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SUMMER 2014 ISSUE
EQUINE MATTERS
8
MUSCL E D I SEASES
A heritable disorder found in QHs and
Paint Horses. Affected foals may be
aborted, stillborn or die suddenly when
very young. A muscle biopsy is diagnostic
and a blood test is possible to identify
carriers for this inherited trait.
3.
Glycogen Branching
Enzyme Deficiency
RER (figure 1) is likely an inherited trait, as
it affects approximately 7% of UK racing
Thoroughbreds, with young fillies being
over-represented. Horses suffering from an
episode of RER will have sudden onset
muscle cramping, with reluctance to
move, sweating and marked pain on
palpation of affected muscles.
Blood sampling will reveal that muscle
enzymes are significantly raised. Muscle
biopsy results will help confirm a suspicion
of RER. Almost all animals return to athletic
function, though this can take from one
week to months for full recovery.
Managing a horse with RER requires
minimising stress for that horse, supplying
the majority of calories as fats not
carbohydrates, along with keeping regular
exercise and management routines
consistent. A drug called dantrolene is
sometimes used to help manage patients
suffering frequent recurrences.
4.
Recurrent Exertional
Rhabdomyolysis (RER)
Acquired Disorders
This is a non-painful cause of mechanical
hindlimb lameness most obvious at the
walk. The affected animal can be seen
to slam the foot down to the ground
prematurely during each stride. It can be
caused by repeated tearing of muscle
fibres in the back of the thigh during
excessive exercise, after pulling back
abruptly and whilst tied up or during sliding
stops; after intramuscular injections or it can
develop due to trauma during birth.
Surgical management involves cutting the
tendinous attachment of the affected muscle
(figure 2) under general anaesthesia (GA),
which can improve the condition, although
complete resolution is uncommon and
recurrences can occur.
1.
Fibrotic Myopathy
Horses are heavy animals and occasionally
when undergoing a GA, their large mass
squashes the blood supply to their own
muscles, resulting in tissue damage. This
results in hot and swollen muscles that
are very painful to touch, with the patient
having great difficulty getting up and
remaining standing for any length of time,
often requiring assistance. For this reason
great care is taken to evenly position
patients on a soft, well-padded operating
table during surgery, ensuring their legs
are well supported.
2.
Post-anaesthetic
Myopathy
This is a highly fatal muscle disease seen
in horses at pasture. According to recent
research published in 2013, it is caused
by ingesting a toxin present in sycamore
tree seeds. Outbreaks occur in the Autumn
and in the Spring following spells of
autumnal-like weather. Horses that develop
AM are usually kept in sparse pastures
with an accumulation of dead leaves,
dead wood and trees in or around the
pasture and are often not fed any
supplementary hay or feed. Identifying a
possible cause is a huge step towards the
prevention of this disease, as we can now
avoid horses grazing on pasture where
the seeds fall, or at least provide
supplementary feed if access to seeds
cannot be restricted. Treatment is difficult
as severe muscle damage and kidney
failure develop rapidly, with only about
10% of patients surviving.
3.
Atypical Myopathy (AM)
Figure 2:
The tendon attachment to the
muscle overlying some forceps prior to
being cut in a case of fibrotic myopathy
Figure 1:
Muscle breakdown products
(myoglobin) are present in the urine of
horses with RER, causing red coloured urine
and in some cases serious kidney damage
This condition results from a long term
(chronic) vitamin E deficiency, resulting
in severe muscle wastage and nerve
damage. As well as marked weight loss,
constant weight shifting and muscle tremors
are seen and affected animals may have
an abnormally high tail carriage. EMND
is a rare condition, but occurs in horses
without access to grass and fed poor
quality hay. Treatment is with vitamin E but
horses with clinical signs rarely recover.
4.
Equine Motor Neurone
Disease (EMND)
Summary...
When we think about muscles,
we generally consider the
effect of injuries that involve
strains or tears sustained
during ridden or pasture
activities. Whilst these are
relatively common, there are
a number of less common, but
equally as important inherited
disorders, along with several
conditions caused by factors
other than exercise, that
affect muscle metabolism.