It started innocently enough. The morning feeders noticed that a couple of horses were acting abnormally. Two were down in the pasture and had difficulty getting up. Another one was walking very erratically, wobbling and weaving as if drunk.

When the feeders entered the barn, most of the horses were standing at the front of their stalls, looking for their breakfast. However, a couple of them were very depressed looking, with drooping heads and showed no interest in food. The owner of the farm was called immediately and she quickly arrived to assess the situation.

While she walked the barn and the pastures, the feeders went about their work, feeding the horses. Alarmed by what she saw, the owner called her veterinarian. Out of approximately 40 horses on the farm, five seemed to acting abnormally. The veterinarian examined the horses, took blood samples and conferred with the owner. They reviewed the vaccination history of the affected horses and all were up to date on vaccinations. Many of the symptoms could be caused by a variety of causes, so the veterinarian would call back as soon as he had the blood results and they would go from there. None of the horses were showing gastrointestinal problems aside from one horse that had diarrhea. The symptoms were more neurological in terms of the wobbly gait and hind end weakness shown by one of the pasture horses.

Obviously the list could be very long in terms of what could be going on. West Nile, Encephalitis, and the neurological form of the equine herpes virus could result in the same symptoms, but the horses had been vaccinated regularly. While vaccinating doesn’t guarantee protection, most horses are adequately protected by their vaccinations. Furthermore, none of the horses were running a fever — at least so far. Three of the horses were kept outside on pasture and two were kept in the barn. If the pasture horses had eaten some sort of toxic plant, the horses in the barn should not have been sick.

As the day progressed, the affected horses continued to worsen. More alarmingly, several previously normal horses started showing symptoms — the wobbly gait, muscle weakness, sweating, and labored breathing.

In October 2012, I did a column on botulism poisoning in horses caused by a dead animal that was inside a large round bale that was fed to a group of horses in pasture. The horses displayed muscle weakness, depression, and failure to get up once they went down. In that instance, the owner lost several horses before botulism was confirmed.

However, in this case of multiple horses sick simultaneously, the pasture horses were not fed hay, while the barn horses did get hay and alfalfa, depending on their feed requirements.

The veterinarian reported that the kidney values were abnormal in several of the horses, plus there were signs of muscle deterioration and damage reflected in high muscle enzyme levels. This very likely pointed to some sort of toxin or poisoning that was breaking down muscles. The only thing that was common to all of the affected horses was they all received the same grain. Also, significantly, all of the horses that did not receive grain were acting normally. The owner used a custom mix from her local feed mill for her horses. She immediately stopped all feeding of grain and submitted samples from the remaining bags in the feed room for analysis.

The Culprit

When the grain analysis came back, the results identified the toxin responsible — Monensin — also known as Rumensin in the U.S. and Coban in Canada. Sadly, and the reason I wanted to highlight this story, is that every year, several times a year, there is a story just like this, about Monensin poisoning in horses. Therefore, the more people understand about Monensin and how this can happen, the greater likelihood that horse owners can be alert and prevent this in their barn. Or at minimum, recognize the signs and quickly pinpoint the cause before more harm is done.

Monensin is an antibiotic routinely used in livestock and chicken feed to increase growth and development. The problem is that horses are extremely sensitive to Monensin compared to other livestock species. Monensin is an ionophore — a chemical that controls ion transport between cells. In cattle and chickens, for example, it can also control some transport of parasites. However, in horses, it interferes with sodium and potassium ion transport in the muscles, particularly in the heart. This ultimately can result in damage to the muscles but also in cardiac failure.

Monensin has been used routinely in livestock feed since the 1970s, however, it has also been poisoning horses for the same amount of time. It is illegal to add Monensin to horse feeds. What generally happens is that a mill running feed for livestock does not adequately clean out all of the equipment before running horse feed and the horse feed becomes contaminated. That is the most common occurrence for Monensin poisoning. However, on working ranches, feedlots, and other operations that have horses and cattle in close proximity, it is possible for staff to mistakenly feed horses grain meant for cattle, or for horses to gain access to spilled cattle feed.

Monensin is not only toxic to horses, but dogs as well. Because dogs only have a single stomach (as do horses) they process Monensin differently. In a case history of a dog poisoned by Monensin, the amount of Monensin found in the contaminated feed was 150g/ton of feed. The dog food consumed had been stored in bins that had previously contained livestock feed with Monensin. Symptoms of Monensin poisoning in dogs include ataxia (unsteady gait), possible partial paralysis, muscle damage and cardiac damage. Therefore, it is also important to know where your dog food is milled and what precautions the mill takes when running dog food in a facility that also processes livestock feed. If your dog likes to eat grain and there is livestock feed on the premises, there is the potential for dogs eating spilled grain in livestock pens or feeding bins to become poisoned. In Australia and New Zealand, there are also anti-bloat capsules that are used in cattle that contain Monensin. Case histories of dogs consuming or partially consuming these capsules have also resulted in canine deaths.

What Happened?

After the initial discovery of the sick horses, the grain wasn’t identified as a possible culprit for 36 hours. This gave the horses who continued to consume the contaminated feed a higher dose of the toxin. Cattle can consume 10 times the amount of Monensin than horses with no ill effects. Therefore, with the level of Monensin in the feed calibrated for cattle, it takes only a very small amount to poison a horse. Even if there was just a trace amount of Monensin in the horse feed, eating it daily over time could cause devastating results. Unfortunately, since the symptoms of Monensin poisoning mimic many other conditions, it may be several days before feed contamination is suspected.

