Ionophore antibiotics such as monensin and lasalocid are commonly used feed additives for controlling coccidiosis, bloat and improving efficiency of weight gain in cattle. They are sometimes incorporated into pellets fed on ships.
Toxicity may result from mixing errors and consequent inadvertent over exposure of animals to these compounds. Affected animals suffer muscle damage leading to acute heart failure and possibly skeletal muscle effects (paresis and paralysis). Deaths in cattle can be expected to commence at intakes of 10 mg/kg bodyweight.
There is usually sudden onset of feed refusal, recumbency, tachypnoea (fast breathing), ruminal atony, diarrhoea and brown urine. In the subsequent days to weeks there may be development of illthrift and signs of congestive heart failure including submandibular and brisket oedema, and distended and pulsating jugular veins.
Necropsies performed on animals dying acutely may not reveal any gross lesions. In longer surviving animals there may be pallor of the myocardium especially of the left ventricle reflecting areas of myocardial necrosis. There may also be signs related to congestive heart failure: pulmonary oedema, hydrothorax, ascites and hepatomegaly (enlarged liver).
Laboratory confirmation is based on specimens of myocardium and skeletal muscle submitted in buffered formalin for histology, and identification of toxic concentrations of an ionophore in samples of feed.
No antidote or specific drug treatment is available. Nursing care and careful handling is required. Survivors should be marked for salvage slaughter once residues periods have passed. In an outbreak, decisive measures to replace, reduce or dilute toxic feed are required.
Prevention relies on preventing mixing errors in formulated rations.