RESEARCHERS at the Royal
Veterinary College have developed a
test which may reliably identify
horses susceptible to exertional
rhabdomyolysis syndrome before
they enter training.
Dr Richard Piercy, senior lecturer in
equine medicine at the
RVC, described
progress with the project at a
symposium at
Cheltenham in
November focusing on
research supported by
the Horserace Betting
Levy Board.
The condition, also
known as setfast or
tying up, affects
between five and seven
per cent of
thoroughbred horses
in training and has a
seriously detrimental effect on racing
performance.
It causes repeated episodes of
muscle damage, resulting in signs
ranging from muscle stiffness to severe
myoglobulinuria with a potentially fatal
outcome. Its cause is still unknown
although it is thought likely to result
from a combination of genetic and
environmental factors. As well as
improving diagnosis, the test may be
helpful in identifying the exact cause
and in testing novel treatments, he said.
Metabolism defect
Previous studies by US researchers have
identified a defect in calcium
metabolism in horses with a propensity
to develop the condition, which is
normally diagnosed in clinical cases by
measuring creatine kinase and aspartate
aminotransferase levels in the blood.
Muscle fibres taken from these
horses show an abnormal sensitivity to
the effects of caffeine and other agents
which affect muscle contraction by
increasing cellular calcium levels.
However, this test is based on a biopsy
sample from the intercostal muscles
which not only leaves a scar, but it can
also result in a pneumothorax if the
sampling technique is poor.
Dr Piercy’s team have developed a
minimally invasive alternative based on
small skin biopsy samples which can then be cultured in the laboratory and
converted into muscle cells through the
action of a muscle-specific transcription
agent known as MyoD.
The relevant gene for this protein
has been cloned into a virus which is
then used to infect the cultured skin cells and change their
morphology. Lab tests
have shown that these
newly created muscle
cells have the same
response to caffeine
and similar agents as
normal muscle cells.
A wide range of
treatments and nutritional
supplements have
been marketed to
thoroughbred trainers
as being protective
against the effects of exertional rhabdomyolysis in their animals.
However, the only treatment with
proven efficacy is dantrolene, a drug
which appears to have an opposite
effect to caffeine on the ryanodene
receptor involved in calcium transport
within muscle cells.
Dantrolene is often given by
trainers to horses with a history of the
condition to prevent further damage
while they build up fitness in
preparation for a race, although
standard withdrawal times must be
observed in order to avoid infringing
the rules of racing.
Dietary adjustments, including a
high fat, low carbohydrate diet and
avoiding sudden changes in the training
schedule, have also been suggested as
strategies which may protect against
muscle damage.
“Other than these
recommendations, there is very little
convincing evidence that any other
management, supplement, drug or diet
is beneficial. Our hope is that our new
lab-based technique will allow us and
others to investigate novel treatments
and perhaps even a cure,” he said.
A new laboratory method has also
been approved this year for inclusion in
the codes of practice for the prevention
of contagious equine metritis and other
venereal conditions in breeding units,
Chris Rea from the Three Counties Equine Hospital in Tewkesbury told the
meeting.
Mr Rea sits on the HBLB’s
veterinary advisory committee, which
has recommended the use of the
polymerase chain reaction technique for
use in testing swabs taken from mares
for the causative organism of CEM, the
bacterium Taylorella equigenitalis. It
may also be used in testing semen
samples for the equine viral arteritis
agent.
In the case of the CEM organism,
the new test has advantages over
traditional culture methods in providing
a result within two days rather than
seven.
It can also be used on samples that
have become too degraded for
conventional culture and will distinguish
between the pathogenic bacterium and
the closely related but benign organism,
T. Asinigenitalis, found mostly in
donkeys.
This latter benefit is important
because it protects the UK’s important
export trade from future disruption.
The greater precision will dissuade
importing countries placing restrictions
on admitting horses from states where
there have been incidents of samples
wrongly classed as positive to T.
equigenitalis on culture.
Mr Rea said the code has been a
major factor in successfully guarding
Britain against major epidemics of
equine reproductive diseases over the
past 30 years and encouraged vets and
horse owners to continue supporting
regular sampling from adult horses of
all breeds.
Anthelmintic resistance
Testing in equine practice laboratories
will also help to protect the UK horse
industry against the damage that may
be caused by the emergence of
anthelmintic-resistant strains of small
cyathastome worms, said Professor
Jackie Matthews of the Glasgow
veterinary school.
Preventing the further spread of
resistance to the only three groups of
anthelmintics currently available will
depend on the development and use of
sustainable worming strategies, she said.
These will be based on routine
testing to assess the susceptibility of
resident worms to the drug class
intended for use. Analyses of the worm
burden in individual animals will also
help to maintain an effective armoury
of drugs by focusing treatment on the
20% of individual animals that carry
80% of the worm burden on any
equine premises.
Professor Matthews reviewed
progress in developing new, more
effective methods for analysing worm
counts and assessing the likely efficacy
of treatment, including new DNA-
based methods for detecting the genes
responsible for anthelmintic resistance.
But education of horse owners was
an equally important element in
persuading owners to pay for
susceptibility tests rather than
implementing blanket treatment
strategies for all horses.
Targeted treatment will reduce the
costs of treatment even on holdings
where owners would have to pay for
individual faecal egg count tests on
dozens of animals, she said.
