Acquired hypertrichosis (excessive hair growth) in horses is associated with the endocrine disorder pituitary pars intermedia dysfunction (PPID). Definitive diagnosis is based on measurement of the blood endogenous ACTH concentration, either to obtain a baseline value or after a thyrotropin-releasing hormone stimulation test. PPID is controlled by the administration of oral pergolide. Hypertrichosis can be managed by frequent body hair clipping.
Hypertrichosis is excessive growth of the nonshedding coat (see image) that occurs in older horses (typically ≥ 18 years old). It is invariably associated with pituitary pars intermedia dysfunction (PPID) caused by an adenoma of the pars intermedia of the pituitary gland.
Courtesy of Dr. Janice Kritchevsky.
In contrast to hypertrichosis, hirsutism is increased hair growth that occurs secondary to elevated blood concentrations of androgens. Therefore, "hypertrichosis" is the more correct term to describe the increase in hair length and lack of shedding that occurs in horses with pituitary disease.
Horses of any breed can be born with a curly coat mutation, and horses of the Bashkir Curly breed have longer coats than those of most horses. However, the hair of a Bashkir Curly horse does not change as the animal ages.
Pituitary adenomas may compress the overlying hypothalamus, which is the primary center for the homeostatic regulation of body temperature, appetite, and cyclical shedding of hair. In addition, pars intermedia adenomas secrete increased amounts of alpha-melanocyte-stimulating hormone, a factor in the growth of a long coat in winter.
Clinical Findings of Hypertrichosis in Horses
In addition to hypertrichosis, the most common signs of PPID include the following:
polyuria and polydipsia
poor muscle tone
weakness
somnolence
abnormal distribution of adipose tissue
swelling of the periorbital fossa
increased susceptibility to infections
intermittent pyrexia
generalized hyperhidrosis
If insulin dysregulation is also present, horses with PPID are at high risk for laminitis as well.
Common and uncommon signs of PPID are summarized in the table Clinical Signs of Pituitary Pars Intermedia Dysfunction.
Clinical Signs of Pituitary Pars Intermedia Dysfunction
Common | Uncommon |
|---|---|
Hypertrichosis Increased sweating Loss of seasonal shedding; delayed shedding Lethargy; decreased performance Regional adiposity; increased fat on neck, topline, and tail head; bulging supraorbital fat pads Recurrent infections, particularly of teeth and hoof abscesses Loss of musculature; pendulous abdomen Polyuria and polydipsia Tendinitis or desmitis Laminitis, if insulin dysregulation is also present | Blindness Hyperglycemia (diabetes mellitus) Seizure; neurological disease Mammary gland secretions; inappropriate lactation |
Hypertrichosis often becomes evident because of a failure in the cyclical seasonal shedding of hair. Before generalized hypertrichosis is observed, horses may have longer hair on the legs, ventral abdomen, and throat latch. Eventually, the hair over most of the trunk and extremities becomes long (up to 10–12 cm), abnormally thick, wavy, and often matted.
Pars intermedia adenomas are the most common pituitary tumors in horses. They are yellow to white and compress the pars nervosa.
Horses with PPID may have hyperglycemia (in the context of insulin dysregulation) and glucosuria. It is not known why some horses with PPID become insulin resistant and others do not. However, cortisol and other hormones that may be present in increased concentrations in horses with PPID are insulin antagonists.
In cases of PPID, concentrations of plasma immunoreactive adrenocorticotropin and alpha-melanocyte-stimulating hormone may range from modestly to extremely increased. Blood cortisol concentrations remain in the normal range; however, they lack the normal diurnal rhythm and escape suppression by the administration of dexamethasone much more quickly than in healthy animals.
Diagnosis of Hypertrichosis in Horses
Measurement of resting endogenous ACTH concentrations
Insulin insensitivity is the hallmark of pars intermedia adenoma in horses. However, because they also occur in horses with equine metabolic syndrome or other insulin dysregulation syndromes, these signs are not diagnostic of PPID.
In horses with pars intermedia adenoma, other nonspecific findings include absolute or relative neutrophilia, eosinopenia, and lymphopenia; lipemia; hypercholesterolemia; and mild, normochromic, normocytic anemia. Liver enzyme activity may be increased. Electrolyte concentrations are usually normal. Findings on urinalysis are unremarkable except for occasional glucosuria and a low to normal urine specific gravity.
Definitive diagnosis of PPID is based on evocative testing or measurement of resting endogenous ACTH concentrations.
A dexamethasone suppression test is an evocative test used to evaluate horses with suspected PPID. First the baseline cortisol concentration is measured; then dexamethasone is administered (0.04 mg/kg, IM, once), and the cortisol concentration is measured at one or more time points afterward, depending on the testing protocol.
In horses with PPID, cortisol concentration is often not suppressed to 30% of baseline (< 1 mcg/dL) 12–20 hours after dexamethasone administration; in healthy horses, cortisol concentration does decrease to 30% of baseline (< 1 mcg/dL) 12–20 hours after dexamethasone administration.
