Cushing’s difficult to diagnose early - Veterinary Practice
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InFocus

Cushing’s difficult to diagnose early

reports on a paper presented at a recent conference on laminitis in the USA

“BECAUSE horses afflicted with
equine Cushing’s disease often
develop laminitis, advances in
diagnosis and treatment of
Cushing’s can help veterinarians
better prevent and/or treat the
laminitis that stems from it,” Dr
Nora Grenager, of
the Steinbeck Country Equine Clinic, in Salinas, California, told delegates at the Laminitis West Conference held in California in September.

Horses with
Cushing’s (pituitary
pars intermedia dysfunction or PPID)
have an eight-fold decrease in levels
of dopamine and its metabolites
compared with non-afflicted horses
of the same age, she said. This was
most likely due to localised oxidative
stress but she commented that
evidence was lacking that this
decrease was caused by systemic
inflammation or systemic oxidative
stress.

Other, less likely, factors that have not yet been well-evaluated could
include toxins or pesticides, genetic
predisposition, chronic inflammation
or abnormal systemic metabolism,
and a diet lacking enough selenium or
protein.

Cushing’s horses are frequently
insulin-resistant, a condition that also can
be a factor in non-
Cushing’s-related
laminitis cases, but a
simple blood test can
often identify horses
with insulin resistance.
Part of the problem
with Cushing’s is that
it can be difficult to diagnose early because
horses can present with widely divergent clinical signs.
“The ones where we need help are the ones where the diagnosis is not so
clear,” Dr Grenager continued. “The
bad news is that we don’t yet have a
great test for those horses. That’s
what researchers are working on.”

She went on to discuss common
Cushing’s tests, particularly the
dexamethasone suppression test
(DST) and the resting endogenous adrenocorticotropic hormone
(ACTH) test. These tests are usually
used in an attempt to diagnose horses
that don’t show dramatic clinical
Cushing’s signs.

While the DST can be effective, it requires two veterinary
visits and administration
of steroids (which can on
occasion trigger laminitis
in susceptible horses). It
can also give early false
negatives and show
seasonal variations – false
positives are more likely
to occur when testing in
late summer and autumn.
A combined
DST/thyrotropin-
releasing stimulation test
might show fewer false
positives, but it is a 24-hour test that
requires four blood samples.

The ACTH test, another effective
test, requires only one veterinary visit
and no steroids.

Dr Grenager prefers the newer
domperidone stimulation test because
it is a safe test that doesn’t take long
and has been shown to be more
effective year-round, avoiding the seasonal variations in test results. It is
more expensive, however, and its
reliability is not yet well-documented.

Regarding Cushing’s treatments,
the speaker noted that pergolide is the
most effective treatment, but it can be expensive;
cyproheptadine is less
expensive yet not as
effective. She said that
more studies are needed
on both medications, and
on newer medications
that work on Cushing’s
in humans and could
potentially be explored
for use in horses.

It is usual to
prescribe specific diet
and exercise regimens,
which can help minimise

the risk of laminitis.
Dr Grenager also recommended an
emphasis on good foot and dental
care, routine faecal egg count testing,
and body clipping if medication is not
controlling the long hair coat. She
advised watching for clinical signs and
diagnosing and treating affected
horses as soon as possible to avoid
Cushing’s-associated laminitis.

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