South African Equine Veterinary Association Congress 2015 Protea Hotel Stellenbosch
Also unlike Cushing‟s disease in humans and dogs, adrenocortical hyperplasia accompanying
equine Cushing‟s disease is relatively uncommon, occurring in ~20% of affected horses. These
differences in location and pathophysiology between human, canine, and equine pituitary
adenomas have lead several authors to suggest that the disease in horses should not be called
equine Cushing‟s disease; rather, pituitary pars intermedia dysfunction (PPID) has been advanced
as a more appropriate descriptor.
Clinical signs: The classic clinical sign of PPID in horses is hypertrichosis, a long and curly hair
coat that fails to shed. In some affected horses, coat colour changes have also been observed. The
pathogenesis of hypertrichosis, characterized by arrest of hair follicles in anagen, remains unclear.
Abnormal sweating, both hyperhidrosis and anhidrosis, is also observed in up to two-thirds of
horses with PIPD. Weight loss and lethargy, or poor performance, can also be observed in horses
with PPID. In addition to true weight loss, protein catabolism due to increased cortisol activity
leads to loss of muscle mass. This is most notable in advanced cases as a loss of epaxial and rump
musculature. Despite weight loss, appetite in affected horses is normal or even increased
(polyphagia). However, dental abnormalities, leading to painful mastication and quidding, may
compromise feed intake and contribute to weight loss in some horses. Combined with, or often
preceding, loss of muscle mass in some horses is insulin resistance and deposition of fat along the
crest of the neck, over the tail head, and in the sheath of male horses. Another area where
abnormal fat deposition may occur is above and behind the eyes (supraorbital area). Horses with
PPID have also been described as overly docile and more tolerant of pain than normal horses. The
latter signs have been attributed to increased plasma and cerebrospinal fluid concentrations of endorphin that are 60- and more than 100-fold greater, respectively, in horses with PPID than in
normal horses.
Chronic, insidious-onset laminitis is perhaps the major clinical complication of PPID with more
than 50% of horses affected in most reports. Although the condition is more amenable to
management in ponies due to their lower body weight, chronic or recurrent pain with exacerbation
of laminitis or associated foot abscesses is often the reason for euthanasia. Polydipsia and
polyuria (PU/PD) develops in about one-third of horses with PPID. Equids with PPID tend to
have delayed wound healing and are frequently affected with secondary infections. Commonly
recognized infections include skin infections (e.g., refractory “scratches” and fistulous tracts),
recurrent subsolar abscesses, conjunctivitis, sinusitis, gingivitis, alveolar periostitis, and
bronchopneumonia.
Other signs that have been reported in horses with PPID include persistent mammary secretions
and infertility. Central nervous system (CNS) dysfunction, including ataxia, blindness, and
seizure-like activity, are occasionally observed in equids with PPID. A major complication of
hypercortisolism in affected human patients is osteoporosis. Although occurrence of this
complication has not been investigated in horses, it is interesting to note that euthanasia of horses
with PPID has been reported due to development of pelvic, pedal bone, mandibular, and multiple
rib fractures.
Clinicopathologic findings: Abnormal laboratory data in horses with PPID may include mild
anaemia, an absolute or relative neutrophilia, and an absolute or relative lymphopenia. As well as
being increased in number, neutrophils in affected animals may appear hypersegmented. This
finding reflects maturity of neutrophils and can be attributed to a longer half-life of circulating
neutrophils because cortisol excess limits diapedesis from the vasculature. The most common
abnormality detected on serum biochemical evaluation is mild to moderate hyperglycaemia,
reported in 25-75% of cases, depending on the upper end of the reference range used. Additional
60