Chronic weight loss is a common complaint from horse owners,
especially when dealing with an older animal. The clinical
problem suggests that a horse has lost weight over a known period
of time and can be broken down into
1) normal physiological loss of weight or
2) pathological causes for weight loss.
The common physiological causes of weight loss include:
1) early stages of lactation.
2) high level of exercise-related performance.
3) breeding season.
Many breeding stallions lose weight during the breeding season
and gain it back again in the fall and winter. Weight loss is due
to increased metabolic rate and calorie consumption and reduced
intake. Many stallions fret throughout the breeding season and do
not eat as well as the normally would during the off-season.
Late pregnancy and lactation also increase demand for nutrients,
and weight loss results if dietary adjustments are not make. A
pregnant mare needs about 20% more nutrients in late pregnancy
than for normal maintenance, and at peak lactation requires up to
Other than these physiologically induced states of increased
demand, some rare metabolic diseases result in weight loss.
Included are hyperthyroidism (not documented in horses) and
Cushing's syndrome. Nuff said.
Late pregnancy can be associated with a loss of condition rather
than a loss of weight.
Weight loss is most commonly associated with one or more of the
2) increased nutrient demands
3) protein calorie malnutrition
Anorexia is usually secondary to a primary disease. Increased
nutrient demands maybe associated with physiologic conditions
(winter weather, exercise, pregnancy and lactation) or with
pathologic processes (sepsis, trauma, parasitism, thermal injury,
neoplasia, organ failure). Inadequate feed quality and/or
quantity commonly results in protein calorie malnutrition of
varying degrees. Weight loss can also occur with deficiency of
essential micronutrients such as copper, cobalt (Vit B-12), or
Mechanisms of Weight Loss:
Anorexia by definition is the loss of appetite or lack of desire
for food, it may be partial or complete. I don't recall too many
horses I've ever known who turned away from the feed trough
unless it was associated with a primary disease condition. It may
be differentiated from dysphagia by observation. Many diseases
that cause anorexia have in common dehydration, electrolyte
imbalances, and/or acid-base disorders. Acute, complete anorexia
results in a dramatic weight loss whereas partial anorexia over a
long period of time may exhibit only subtle weight loss.
Therefore anorexia will result in a secondary or conditional
state of PCM.
Increases in nutrient requirements have been established in
humans with the following conditions:
Condition % change above normal
Elective surgery 10%
Severe infection/sepsis 30-60%
Major thermal injury 50-110%
Though we cannot directly extrapolate from this data to the
horse, these guidelines may give you an idea of the degree of
change as a result of disease or injury.
Protein calorie malnutrition continues to be a persistent problem
in all domestic animals. The lack of the major nutrients
obviously results in weight loss. Several mechanisms are
1) Inadequate availability of feed to meet dietary requirements,
ie frank underfeeding otherwise known as agroceriosis, miss-meal
colic, or hollow belly.
2) Inadequate feeding facilities, ie mixing age
3) Quality of the diet.
Prehension, Chewing, and Swallowing
These functions are integrated, and abnormalities in one or more
can lead to reduced food intake and loss of weight. Review your
notes on dysphagia. Don't forget to look at those teeth. Abnormal
dental wear leading to the development of sharp points on the
cheek teeth can cause abrasions to buccal mucosa and the tongue,
making chewing painful. Nuff said.
Chronic painful conditions:
Severe arthritis, chronic laminitis, deep nonhealing wounds, and
invasive tumors are considerations. Pain in these conditions
leads to depression, which results in partial anorexia.
Conditions such as severe arthritis and chronic laminitis also
reduce the horse's mobility, reducing the amount of grazing in
pasture raised animals. Analgesics or other more specific
treatments, in certain conditions, might lead to increased food
consumption and reversal of any weight loss.
Such as EIA, internal abscesses, or pleuritis may lead to
recurrent fevers, depression, and partial anorexia. Clinical
signs vary and clin path data is usually nonspecific. The
erythron usually shows some degree of anemia. Time to get Duncan
and Prasse out and look up the details of the "anemia of chronic
infection." Chronic bacterial infections usually show a
leukocytosis with a mature neutrophilia. Serum fibrinogen is
usually elevated. Abdominocentesis, rectal examination,
thoracocentesis, thoracic percussion, and radiographs will be
useful in diagnosis. Chronic EIA, no treatment, diagnose with
AGID test (Coggins test).
Chronic obstructive pulmonary disease in the later stages is
associated with weight loss. These animals are usually severely
dyspneic and reluctant to move. The work of breathing may be so
strenuous that the animal does not take sufficient time to eat
and maintain body weight.
Neoplastic disease, especially that of the GI tract, is commonly
associated with weight loss. Some of the weight loss is
attributed to reduced intake (systemic effects of TNF), from pain
caused by the tumor, or from physical obstruction (pharynx).
