EQUINE | Equine Disease Quarterly
disease causing tying-up in Arabian horses. Advanced
molecular techniques are also being applied to some
disorders to determine exactly what goes wrong at
the cellular level, to help define the basis of clinical
signs and to better direct the development of effective
genetic tests.
Currently, the most clearly defined muscular disorders
of athletic breeds are polysaccharide storage myopathy
(PSSM), hyperkalemic periodic paralysis (HYPP), and
glycogen branching enzyme deficiency (GBED),
because the cellular nature of these diseases and the
underlying causative genetic mutations are largely
defined. PSSM is common in Quarter horses and related
breeds, and typically causes signs of tying-up. It is also
linked to high body condition, which can be desirable
in those breeds. HYPP causes weakness and paralysis
and, occasionally, death. It is also associated with high
muscle mass, and therefore became prevalent due to
positive selection pressure for this trait. GBED is an
inevitably fatal disorder of Quarter Horses and Paints,
causing weakness and death in the first days to weeks
of life. It is uncommon, and association with a positive
trait is not recognized. All three of these disorders can
be readily tested for on hair or blood.
Standardbred and Thoroughbred horses appear to
share a disorder causing tying-up during training,
with clinical signs rarely or never seen during actual
racing exercise. This disease is referred to as recurrent
exertional rhabdomyolysis (RER). It appears to be more
common in faster horses because it might permit more
rapid muscular processes at the cellular level, hence
there has been positive selection pressure on this
disease. The genetics of RER have proven frustrating
to define despite many years of work. Warmblood and
Arabian horses might also share a disorder recently
coined as “myofibrillar myopathy” (MFM). Signs of this
disease appear to consist of tying-up in Arabian horses,
and poor performance and poorly defined movement
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abnormalities in Warmblood horses. The genetic cause
of MFM is not known and no scientifically validated
genetic tests exist for MFM or RER.
In summary, muscular disorders are common in
athletic horses, and can be frustrating and even fatal
under some circumstances. Association with specific
positive traits has contributed to the prevalence of
some of these diseases. Genetic testing is available
for many heritable equine diseases, and it is likely
that testing for less clearly defined disorders will be
developed in the coming years. Genetic stewardship
relies on breeders being educated about heritable
disorders, and performing proactive breeding selection
processes to produce horses that are not only high
performers but also genetically sound.
CONTACT:
Erica McKenzie, BSc, BVMS, PhD, DACVIM, DACVSMR
[email protected]
(541) 737-2858
Carlson College of Veterinary Medicine
Oregon State University
Corvallis, OR
Equine Renal Disease
Renal function is an important component of overall
health in any species. The kidneys perform several
important functions, including waste removal,
electrolyte balance, blood pressure maintenance,
a calcium source for bone health and production of
factors for red blood cell stimulation, to name a few. In
addition, the kidneys receive substantial blood flow—
approximately 25% of the cardiac output. Therefore,
changes in blood flow, either increased or decreased,
can have a significant impact on renal health.
Equine necropsy cases submitted to the University of
Kentucky Veterinary Diagnostic Laboratory (UKVDL)
• Equine Health Update •