Published March, 2012

A recent recall of horse feed contaminated with the ionophore antibiotic monensin has prompted many questions about this feed additive and its risks to horses. 

Ionophore antibiotics are feed additives used to improve weight gain and control coccidiosis (a protozoan infection) in ruminants, swine, and poultry. Ionophores can also potentially reduce the incidence of a number of digestive and respiratory conditions in cattle. Several ionophores are approved for use in the United States, including monensin, lasalocid, salinomycin, narasin, maduramicin, laidlomycin, and semduramicin. 
However, horse feeds should never intentionally contain ionophores, as ingesting large amounts can be fatal to a horse. Occasionally, horse feeds are contaminated; the risk this poses to horses depends on how much ionophore is in the feed and how much the horse ingests. 
Many cases of ionophore poisoning occur in non-target species (i.e., species for which ionophore use is not approved). However, poisoning can also occur in the intended species if an animal ingests excessive amounts. Most often this occurs when excessive amounts of ionophores are accidentally added to feeds, or when animals have accidental access to concentrated pre-mix formulations. 
Horses are much more susceptible to ionophore toxicity than are other species. For example, horses are nearly 20 times more sensitive than cattle and 200 times more sensitive than poultry to monensin toxicity, on a mg monensin per kg of body weight basis. However, not all ionophore exposures are necessarily dangerous to horses; risk depends on the amount ingested. If a horse eats a just few mouthfuls of cattle feed containing the approved amount of monensin for cattle (33 ppm or parts per million), for example, the horse will suffer no adverse effects. Even if the horse eats a small amount of this feed every day for several weeks, the horse might develop only transient anorexia and nothing more serious. However, if a horse ingests several pounds of feed containing very large amounts of monensin, such as 200-300 ppm, this could easily cause death. Studies have indicated that dosages of 1.4 mg monensin per kg of body weight can be fatal to a horse. Minimum toxic dosages for many of the other ionophores have not been well established in horses.
Ionophore intoxication damages heart and skeletal muscle and has other effects by mechanisms that are not well understood. Clinical signs of ionophore poisoning in horses vary depending on the dosage ingested, but can include poor appetite and feed refusal of the grain product, diarrhea, weakness, rapid heart rate, depression, wobbly gait, colic, sweating, recumbency, and sudden death. Animals that recover from sublethal poisoning can develop chronic heart failure resulting in exercise intolerance, poor performance, and death. Diagnosing ionophore poisoning can be difficult, but suspect a feed-related toxicity if horses becoming sick after being fed a new batch of feed and/or multiple horses are affected at the same time. Contact your veterinarian immediately if you suspect your horse might have been exposed to ionophores. Diagnostic tests can be performed on the patient and on the feed to help determine if exposure has occurred. Remove the suspected feed immediately, but save it for testing if needed. As discussed above, the mere presence of an ionophore in the feed is not proof of poisoning; the amount matters, so quantitative analyses should be performed on a representative sample of the suspect feed. 
If a veterinarian can initiate treatment immediately after exposure, decontamination can be attempted. However, in most cases the exposure is not realized until after clinical signs have started. There is no antidote for ionophore poisoning, but veterinarians can initiate symptomatic and supportive care. Some horses might die regardless of treatment, others might recover over days, weeks or months and return to performance, and some will develop permanent heart damage and never fully recover. 
Prevention is important. Do not allow horses access to feeds or feed pre-mixes prepared for other species, and purchase all horse feeds from reputable sources that practice good quality control measures. Consider signing up to receive automated feed recall notifications from the FDA (see the FDA Center for Veterinary Medicine website for further information on recall notification). 
Cynthia Gaskill, DVM, PhD, clinical veterinary toxicologist at the University of Kentucky Veterinary Diagnostic Laboratory, provided this information. Contact information her via phone: 859/257-7912, or e-mail: