Page 937 - Equine Clinical Medicine, Surgery and Reproduction, 2nd Edition

912 CHAPTER 6

VetBooks.ir axis more sensitive to decreases in serum calcium, proopiomelanocortin (POMC), a precursor
peptide that is split into adrenocorticotropic
allowing more rapid mobilisation of calcium from
bone in times of acute need. Calcium supplementa-
tion should be provided during times of acute loss hormone (ACTH), α-melanocyte-stimulating
hormone (α-MSH), corticotropin-like immuno-
(e.g. during exertion that causes profuse sweating reactive peptide and β-endorphins. The reason
or during early lactation). for the decrease in hypothalamic inhibition is
currently unknown, but recent studies suggest
Prognosis that inflammation and oxidative stress over time
If properly recognised and treated, the prognosis for lead to neurodegeneration. Certain breeds, such
recovery from hypocalcaemia is excellent. However, as Morgan horses and ponies, may be genetically
if hypocalcaemia is unrecognised in severely affected predisposed to developing PPID.
(e.g. recumbent) horses, the prognosis is poor, and The exact consequences of the decreased inhibi-
death may occur before the results of plasma calcium tion of the melanotrophs and increased secretion of
measurements are known. Horses with primary ACTH, α-MSH and β-endorphins is poorly under-
hypoparathyroidism can be managed successfully stood. The traditional view of PPID is that clini-
with oral calcium supplementation. cal signs are caused by overproduction of ACTH,
leading to excessive stimulation of the zona fas-
PITUITARY PARS INTERMEDIA ciculata of the adrenal gland producing glucocor-
DYSFUNCTION ticoids, such as cortisol. However, baseline plasma
concentrations of cortisol are usually within the
Other and previous names: equine Cushing’s disease, normal range in horses with PPID. Recent studies
pituitary adenoma, pituitary hypertrophy, pituitary have suggested that clinical signs could be caused
hyperplasia. by a loss of the normal cortisol circadian rhythm or
by other metabolites and peptides also produced in
Definition/overview excess (e.g. α-MSH).
PPID is a common disease of older horses affect-
ing more than 25% of horses older than 15 years Clinical presentation
of age. This condition, originally described as a The traditional clinical presentation is a lethar-
benign tumour of the pars intermedia of the pitu- gic older horse (>15 years of age) with a pendulous
itary gland, is probably more accurately described as abdomen and long, shaggy hair coat that does not
hypertrophy or hyperplasia (adenoma or adenoma- shed normally in spring (hypertrichosis) (Fig. 6.9).
tous hyperplasia) of the pituitary gland. The main Less advanced cases may have subtle clinical presen-
clinical significance of the disease are the associated tations including poor performance, loss of topline,
complications such as laminitis and recurrent infec- hoof changes in the absence of lameness and slightly
tions. The latter is reflected in an increased incidence delayed coat shedding. As the disease evolves, there
of sole abscesses, non-healing skin lesions, dental is a tendency for fat to accumulate in the crest of
disease, GI parasitism and non-responsive respira- the neck, over the rump, in the sheath of geldings
tory infections. In rare instances, the neurological and behind the eyes. Despite this fat accumula-
deficits, presumably due to the mass effect of a large tion, weight loss can occur due to muscle wasting.
tumour, may be the initial presenting clinical signs. Affected horses may be hyperhidrotic, secreting
sweat that tends to be greasy (Fig. 6.10). The long
Aetiology/pathophysiology hair coat traps sweat, exacerbating skin problems,
PPID is caused by neurodegeneration of the particularly Dermatophilus congolensis infection.
inhibitory dopaminergic hypothalamic neurons A fairly uncommon manifestation is lactation in
resulting in adenomas or adenomatous hyper- non-pregnant mares probably related to altered
plasia of the pars intermedia melanotroph popu- prolactin secretion. While it is possible for affected
lation. The melanotroph population produces mares to carry foals to term, mares are more