Whinny’s Wisdoms

Springhill Equine Veterinary Clinic

Hi everyone, Whinny here! Halloween’s over but today I want to talk about a really scary thing for your horse’s health. Don’t we have enough things to be terrified of when it comes to horses, you say? Well yes, I agree but we’ve got to add one more. What’s kind of crazy is that a lot of people have never heard of it. Many of you have cattle, chickens, or other farm animals on the property with your horses. Do you know how super dangerous their feed can be if your horse eats any of it? That’s because of ingredients called ionophores, and they can be fatal if even a small amount is ingested. New fear unlocked, right?

What Is An Ionophore?

Ionophores are chemicals that transport ions across cell membranes. They have antimicrobial effects on parasites called coccidia, and so are used as feed additives for cattle, goats, swine, and poultry to improve weight gain and control protozoan infection. There are several kinds of ionophores used in feed, including monensin, lasalocid, salinomycin, narasin, maduramicin, semduramicin, and laidlomycin propionate. Monensin is a the prototypical ionophore and is used extensively in the beef and dairy industries. It’s also known by its trade name, Rumensin.

Why Is It So Bad For Horses?

Here’s the big problem for horse owners – while cattle and poultry can safely ingest relatively high levels of monensin in their feed, for horses, it’s a very different story. Horses are nearly 20 times more sensitive than cattle and 200 times more sensitive than poultry to monensin. The toxic dose for monensin in horses is less than 2.0 milligrams per kilogram of body weight, or about 1 gram for an average sized horse. Why is it so toxic for horses compared to other livestock? We don’t really know.

Whinny Wisdom: What’s good for the goose might be good for the gander, but it’s bad for the horse. When it comes to your animals, not sharing is caring!

 The symptoms of toxicity vary depending on the amount of monensin ingested. Trace amounts may cause a horse to go off his feed, show signs of colic and appear unwell for a few days. Larger amounts will cause a horse to show more serious symptoms within a few hours including colic, weakness, sweating, incoordination, collapse and sudden death.

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The most important result of monensin toxicity is damage to heart muscle. In a healthy heart, ion fluxes of sodium and potassium allow the heart to contract normally with each beat. It’s believed that monensin inhibits sodium and potassium ion transport across the cell membrane, preventing the heart from working properly and leading to cardiovascular collapse. Horses that recover from sublethal poisoning can develop chronic heart failure resulting in exercise intolerance, poor performance, and death. Sometimes horses die very quickly with acute heart failure. Ingestion of a large dose at one time can result in death within a few hours of eating the feed.  In other cases, they may die of heart failure in a few days or even weeks.

How Does It Happen?

Accidental poisonings can occur if there is a mixing error at a feed mill. If the mill makes cattle or poultry feed and ionophore ingredients are accidentally added to horse feed instead, the contaminated product may be distributed to horse farms. However, only a small proportion of poisoning cases are caused by feed mill errors. More commonly, a horse gets access to cattle feed by accident – either eating spilled feed or gaining access to where the feed is stored on his farm. Some owners may simply not realize it’s even a problem and offer it to their horse, assuming that feed that is good for their cattle or chickens is fine to feed to their horse as well.

Can It Be Treated?

Unfortunately, there is no antidote for monensin toxicity. The best thing you can do is to prevent any possibility of your horse getting access to feed made for other species and purchase your horse feed from a reputable company that uses strong quality control measures. If the worst happens though, call your vet immediately. There’s no time to waste here. If the ingestion was recent, your vet will pass a tube into the horse’s stomach and attempt to pump out as much as the feed as she can. She will probably administer mineral oil or activated charcoal to try to reduce absorption of the chemical. Your horse may be put on intravenous fluids for supportive care. The problem is that the toxin acts quickly, sometimes before you are even aware of the ingestion or have called your vet. And unfortunately, once the damage occurs, it is permanent. Even if the horse survives in the short term, damage to the heart muscle may cause heart failure in the future.

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Can You Test For It?

The clinical signs of poisoning can be non-specific, meaning it’s not always obvious that the horse’s sickness is due to ionophores. If multiple horses become sick after starting a new batch of feed, a feed-related toxicity should be suspected. An exam of the horse’s cardiovascular system may give evidence – the heart rate may be abnormally fast or irregular and an ultrasound exam of the heart may show dysfunction. Blood and urine samples should be run to look for abnormal muscle enzyme levels. But there may not be obvious signs that are specific for a diagnosis of ionophore toxicity. The ionophore chemicals themselves break down quickly, meaning they can be difficult to test for in the horse’s body, especially if the ingestion happened several days ago. If contaminated feed is suspected, it is usually easier to test the feed for the presence of ionophores. The suspected feed should be saved for testing, along with all receipts, feed bags, and labels.

So pretty scary, right? And not fun Halloween scary, it’s scary scary. But I really want you to be aware of the risk so you can be super careful about keeping livestock feed far, far, far away from your horses and never have to experience that nightmare.

Until next week,


P.S. There are a ton of great videos over on my YouTube Channel. Have you checked them out? Between the videos and the podcast the humans around here do, Straight from the Horse Doctor’s Mouth (which is the biggest horse podcast in the world, if I may toot their horn!) you can get a free graduate degree in horse care just by watching and listening to my docs while you ride or clean stalls. So make sure you’re taking advantage of all these resources!

Whinny’s Wisdoms is the official blog of Whinny the Clinic Mouse at Springhill Equine Veterinary Clinic in Newberry, Florida. If you liked this blog, please subscribe below, and share it with your friends on social media! For more information, please call us at (352) 472-1620, visit our website at SpringhillEquine.com, or follow us on Facebook!

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