Depending on the amount on Monensin consumed, horses may suffer acute heart damage and die quickly from heart failure. Others may have chronic muscle damage that causes chronic heart problems resulting in debilitation and possibly death over the course of days or weeks.  Unfortunately, in this case, it appears that the contaminated feed had been fed for a number of days, with the result that out of over 40 horses, nearly half either died or were euthanized.

How is Monensin Poisoning Diagnosed?

Since the ionophore breaks down in the horse quickly, tissue samples are not generally diagnostic. Since it most seriously affects the heart, horses with very high heart rates, combined with labored breathing and difficulty in walking or getting up and down must be considered suspect. Again, these same symptoms could be seen in a severe colic. However, if multiple horses are showing signs at the same time, it would be very unusual to have multiple severe colics happening simultaneously.

Blood work that identifies an increase in enzymes connected with muscle damage is significant as well as high myoglobin (muscle protein) levels in the urine. This type of bloodwork can also be indicative of high lactic acid levels and muscle damage found in horses with tying-up syndrome (exertional rhabdomyolysis). These horses usually are reluctant to move or walk, have a very stiff gait and can sweat profusely. Again, multiple horses having this simultaneously at a single farm would be unlikely.

Therefore, taking blood and urine samples in this sort of situation would be helpful in trying to narrow down the diagnosis. However, sometimes the only conclusive diagnosis might be obvious damage to the heart found on necropsy.

The best way to definitively diagnose Monensin poisoning is to have the feed tested.

What To Do in The Event of Suspected Poisoning?

First of all, use logical thinking. When faced with multiple horses in distress, thinking clearly may be difficult. A single sick horse is probably not indicative of contaminated feed. Multiple horses in a variety of living situations (stall, pasture, paddock) all showing the same symptoms is cause for concern.

If there are horses NOT being fed grain, determine if they are affected or not. If not, it increases the chances of grain being the culprit.

If you have multiple horses all showing the same symptoms, immediately take all feed away and save it for analysis. Save empty feed bags if no feed is available. Have your veterinarian send samples to an independent laboratory for testing.

The veterinarian will likely begin supportive therapy. However, there is no antidote to Monensin, so supportive therapy is all that can be done.

Short Term/Long Term Effects

Depending on the amount of Monensin consumed, the effects could be very quick and very tragic. Since horses are 10 times more sensitive to the amount of Monensin used for cattle and 100 times more sensitive to the amount used for chickens, consuming even minimal amounts can result in cardiac failure and death.

If the horse has not consumed a fatal amount, or has consumed trace amounts over time, damage to the heart muscle or other muscles can occur and will be permanent. Horses that recover from Monensin poisoning can suffer from heart or muscle problems when exercised. This can result in a horse that potentially could never be used for riding or driving. Since the muscle damage is not visible or particularly measurable, there are many reports of horses dying months or years after exposure.

Therefore, anyone who sells a horse that has recovered from Monensin poisoning should disclose that fact when selling the horse.

How To Prevent Monensin Poisoning?

Unfortunately, since the seventies, there have been countless cases of Monensin poisoning involving large and small feed mills. If you buy a custom feed made by a regional or local mill, it is important to talk to them about how they run the mill. Don’t be afraid to ask what their protocol is for cleaning out the mill between livestock and horse runs of feed. Ask if they use Monensin in their mill and what steps they take to prevent contamination. Ask if they have ever had an instance of contaminated feed. Take control of the feed you use and make sure you know what the feed mill is doing.

If you buy a commercial feed from a mill that ONLY produces horse feed, the chances are minimal that your feed could be contaminated. However, it is still a good idea to check with the company and find out what their involvement is (if any) with running livestock feed through their mill. If they run any livestock feed at all, you must consider that a risk factor.

Horses and cattle go together like peanut butter and jelly. Therefore, if you have livestock (sheep, swine, cattle, or chickens) along with your horses, make sure the feed for the horses is kept separately from the livestock feed. Make sure your employees know the difference and do not confuse feeds. Any spillage of livestock feed must be cleaned up immediately. Do not allow your horses access to livestock feed. For example, no wandering of horses in areas where there are troughs where livestock is fed. Do not feed horses and cattle in the same environment unless you know for certain that there is no Monensin in the feed. Check all supplements that are used for livestock before use — some supplements that are not grain based can contain Monensin.

Again, if Monensin poisoning is even suspected, immediately pull all grain at the first signs until testing can occur. Your horse can live quite well without grain, but can die quickly if the grain is contaminated. And, as there have been multiple instances, year after year, decade after decade, it is clear that many horse owners do not understand the dangers of Monensin in feed.

Monensin certainly increases the health and productivity of livestock and poultry, but can never be around horses. It will continue to be used in the livestock and poultry industries, so it is up to horse owners to become educated and keep it away from their horses.

December is the holiday month, whether you are grilling shrimp on the “barbie” at the beach in Sydney, or having roast beef with Yorkshire pudding in London. Turkey may be on the menu here in the United States, but roast goose or even venison might make an appearance as well. As we look forward to a festive holiday meal, we also make sure that what we eat is safe for us. It is important that we do the same for our horses.