Monensin is a monovalent carboxylic acid, used in the form of the sodium salt (monensin sodium), where it is typically purchased in the form of feed additive pellets. Monensin is one of several classes of antibiotics exclusively for veterinary use, known as ionophores. They are used as anticoccidial feed additives for poultry and cows, as growth promoters, and for improved feed efficiency in ruminants.

Cattle and poultry can ingest relatively high levels of monensin (Chickens - 200 mg/kg and Cattle 50-80 mg/kg) in their feed without any negative impact on their health. Horses and donkeys, however are highly sensitive to monensin. The LD50 value for oral ingestion of monensin in a donkey or horse is 2 mg/kg. For a 1100-lb (500kg) horse, this would require only 0.035 ounces of product containing monensin.

Unfortunately, incidents of poisoning in horse feeds occurs somewhat frequently. Toxic effects of ionophores are directed mainly against skeletal and/or cardiac muscle as a result of disturbances in muscle cell calcium homeostasis followed by increased intracellular Na+ concentration. The possibility of delayed effects on the heart is of major concern in horses exposed to monensin. Horse feed contaminated with monensin can sometimes be detected through smell of the feed. It is usually associated with a strong, 'bitter' odor.

Incubation Period
The incubation period varies depending on the amount of the toxin the horse ingests. Horses can start to develop signs from several hours to 4 months from first consuming the contaminated feed. Most horses that consume toxic levels of monensin, die within 48 hours of onset of signs.