South African Equine Veterinary Association Congress 2015 Protea Hotel Stellenbosch
(McKenzie et al. 2002). In comparison with controls, normal Thoroughbreds administered
frusemide and sodium b i c a r b o n a t e developed lower plasma calcium, chloride, magnesium
and potassium concentrations. Following exercise, serum CK activity was found to be
significantly higher in the treated group (Freestone et al. 1991). Some evidence therefore
suggests that electrolyte imbalance may play a role in the development of certain forms of
the disorder.
Hormonal influence
Many studies report a higher incidence of ER in female horses compared with males
(Harris 1991; MacLeay et al. 1999a; McGowan et al. 2002). No correlation was found
however between the stage of the oestrus cycle and plasma CK activities in thoroughbreds in
training (Frauenfelder et al. 1986), suggesting that a direct association of female sex hormones
with ER is unlikely. The higher incidence of ER in females remains unexplained, although it
may relate to differences in temperament.
Infectious causes
Equine herpes virus 1 (EHV1) infection was proposed as causing an outbreak of ER in a
training yard, where several horses seroconverted to the virus (Harris 1990). Equine Influenza
Virus (EIV) has also been diagnosed by seroconversion as the potential cause of certain
equine myopathies (Freestone and Carlson 1991). Both EIV and EHV1 infections are common
in groups of young racehorses and cause and effect have not been fully established; as with
other acquired causes, viral infection may modify the phenotype of genetically susceptible
horses.
Inherited causes
We currently recognise several forms of exercise-related myopathy in horses with underlying
genetic causes. These include a condition examined extensively in a small group of
Thoroughbreds in the USA that has been termed „recurrent exertional rhabdomyolysis‟ (RER)
(Lentz et al. 1999) and another condition, termed „polysaccharide storage myopathy‟ (PSSM
or EPSM) (Valberg et al. 1992; Valentine et al. 2000). These conditions have clinical and
clinicopathological similarities and are managed similarly, though they also have key
differences and breed susceptibilities (Valberg et al. 1999) and aetiologies.
Recurrent exertional rhabdomyolysis
The word „recurrent‟ in Thoroughbreds with exercise-related myopathy has been used variably
to describe certain Thoroughbreds with documented abnormalities in muscle calcium
regulation (Lentz et al. 1999), a wider group of Thoroughbreds with an apparent inherited
form of ER (MacLeay et al. 1999b) and all Thoroughbreds with a susceptibility to the
syndrome. In humans there are many genetic causes of rhabdomyolysis (e.g. mutations in
genes encoding enzymes involved in cellular metabolism or structural proteins); it is currently
unknown how many different forms of rhabdomyolysis affect Thoroughbreds.
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