39 LABORATORY TESTING FOR EQUINE PITUITARY DISEASE
1 What causes equine Cushing’s syndrome?
Equine Cushing’s syndrome is due to the presence of chronic excessive circulating glucocorticoids. The syndrome is almost always associated with a tumor or hyperplasia of cells in the pars intermedia of the anterior lobe of the pituitary gland. The tumor causes production of excess adrenocorticotropic hormone (ACTH, adrenocorticotropin) and related peptides. These stimulate the adrenal gland to overproduce glucocorticoids and results in loss of the normal circadian regulation of cortisol in affected horses.
2 Are there typical complete blood count and serum biochemical abnormalities in equine Cushing’s syndrome?
Complete blood count (CBC) often, but not always, shows a mature neutrophilia and lymphopenia. Mild to moderate hyperglycemia is the most common serum biochemical abnormality in this equine pituitary dysfunction, often accompanied by glucosuria.
3 What is the best test to diagnose pituitary pars intermedia dysfunction in horses?
As with other endocrine diseases, several different tests and protocols may be used to diagnose equine pituitary pars intermedia dysfunction, and none of them is always accurate in making the diagnosis. The tests are most diagnostic when used in conjunction with observation of appropriate clinical signs and history.
The test recommended most often is the dexamethasone suppression test (DST). In horses, DST is best performed as an overnight test, with a baseline plasma sample taken before intramuscular injection of 40 μg/kg body weight of dexamethasone. Because of the normal diurnal pattern of cortisol production, DST should be started in the late afternoon at around 5 PM. A single follow-up plasma sample is taken at 19 hours after stimulation. Normal horses will have plasma cortisol levels suppressed to less than 1.0 μg/dl at this time. Cortisol concentrations greater than 1.0 μg/dl at 19 hours are considered diagnostic for equine pituitary pars intermedia dysfunction. Some horses show partial suppression and may be in earlier stages of disease or have less severe disease, although this has not been well documented.
Although concerns about exacerbation of laminitis have been reported with this DST protocol, it does not appear to be a common finding.
4 Is the ACTH stimulation test useful for diagnosing equine Cushing’s syndrome?
Although this test has been used in horses, ACTH stimulation does not reliably distinguish between normal and affected horses and is not recommended.
5 Can measurement of endogenous plasma ACTH diagnose equine pituitary pars intermedia dysfunction?
Mean plasma concentrations of ACTH have been shown to be significantly higher in affected horses than in normal horses. An ACTH concentration greater than 50 pg/ml may be used as an indicator of disease, although this varies among laboratories. This test appears to have fairly good sensitivity and specificity. Although equine ACTH may be somewhat more stable than canine ACTH when not immediately frozen, samples still need to be handled with care, including collection into plastic tubes, prompt separation, and timely shipping of frozen samples to the laboratory. There is variability in the assay, and the reagents used and interpretation of results should be based on validation and reference range studies performed at each laboratory.
The plasma ACTH test may be a good substitute for the DST in horses considered at high risk for the development of laminitis.
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