South African Equine Veterinary Association Congress 2015 Protea Hotel Stellenbosch
Overexertion
Extreme or unaccustomed exercise predisposes horses to rhabdomyolytic attacks. Muscle
damage in such cases may include a combination of physical damage incurred during
excessive eccentric contractions, metabolic exhaustion and oxidative injury.
Eccentric-contraction
Delayed onset muscle soreness in humans is associated with damage caused by excessive
eccentric contractions (contraction during muscle lengthening). Although the mechanism is
poorly understood there are prominent signs of damage within a muscle following eccentric
contractions that include disruption of sarcomeres. This damage is reflected by (sometimes
considerable) elevations in serum muscle enzyme activities and triggers a local inflammatory
response accompanied by oedema. It is unknown whether horses experience this syndrome,
however it seems likely.
Metabolic exhaustion
Exceeding the level of training in a horse either by excessive endurance exercise or
overexertion when galloping is believed to be a common cause of acute ER in horses (Snow
and Valberg 1994). Although the causes are numerous and likely involve electrolyte
imbalances and hyperthermia, in certain cases a presumed underlying factor is the deficiency
of ATP, which results in an inability to maintain ion homeostasis.
Antioxidant status
Free radicals are widely believed to cause post-exercise stiffness and fatigue in muscle,
through several deleterious mechanisms that include the peroxidation of lipid membranes. Cell
damage is normally minimised by the action of a complex cascade of free radical scavengers
and antioxidants, including vitamin E and the selenium-dependent enzyme, glutathione
peroxidase. When antioxidants fail to quench free radicals sufficiently, the body is subjected to
so-called „oxidant stress‟(Ji and Leichtweis 1997).
Evidence suggests that exercise-induced oxidative stress occurs in horses, but there is no clear
association with muscle damage (Chiaradia et al. 1998). This is supported by studies
demonstrating that antioxidant supplementation fails to attenuate exercise-induced elevations
in plasma CK activity in horses, (Brady et al. 1978; Siciliano et al. 1997; White et al.
2001). There is therefore no good evidence that oxidant stress plays an important primary
role in ER. Despite this, during conditions likely to result in significant oxidant stress, such
as very strenuous or prolonged exercise, antioxidant deficiencies may exacerbate muscle
damage caused by other mechanisms.
Electrolyte imbalance
Published and anecdotal reports of improvement following correction of electrolyte
clearance ratios (Harris and Colles 1988) underlie the interest in this area. In a group of 38
ER-susceptible Thoroughbreds, about a third had potassium clearance ratios of less than 30%
or low chl ori de clearance (Beech et al. 1993). These differences could potentially reflect
differences in the handling of electrolytes by ER-susceptible animals, but ER-prone horses
exhibit the same dietary-induced alterations to electrolyte clearance ratios as normal animals
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