I recently became aware of a situation in which horses might have been exposed to ionophores in an equine feed. Ionophores (salinomycin, monensin, lasolocid, semduramicin, narasin, etc.) are types of antibiotics added to cattle, swine and poultry feed that help to prevent infection, promote growth and increase feed efficiency. They act by altering the electrical activity across cell membranes.
In horses, even small amounts of these antibiotics can be toxic. For example, the LD50 (the dose that is lethal in 50% of the population) of monensin in poultry is 214 mg/kg BW of the animal, 26.4 mg/kg BW in cattle, 16.8 mg/kg BW in pigs, and only 1.4 mg/kg in horses. Therefore, poisoning in horses will depend on the both the concentration of the compound in the feed, and the amount of feed consumed. For example, cattle feed typically contains 33 ppm monensin – or 33 mg/kg of feed. The LD50 for a 500 kg horse would be 700 mg (500kg BW x 1.4 mg/kg BW). So a horse would need to eat 21 kg of that product to reach the toxic dose. However poultry feed might be closer to 250 ppm monensin – so a horse would only need to eat 2.8 kg of poultry feed. Also recall that the LD50 of 1.4 mg/kg BW is an “LD50” – which means that 50% of animals are susceptible to even smaller amounts.
So how does this get into horses? Sometimes there is an outright feed mixing error. Instead of one compound added to a mixed feed/supplement, another might be added in error. Or an error in delivery could place the ionophores in a different storage bin, potentially labeled as something else. Typically, however, it happens when feed (or a supplement) is made at a facility that also produces products intended for livestock species. Because of this, ionophores will be at the facility, and errors can occur. Also, if the mixing and pelleting machinery is not adequately cleaned between feed types (for example, cattle to horse feed) there could be some residual ionophores that can contaminate the equine product.
If a horse is exposed to ionophores, the changes in electrical activity in heart and muscle cells can cause heart attacks, colic, respiratory problems, weakness, etc. Horses may die suddenly, while others may survive but have long lasting damage that may be undetected.
If ionophore toxicity is suspected, you should remove the suspected feed immediately and call your veterinarian, who will likely flush out the digestive tract to prevent any additional absorption of the compounds. It is also strongly recommended that you have that suspected feed sent to an independent facility to test for all known equine toxins.
So how do you prevent ionophore toxicity? I recommend finding out where your feeds are actually produced. Are they made in a facility that also makes livestock products? I typically try to only purchase feeds or supplements that come from mills that only make equine products. That way there is a far lower risk that any ionophores or antibiotics will be at the property, or inadvertently mixed with your horse feed.