In horses with PPID, cortisol concentration returns to ≥ 80% of baseline 24 hours after dexamethasone administration; healthy horses still have suppressed cortisol concentrations 24 hours after dexamethasone administration.
False negatives and false positives are possible with dexamethasone suppression testing.
The thyrotropin-releasing hormone (TRH) stimulation test is believed to be the most sensitive means to detect a pituitary tumor. Plasma endogenous ACTH concentration is measured 10 minutes after administration of TRH (1 mg/horse, IV, once).
ACTH concentrations < 110 pg/mL after TRH administration are normal.
ACTH concentrations > 200 pg/mL are considered a positive indicator of pituitary adenoma.
ACTH concentrations of 110–200 pg/mL are equivocal (horses should be retested later).
Horses experience a seasonal rise in endogenous ACTH concentration in the fall (August through November in the Northern Hemisphere). Healthy horses are less likely to have a decreased cortisol concentration in response to dexamethasone administration, and they may also demonstrate an exaggerated response to TRH in the fall.
In light of these seasonal fluctuations in horses, only endogenous ACTH concentrations should be examined between August and December and compared with season-specific normal values. Evocative testing should be performed only from January through July because of the high percentage of false-positive results in the fall.
Pearls & Pitfalls
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For horses with suspected PPID, there are a number of diagnostic considerations:
Other conditions resulting in chronic debilitation (eg, poor management and nutrition, parasitism, and chronic systemic diseases) should be ruled out.
Polyuria and polydipsia in cases of suspected PPID must be differentiated from polyuria and polydipsia attributable to chronic renal disease or diabetes insipidus.
Hyperglycemia, glucosuria, polyuria, and polydipsia in cases of suspected PPID must be differentiated from those attributable to primary diabetes mellitus.
High insulin concentrations or an increased glucose:insulin ratio must be differentiated from primary hyperinsulinemia (insulin dysregulation, equine metabolic syndrome).
Pheochromocytomas may cause hyperhidrosis, hyperglycemia, and tachypnea; however, these tumors usually are nonfunctional and found only incidentally at necropsy.
Differential diagnosis for hypertrichosis includes being of the Bashkir Curly breed or having a congenital curly coat abnormality. Some horses, particularly ponies, grow extremely long coats in the winter, which can be mistaken as resulting from hypertrichosis. These animals shed out to normal coats in the spring, however.
There is no other recognized condition besides PPID in which adult horses acquire a long, curly coat. For this reason, hypertrichosis can be considered pathognomonic for PPID.
Treatment of Hypertrichosis in Horses
Pergolide
Good husbandry
Horses with PPID are relatively fragile and have poor immune function; therefore, most PPID patients require diligent attention to good husbandry.
Pergolide (starting dosage, 2 mcg/kg, PO, every 24 hours, then titrated according to individual response), a dopaminergic receptor agonist, is currently the only agent demonstrated to decrease endogenous ACTH concentrations in horses with PPID. Clinical improvement is expected within 6–12 weeks. If clinical signs and endocrine test results fail to improve, the dosage may be increased gradually in increments of 500 mcg/horse (up to a maximum of 4 mcg/kg, PO, every 24 hours). Most horses are stabilized with a final dosage of 500–1,000 mcg/horse (0.5–1 mg/horse), PO, every 24 hours.
Reported adverse effects of pergolide treatment in horses include listlessness and anorexia. Often, these signs are transitory and resolve over time. If they do not, the dose may be decreased temporarily or split and administered every 12 hours.
Administered alone, cyproheptadine (0.25 mg/kg, PO, every 12–24 hours) has not been reported to improve clinical signs of PPID in horses; however, it may exert synergistic effects when combined with pergolide. In the author's experience, the combination may result in outcomes better than those achieved with pergolide alone.
Treatment with trilostane, a competitive 3-beta-hydroxysteroid dehydrogenase inhibitor, has not been investigated adequately in horses. However, administration of 0.4–1 mg/kg, PO, every 24 hours has improved clinical results.
Hypertrichosis in horses can be managed by frequent body hair clipping, which can also ameliorate the increased sweating that can occur in horses with long coats.
Key Points
Hypertrichosis, which develops in some older horses, is invariably caused by pituitary pars intermedia dysfunction (PPID).
PPID is treated with pergolide.
PPID causes other morbidities more serious than hypertrichosis, including desmitis, tendinitis, and increased susceptibility to infections.
Good husbandry is essential for horses with PPID, including regular farriery, dental care, and parasite management.
Horses with hypertrichosis should be tested for insulin dysregulation, which can accompany PPID and lead to laminitis.
For More Information
Equine Endocrinology Group: Pituitary Pars Intermedia Dysfunction.
McFarlane D. Equine pituitary pars intermedia dysfunction. Vet Clin North Am Equine Pract. 2011;27(1):93-113. doi:10.1016/j.cveq.2010.12.007
Kirkwood NC, Hughes KJ, Stewart AJ. Prospective case series of clinical signs and adrenocorticotropin (ACTH) concentrations in seven horses transitioning to pituitary pars intermedia dysfunction (PPID). Vet Sci. 2022;9(10):572. doi:10.3390/vetsci9100572
