On a very long list of things horses cannot eat, onions are very high!
Horses cannot onions. Even small amounts can make them ill, and if horses ingest onions in high quantities it can ultimately lead to death.
So how does a seemingly harmless vegetable that makes us cry, do so much more damage to our equine partners?
You’d be surprised to know how harmful the interaction is between the compounds contained in onions and the body systems of not only horses but other animals like dogs and other cattle.
Here’s the science …
What Makes Onions Toxic
Onions are a part of the allium species which can be found worldwide – both cultivated and in the wild.
The three compounds that are common in the allium species – which includes bulb onion, common onion, and garlic – are N-propyl sulfide, alkyl disulfide, and methyl disulfide [Source].
These compounds themselves are not the causation of the medical issues that horses encounter when they ingest onions.
Instead, it’s what these compounds do to red blood cells that causes the most damage that can lead to serious illness and potentially death.
Horses tend to actually avoid these plants when another suitable forage is available to them. Onion ingestion is usually either by someone feeding their horses culled onions due to lack of knowledge that they are onions or that they are poisonous.
The other reason horses may ingest onions is if there is a lack of other available vegetation to eat, so they resort to eating onions as nourishment.
Oxidative Damage to Red Blood Cells
So what is this oxidative damage and why do we care about it?
Well those of us who never took a hematology class in school may not understand the mechanism that causes these chemical compounds to cause so much damage. However, if you have ever have piled meatballs on a dinner plate – you’ll understand this analogy!
Imagine your dinner plate as your red blood cell and your meatballs as your molecules (or units) of oxygen that are transported to different body tissues like your heart, your gut, and your brain.
Think of the compounds contained in onion as chipping away at your dinner plate. It would be quite difficult to get your meatballs from the stove to the dinner table without them falling off the broken/chipped plate to the floor.
Once that meatball hits the floor, it can only go one other place – in the trash!
So with all your damaged red blood cells, your body experiences a decreased volume of oxygen being transported to important body tissues.
Impaired delivery of oxygen to these tissues now causes cell death and damage to these larger tissues [Source].
Anemia in Horses with Onion Toxicity
Anemia is simply a condition in which your body does not have enough healthy red blood cells to carry oxygen to the places in your body that need it.
Think of anemia as a kitchen full of broken plates.
In a blood test, anemia is reflected as low hemoglobin (the molecules that oxygen attaches to) and low hematocrit (the ratio of the volume of red blood cells to the total volume of blood) [Source].
Onion toxicity causes what’s known as hemolytic anemia which is when red blood cells are destroyed faster than they can be replaced.
Again going back to the analogy, you have all these broken plates that continue to be damaged and you’re still waiting for your replacement plates to be delivered.
Signs and Symptoms of Onion Toxicity
Some signs of onion toxicity can present as any other illness.
For example, depression and anorexia are common symptoms of a variety of conditions from fever to an upset stomach.
Other signs that may make you lean more towards onion toxicity are weakness, signs of hematuria (blood in the urine), and exercise intolerance [Source].
If your horse is experiencing any of these signs or symptoms regardless if the cause is known, you should contact your vet as soon as possible.
If you have a suspicion that your horse may have ingested onion, that is an important detail that should be disclosed to your vet if not during your initial call, definitely, once your vet arrives.
Treating Onion Toxicity
With any type of poisoning your first step is to remove the source of the toxin.
If this is onion poisoning, this is not something you are actively feeding your horse (hopefully). Therefore, removing the toxin is not as easy as taking it out of your horse’s daily feed.
That being said, the source is more than likely out in the field – in particular the field where your horse has recently spent the most time.
If you’re not sure where the source is coming from, it’s important to control your horse’s environment.
You should examine your horse’s turnout very closely before letting them have free range or choose a different area for them to go out that has also been checked.
Besides removing the onion from their environment, the onion needs to be removed from their body.
Your vet can administer activated charcoal through a tube that goes up their nose, down their throat, and into their stomach [Source].
This action although slightly uncomfortable helps to decrease the amount of toxin that is absorbed into the circulation and that circulates the body to damage various organ tissues.
Other therapies your veterinarian may also consider are intravenous (IV) fluid therapy or a whole blood transfusion. The IV fluids help dilute the toxin and flush it out of the circulation as well as give your horse some valuable hydration. [Source]
If your horse is exhibiting more detrimental signs of anemia like tissue hypoxia and organ dysfunction, a whole blood transfusion may be warranted.
Like we have previously discussed, hemolytic anemia is defined as an excess of broken or damaged red blood cells without enough healthy replacement red blood cells[Source].
So think of this blood transfusion as your shipment of plates (red blood cells) that have finally arrived to replace your broken plates. Now your meatballs (your oxygen) can make it to the dinner table (the body tissues).
Prognosis of Onion Toxicity in Horses
The prognosis of onion toxicity is dependent on the horse’s current health and co-morbid conditions, other pre-existing conditions, the amount of onion the horse has been exposed to, and the severity of their symptoms and anemia.
Some of the damage done to organs that do not get sufficient oxygen can be permanent and cause further medical problems down the road.
Preventing Onion Poisoning
The best way to prevent this type of poisoning is to limit your horse’s potential access to this plant.
If you’re not certain what you’re looking for, it may be best to pull up some Google images on your phone so you can spot these harmful plants.
If you have other staff at the barn, make sure they are aware of these plants too. You could also consider hiring a landscaper or a botanist who may have more experience and expertise in identifying plants.
If you do find these plants, you should remove them or relocate them to an area your horse cannot get to.
If you can’t remove the plants for whatever reason, you may have to block off this area with additional fencing, or just move your horse to another area if possible!
Use secure fencing that does not pose as an additional safety hazard and watch your horse in the paddock to make sure there’s no way for them to bypass the barrier and get to the plants. The last thing you want to do is cause your horse to get stuck and suffer a potential injury that could be just as damaging as a toxin exposure!
Being a responsible horse owner involves many things which include prevention of injury and harm!
You should make it a habit of walking the area of your horse’s paddock (if possible) and surveying the area. Look for debris, down fencing, large holes in the ground, and toxic plants!
If you make this a regular habit, you will most likely stop potential dangers before you have problems. So keep your eyes peeled to keep your horses happy and out of trouble!
Acute hemolytic anemia caused by wild onion poisoning in horses. Journal of the American Veterinary Medical Association. https://pubmed.ncbi.nlm.nih.gov/5061884/.
LLC, H. D. V. M. (n.d.). Onions Poisoning in Horses. HorseDVM. http://www.horsedvm.com/poisonous/onion/.
Wild Onion. Wild Onion – an overview | ScienceDirect Topics. (n.d.). https://www.sciencedirect.com/topics/agricultural-and-biological-sciences/wild-onion.