Azoturia (Typing Up, Setfast, Exertional Rhabdomyolysis)
Clinical Signs
Azoturia is a relatively poorly understood disease, resulting in a variable
degree of muscle stiffness and pain, which occurs under a number of different
circumstances. Traditionally the problem occurs relatively soon after the
start of exercise, particularly in fit horses receiving high-concentrate
rations on a day following a rest day. The clinical signs vary from mild
hind-limb stiffness to a total reluctance to move, sweating and obvious severe
pain. The muscles of the back and hindquarters may feel unusually firm.
The exact cause of the condition is unknown. Some horses seem prone to
recurrent attacks. Highly strung individuals, especially mares, seem par-
ticularly susceptible. Vitamin E and/or selenium deficiency has been
suggested as a possible cause, but there is little data to support this
hypothesis and it is probably a multifactorial condition.
What actually happens? Whatever the actual cause, it results in damage of
the muscle cells (myopathy). Their outer membranes become more leaky,
permitting loss from the cells of two enzymes, creatine kinase and aspartate
aminotransferase. As a result, the concentration of these enzymes increase
in the blood. This rise in concentration reflects accurately the severity of
muscle damage. Within minutes of the onset of clinical signs a measurable
increase in the concentration of these enzymes may occur, and their concen-
tration continues to rise for the following twenty-four hours. Thus there is
a reliable blood test which can confirm a clinical diagnosis of azoturia.
If the damage to the cells is severe, a muscle pigment, myoglobin, may be
released into the blood. This is a large molecule which is excreted via the
kidneys causing dark discolouration of the urine. The name azoturia refers
to this. Because of their large size, myoglobin molecules may impair the
filtration mechanism of the kidney and, in extremely severe cases, cause
irreparable kidney damage.
The muscle damage causes release of lactic acid (a substance responsible for
muscle cramp) and pain. Continuing to work a mildly affected horse may
exacerbate considerably the degree of muscle damage and pain, therefore, if
azoturia is suspected, work should stop immediately and the horse returned
carefully to his stall. If some distance from home, the horse should be
transported back if at all possible.
In some horses with a recurrent problem selected blood and urine tests
(creatinine clearance test) indicate a deficiency of sodium, potassium and/or
calcium, and appropriate supplementation of the diet with these minerals has
been most effective. Severely restricting the protein intake by the substi-
tution of bran may be a complicating factor because bran contains a substance
called phytate which can bind dietary calcium and prevent its absorption by
the gut, thus effectively limiting the body's supply of calcium.
Hope this helps clear up some questions.
Joyce