In both beef cattle and chicken meat agriculture antibiotic growth promoters are added to the livestocks feed regularly. A large family of these antibiotics is the ionophore type: monensin (Rumensin), lasalocid (Avatec), salinomycin and narasin. These substances are highly toxic to horses. Symptoms can range from rapid death to a presentation of chronic wasting, weakness, and incoordination. This article describes how horses are poisoned, clinical signs, treatment and prognosis.

Acute Poisoning

Clinical signs will vary depending on which of the drugs is involved and the amount of drug ingested. Salinomycin and monensin can cause death with as little as 1 mg / kg and much lower doses will cause clinical signs. Lasolcid is about 1 / 10 as toxic. Acute poisoning should be suspected when a food change is accompanied by rapid loss of appetite and death in about eight to 24 hours. If the dosage is not too high there may be inappetance, depression, colic, mild diarrhea, incoordination, weakness, and stiffness. There may be signs of brain involvement including blindness and head pressing. Initially there may be frequent urination and hematuria then followed by little or no urination. The horse may become recumbent though make repeated attempts to rise. On physical exam the mucous membranes will be reddish to purplish and the heart rate elevated and perhaps irregular.

Death usually follows the onset of clinical signs within 24 hours though some less severely affected may last two or three days. Those that do not become recumbent may recover. However many of these will develop scarring on the heart muscle and remain intolerant of exercise or die weeks to months later from heart failure.

Chronic Poisoning

With no level long term exposure to ionophore antibiotics horses may lose condition, have low grade fever, develop a poor hair coat, become depressed and develop a weakness or ataxic gait.