This is a degeneration of skeletal and heart muscle as a result of selenium or vitamin E deficiency. Selenium and vitamin E prevent cell damage by oxidants produced during tissue metabolism but have different sites of action.
It is usually a disease of very young animals but can occasionally affect older animals. Myopathy associated with selenium deficiency usually occurs in young animals grazing lush spring pastures in higher rainfall on lighter, acidic soils. Affected animals may show signs on entry to assembly points. Myopathy associated with vitamin E deficiency occurs in young animals fed dry feed, hay or grain over extended periods such as may occur in the live export process.
The disease is triggered by sudden exercise such as moving, yarding or transporting. Continued physical exertion is likely to worsen the condition.
Animals develop a stiff gait or die suddenly from heart failure. Others go down but remain bright and alert and are reluctant to stand. If they stand they have a hunched back and are reluctant to move. Urine is brown from myoglobin.
At necropsy there are areas of muscle that are diffuse, pale and dry and others that contain well defined chalky white streaks or patches. These can be found in the heart and tongue and distributed symmetrically in muscles of the shoulder, back and thighs. The myopathies from selenium and vitamin E deficiencies cannot be differentiated except by investigation of dietary history.
Specimens required for laboratory confirmation include heparinised blood (10 ml) submitted chilled (not frozen) for glutathione peroxidase (GSHPx) activity, from at least 5 animals in the group; at least 2 ml of serum submitted frozen for serum enzymology and vitamin E analysis; and sections of affected and unaffected skeletal muscle and cardiac muscle, in buffered formalin for histology.
Differential diagnoses include enterotoxaemia when there is sudden death and arthritis when there is stiffness.
Injections and oral drenches containing vitamin E and or selenium are available for treatment. The prognosis is good for mildly affected animals.
Selenium deficiency should be confirmed and care must be taken when treating so as not to overdose animals with selenium as the threshold for toxicity is low.
Anthelmintic drenches and vaccines containing selenium are available. A small amount of green pick, even weeds, will avert vitamin E deficiency.