detailed history when presented with a horse with weight loss to help determine the source of the problem. They will look for any additional clinical signs that might point to the cause of weight loss and will ask how much and what brand of food the horse is eating. In addition, they will examine the quality of available roughage (both hay and pasture). Your veterinarian will need to know where the animal is fed, the dynamics of the social hierarchy of the horses in the pasture, and how the horses interact at feeding time. The veterinarian also will need to know when your horse was last de-wormed and with what product.
A thorough physical examination is essential in solving a weight-loss issue. An oral exam with a full mouth speculum should be performed to identify dental abnormalities. Does your horse have other signs of disease? Is there evidence of diarrhea or blood in the manure? Does the horse have a fever? Can the horse swallow; can he pick up his food properly; does he want food when offered?
Knowing the horse’s weight is important, and while a scale is most accurate, the use of a consistent weight tape can provide guidelines as to whether an animal is continuing to lose weight. A body condition score is given based on the location and presence of fat pads on the horse, and ranges from 1 (very poor) to 9 (obese) with an ideal weight around 4.5-to-5 out of 9.
After a history and physical examination, your veterinarian can start narrowing down the potential causes of weight loss in your horse. Is the horse not getting sufficient nutrition because of: underfeeding; competition; inability to chew, swallow, or get hold of food due to a primary disease; or is the horse not interested in the food due to an underlying disease?
Alternatively, your veterinarian will try to identify reasons for weight loss despite adequate intake of food: is the quality of the food poor; is there dental disease; is there an excessive nutritional need; or is there a digestive disorder due to an underlying disease such as cancer or inflammatory bowel disease?
Blood work and a urinalysis are often required to reach a diagnosis. These tests can help to identify several conditions, such as anemia, abnormalities in the total white blood cell count, and increases in the inflammatory proteins. The results also can indicate how well internal organs, such as the kidneys and liver, are working. A fecal float can help identify a parasite load, and a rectal exam can be a way to identify any palpable masses in the abdomen. Imaging such as ultrasound and radiographs can be used to further identify abnormalities within body cavities, as well as the collection of body fluids (i.e. abdominocentesis - a sample of the fluid within the abdominal cavity).
If the clinical investigation fails to identify a cause for weight loss, function tests can be performed to identify if the horse is absorbing or assimilating nutrients. Examples of such tests include an oral-glucose-absorption test in which the horse is fasted overnight and then given a glucose solution through a nasogastric tube, followed by collection of blood samples over a four-hour period. Rectal and intestinal biopsies can be used to identify disease such as inflammatory bowel disease and gastrointestinal cancer that cause poor absorption of nutrients.
Once the cause of weight loss is identified, treatment can begin. Veterinarians will consider the horse’s diet, the micronutrient requirements, and the primary disease to help the animal gain weight. A healthy horse requires 1 to 2 percent of his body weight in total feed a day. Not more than 50 percent of this should be fed with “concentrate,” or grain. A minimum of 1 percent of the body weight should be in forage. For example, a 1,000-pound horse should receive 10-to-20 lbs of total feed a day. (The “easy keeper” receives 1 percent [or slightly less] including his pasture grass for maintenance and the thin horse receives 2 percent or more to gain weight). True complete feeds, such as most equine senior feeds, are not considered concentrates because the majority of their ingredients are forages in pellet form.
One word of caution: When starting to feed the emaciated rescue horse, it is important to start with roughage only, such as an alfalfa or alfalfa/grass mix for the first week, and no complete feeds or concentrates. High-carbohydrate diets can result in “re-feeding syndrome” which can cause significant insulin releases and may lead to phosphorus and magnesium imbalances. These, in turn, can lead to cardiac, respiratory, and renal failure, and death within three to seven days of re-feeding.
Horses can lose weight for many reasons, and sometimes a combination of factors is involved. Keeping track of your horse’s daily eating habits, weight, and activities will be very helpful to the veterinarian in diagnosing and treating the problem.