Horses may lose weight for a number of reasons, but three primary causes are most common: intestinal parasites, malocclusion of the teeth, and malnutrition (improper diet or feeding regimen). The Henneke scale is commonly used to assess the “body condition” of horses, rating them on a scale of one to nine. Body condition scoring (BCS) can indicate whether a horse is underweight to begin with, but an unexplained shift from one body condition type to another that is lower on the scale would indicate weight loss. The lowest score (1) indicates the horse is completely emaciated, and the highest score (9) indicates that the horse is morbidly obese. A BCS of five is ideal for most horses, but some breeds may average slightly higher or lower. For all practical purposes however, a horse that shifts downward on the scale for no known reason my may be suffering from disease leading to weight loss.
Intestinal Parasites:
Intestinal parasites do not rob the horse of nutrition, but they cause inflammation and blood loss in the gastrointestinal tract that may lead to malabsorption of nutrients. Parasites can devastate a foal and can cause general loss of body condition over time in adults. Severe parasite burdens are life-threatening to any age animal. Horses must be protected from parasite infestation by regular testing, appropriate deworming schedules, and proper management of the horses’ environment.
Malocclusion of the Teeth:
Dental problems are a very common cause of weight loss in horses. Proper mastication is necessary to derive all of the essential nutrients from plants. Mastication is the veterinary term for chewing, the first stage of digestion. Food that is not chewed sufficiently passes through the digestive tract unutilized. It may also lead to colic and serious complications like choke.
A horse’s teeth continue to erupt or grow out of necessity, in order to compensate for the huge amount of wear that takes place in consuming abrasive plant material. Abnormal wear of the teeth leads to malocclusion; the teeth do not align properly to achieve complete mastication. Malocclusion also leads to pain when surfaces of the abnormally worn teeth irritate the soft tissues of the oral cavity.
Problems affecting the teeth are many times a result of modern horse husbandry practices, despite the best intentions by owners. Wild equines had to pick through many tougher stalks in unmanaged savannahs to get to tender, more desirable shoots. These plants contain a higher silica content which is abrasive to tooth enamel. The rate of tooth wear was much higher than for today’s horses kept in lush pastures of fine, soft grasses. Wild equines had (and still have) shorter life spans as a result of rapid tooth “wear-out”, but today’s domesticated horses are more prone to tooth overgrowth and subsequent dental disease which can cause weight loss and compromise their extended life expectancy despite better overall provisions and care.
Very old horses may “outlive” their teeth. The rate of tooth wear may exceed the rate of tooth eruption in geriatric animals. Fractured and missing teeth are common and may lead to digestive interruption.
Malnutrition:
Except in the case of abuse, malnutrition is usually not caused by starvation, but it may be a possibility for overly confined horses denied adequate grazing time. More often, malnutrition occurs as a result of improper feeding regimens or an unbalanced diet. Poor quality feed may lead to nutritional deficiencies as well as dental problems that exacerbate GI irritation and malabsorption.
Other causes of weight loss include tumors of the GI system, endocrine glands, and oronasal cavity. Stomach ulcers may lead to malnutrition and diet intolerance. Severe generalized infections and diseases of the kidneys, liver, lungs, and urogenital system are also causative factors.
Any weight loss should be investigated by a veterinarian. Early intervention is much easier to address than advanced disease.