Maldigestion or malabsorption may be caused by the tumor. Large
tumors have significant metabolic demands that consume nutrients.
Gastric neoplasia is not common in horses. Squamous cell
carcinomas usually arise at the margo plicatus and can become
large enough to obliterate almost the entire lumen of the
Gastric granulomas 2-3 cm in diameter are fairly common in horses
and are usually associated with infestations with Habronema
larvae. Draschia spp. are also known to cause gastric granulomas.
Gastrophilus spp, although do not cause granulomatous disease,
can occupy the stomach in large numbers, and are potentials for
gastric irritation, causing pain which may in turn result in
Small intestine neoplasia may result in malabsorption and
possibly maldigestion. Lymphosarcoma, though uncommon, is the
leading cause of GI neoplasia.
Lymphosarcoma fun facts:
1) One of the few disorders associated with low or absent serum
IgM in an adult horse.
2) Rarely exfoliates into the peritoneal cavity-usually confined
to the bowel wall and draining lymph nodes.
3) White blood cell line is usually normal.
4) Serum Ca++ can be elevated in some cases.
Other non-neoplastic diseases which would need to be ruled out
with similar clinical signs include eosinophilic infiltration,
plasmacytic/lymphocytic infiltration, granulomatous enteritis,
and reduced blood flow secondary to parasitic damage to the
arterial blood supply. The causes of conditions other than
parasitic lesions are unknown.
Large-Bowel Maldigestion and Malabsorption:
Large bowel malassimilation could lead to chronic weight loss. If
lymphosarcoma or granulomatous enteritis involves the cecum and
colon, expect to find significant large-bowel dysfunction and
reduced absorption of volatile fatty acids. Large-bowel
associated weight loss is usually associated with chronic
diarrhea. Chronic diarrhea isn't always indicative of total large
bowel malabsorption and many horses with chronic diarrhea
maintain adequate body weight with normal food intake. You will
find chronic diarrhea to be one of the most frustrating
conditions to work up. Near 40% of the time you will be unable to
diagnose a cause or correct the condition. The malabsorption and
weight loss is most likely due to parasitism of the large
intestine, particularly with cyathostomes (small strongyles),
both adults and larvae. Protein-losing enteropathy can be
associated with parasitism, and parasites can directly consume
the host's protein.
The mechanisms by which parasite infestation can result in weight
1) Loss of body fluids and tissues resulting in increased
2) Competition for nutrients in the gastrointestinal tract.
3) Malassimilation, malabsorption.
4) Inflammation causing increased nutrient requirements.
5) Anorexia, advanced stage of the disease.
6) Micronutrient deficiencies.
7) Organ or vascular damage from migrating parasite larvae.
These conditions affect both the small and large intestine.
Maldigestion and malabsorption contribute to the weight loss.
Included in these diseases are granulomatous enteritis, severe
parasitism, and chronic ulcerative lesions (phenylbutazone
toxicity). These usually carry a poor prognosis unless accounted
for by bute toxicity or parasitism. Chronic salmonellosis has
been isolated from horses with large chronic ulcerative lesions
of the large colon, but their role in the cause of the ulcer is
not clear. Weight loss results from loss of large amounts of
protein from these large, open, ulcerated areas.
All of these situations are marked by continued loss of plasma
proteins, and in many cases by abnormal absorption as well.
Laboratory data is not specific. Mild anemia may be present, and
chronic inflammation can be reflected by a moderate leukocytosis.
Serum chemistries are unremarkable except for a low serum
protein, especially albumin.
Chronic renal disease, which may be immune mediated, can lead to
significant protein loss, and this proteinuria accounts for some
of the weight loss encountered. In addition, affected horses are
usually azotemic, depressed, and anorectic. Thus, they have an
accelerated loss of nutrients in the urine and a markedly reduced
intake. The clinical signs associated with chronic renal disease
include depression, anorexia, and P/U and P/D with weight loss.
Peripheral limb and ventral edema can develop secondary to the
marked hypoalbuminemia in advanced cases. These animals may also
be anemic, hypochloremic, hyponatremic, and hypercalcemic.
Urinalysis is often helpful with massive proteinuria and little
or no evidence of urinary tract inflammation.
Oh yes, let us not forget our old friend chronic liver disease.
Just some fun facts:
1) Not all horses with chronic liver disease are icteric.
2) The weight loss is not only due to severe metabolic
derangements, but also to reduced intake resulting from marked
depression in these animals.
3) Note, they are variably depressed, occasionally icteric, may
have ventral edema, and photosensitive over their white or
4) Hepatoencephalopathic? you ask, possibly, exhibiting signs of
aimless wandering, head pressing, convulsions.