Equine pituitary pars intermedia dysfunction (PPID) (Equine Cushing’s disease)
PPID is caused by an adenoma of the pituitary pars intermedia (PIA). This produces excess secretions of a variety of closely related peptides including ACTH, which causes adrenocortical hypertrophy. It is a disease of the older equine (>15years) with hirsutism, weight loss, lethargy, laminitis, polyuria, polydipsia and hyperhidrosis as the most common clinical signs. Basal cortisol levels are often within or below the normal range unless the animal is acutely stressed due to pain possibly caused by an attack of laminitis. Insulin levels are frequently raised but insulin alone should not be used to diagnose PIA. No single laboratory test is the best for diagnosing classic PPID. If possible, more than one of the following tests should be used to avoid the high rates of false positive and false negative results noted with individual tests. Endogenous ACTH is considered to be a very sensitive test and should be done in conjunction with either the straight overnight dexamethasone test or the combined TRH/overnight dexamethasone test.
This test is currently considered the most reliable one. After a significant amount of research with regards to sampling and affect on the results it appears that the ACTH is more stable than first thought. This is because the method used at our laboratory is by RIA and thus more sensitive. Much more of the endogenous ACTH is picked up by this assay. This method also has the benefits that because of this the cut-off range for ‘normal’ and ‘PPID’ cases is significantly different. Thus the now known effect of the ACTH levels being increased in ‘normal’ and ‘PPID’ cases in the Autumn months (August, September, October) does not apply to our RIA method (Copas and Durham 2012). Thus it can be used all year round. The higher sensitivity of the RIA method for ACTH means greater levels of ACTH are picked up and a TRH stimulation test with measurement of Pre and Post ACTH is not necessary.
EDTA plasma or whole EDTA blood (this latter needs to be received by the lab within 24 hours of taking the sample however) may be submitted.
The sample should be chilled but freezing is not necessary. In cases of PPID the endogenous ACTH concentration may be increased to >30pg/ml. Normal animals usually have ACTH values of <100pg/ml. Note that ACTH production by equine pituitary tumours can be variable and some pituitary adenomas do not result in elevated ACTH.
Overnight dexamethasone suppression test
This was considered to be the best test to use but it has the disadvantage that it should not be used in the Autumn months (August, September, October) due to fact that there is an increased number of healthy horses that will have increased cortisol post dexamethasone and thus increased false positives. However at other times it can be used and is considered an economic and efficient test.
- Take a basal blood sample at about 17.00 hours (5pm)
- Inject 0.04mg/kg Dexamethasone i/m
- Take a further blood sample 20 hours later
- Label samples clearly and request cortisol
Healthy horses should suppress cortisol levels to below 30nmol/l. Please note normal horses may give false positive results if the test is done in the Autumn months (August, September, October).
TRH stimulation test
The TRH stimulation test may be helpful in confirming a diagnosis of hyperadrenocorticism (pituitary adenoma) in horses. However, a study suggests that the TRH Stimulation test alone was not able to distinguish between normal and ECD cases (Eiler et al. 1997).
- Take 5ml of clotted or heparinised blood
- Inject 1mg of thyrotropin releasing hormone (TRH) slowly i/v over one minute
- Take two further samples of 5ml clotted or heparinised blood at 15 minutes and 60 minutes post TRH injection
- If possible, separate the serum or plasma before dispatch to the laboratory and label the tubes carefully
Normal horses show up to a 20% increase in cortisol concentration at 15 minutes post TRH stimulation (mean 17% increase) but levels usually return to baseline values by 60 minutes (mean 5% decrease) (Beech and Garcia, 1985).
Horses with PPID show more than a 50% rise in cortisol concentrations 15 minutes post TRH (mean 90% increase) and levels often remain elevated at 60 minutes (mean 58% increase).
Urine cortisol:creatinine ratio
This is a very sensitive screening test to exclude PPID but must not be used to make a diagnosis.
This is because it is affected by the same factors as cortisol levels. Dilute urine in cases with PuPd may not give reliable results.
A morning urine sample is collected for analysis. Normal horses have a CCR less than 20 x 10^6.
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