Equine Cushings disease (HAC) 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 (>7 years) with hirsuitism,
weight loss, lethargy, laminitis, polyuria, polydipsia and hyperhidrosis
as the most common clinical signs.
Urine Cortisol/Creatinine
Ratio
Urine Cortisol/Creatinine Ratio
(UCCR) - This can be useful as a screening test to exclude HAC
but must not be used to diagnose HAC as it is not specific and
other factors such as exercise may give a positive result. A
morning urine sample is collected for analysis. Normal horses
have a CCR less than 20 x 10-6.
Overnight Dexamethasone
Suppression Test
1. Take a basal blood sample at
about 17.00 hours (5 pm).
2. Inject 0.04mg/kg Dexamethasone i/m.
3. Take a further blood sample 20 hours later.
4. Label samples clearly and request cortisol.
Interpretation
Normal horses should suppress cortisol levels to below 30 nmol/L
TRH Stimulation
Test
1. Take basal blood sample.
2. Inject 1 mg TRH (Cambridge Laboratories) i/v. slowly over
one minute.
3. Take two further blood samples at 15 minutes and 60 minutes
later.
4. Label samples clearly and request cortisol.
Interpretation
Normal animals show up to a 20% rise in cortisol concentration
15 minutes post TRH stimulation which usually returns to baseline
at 60 minutes.
PIA animals show more than a 50% rise after 15 minutes and cortisol
levels often remain elevated at 60 minutes.
Combined Dexamethasone/TRH
Stimulation Test
Recent studies have shown this test is thought to be the
most sensitive dynamic test for the diagnosis of PIA in equines.
1. Take basal blood sample.
2. Inject 40ug/kg Dexamethasone i/v.
3 Take another blood sample 3 hours later.
4. Immediately inject 1.1 mg TRH (Cambridge Laboratories)
i/v. slowly over one minute.
5. Take two further blood samples 40 minutes and 19 - 21 hours
later (22 - 24 hours after Dexamethasone).
6. Label samples clearly and request cortisol.
Interpretation
Normal and PIA animals show depression from basal levels at
3 hours (usually > 50%). 40 minutes after TRH, normal horses
depress further but PIA horses show a significant rise in
cortisol (> 50% from 3 hour sample).
At 22 - 24 hours normal horses must depress below 30 nmol/L
and PIA horses remain elevated, usually above the value of
the 3 hour sample.
Insulin
Insulin The analysis of Insulin
on a basal serum sample is very useful as this detects the presence
of peripheral insulin resistance. Insulin levels are usually
very high (>250 mU/mL) in PIA. Normal equines have insulin levels
less than 40 mU/mL.
ACTH - Endogenous
In equines a single sample analysed for ACTH has been shown
to be a very sensitive (100%) test for equine PIA. The sampling
procedure for ACTH is critical. See canine adrenal section
for sampling
procedure.
