Ionophore antibiotics are commonly used in food producing species as a treatment for coccidiosis in poultry and as a growth promotant in ruminants. The latter effect is due to their ability to improve feed efficiency and weight gain in ruminants.
There are several commercially available ionophore antibiotics, including lasalocid, salinomycin, monensin, laidlomycin, maduramicin and narasin. Availability of each is dependent on the specific country. Monensin is the drug most commonly associated with toxicity in horses. Toxicity has also been reported in other ‘non-target’ species, including sheep, pigs, dogs and cats. Horses appear to be highly sensitive to the effects of ionophore antibiotics. The dose that can kill 50% of horses (LD50) has been estimated at only 2-3 mg/kg (toxicity in cattle is 20-34 mg/kg). Lasalocid, a poultry feed additive, is less toxic than monensin in horses (LD50 21.5 mg/kg).
How do poisonings commonly occur?
Most commonly toxicity is caused by providing horses accidental access to feeds intended for target species, such as poultry or cattle. Some cases occur due to misformulation or contamination of horse feeds at feed mills.
How do ionophore antibiotics cause toxicity in horses?
Ultimately these drugs exert their effects by altering ion movement across cell membranes. There is a toxic buildup of calcium inside cells. Within cells it is the mitochondria, the energy powerhouse of the cell, that are most affected. Dysfunction of the mitochondria leads to a cellular energy deficit, and cell death. The toxic effects occur in a range of body systems, including skeletal muscle, liver, kidney, and nervous system. In horses, most of the clinical effects are due to damage of heart muscle.
What are the clinical signs in horses?
Signs begin quickly after exposure, usually within 2-3 hours. The signs include an elevated heart rate, sweating, difficulty breathing, weakness, ataxia, colic, diarrhoea, fine muscular tremors, and collapse. The muscle tremors are worse in the hindlimbs. There may be darkened urine due to myoglobinuria (secondary to muscle breakdown). Other signs of acute cardiac disease include jugular filling with pulsation, weak arterial pulses, and arrhythmias.
Unfortunately for some horses death occurs within 24 hours. Animals who have consumed lesser amounts may succumb days, weeks or even months after the exposure. The chronic signs are less well defined, but include lethargy, exercise intolerance, weight loss, muscle atrophy and tachycardia, with or without arrhythmia. Many of these signs are attributable to progressive myocardial and renal insufficiency. Echocardiography confirms myocardial failure, typified by abnormal (decreased) fractional shortening of the left ventricle.
Treatment
Acute treatment often includes administration of mineral oil or activated charcoal. Isotonic fluids supplemented with potassium should be given intravenously. Intravenous calcium should be avoided. Vitamin E and selenium supplementation may be helpful. Stall rest is recommended.
There is no antidote for ionophore intoxication.
Horses with signs of heart disease after a previous exposure have a poor prognosis.