Vet Tips 2025: Toxins That Can Cause Heart Disease in Horses—What to Avoid 🐴☠️

By Dr Duncan Houston BVSc

Heart disease in horses is rare—but when it does occur, it can be deadly. One often-overlooked cause? Toxins in feed or forage that damage the heart muscle. In some cases, symptoms may be subtle until sudden collapse occurs. Here’s how to prevent toxic heart injury in your horse. 🧠🐎

How Toxins Affect the Equine Heart 💔🩺

Certain compounds in feed, plants, or hay can damage the heart muscle (myocardium), leading to:

• 📉 Poor performance
• 💓 Irregular heart rhythms
• ⚡ Heart failure
• ⛔ Sudden death

Early detection and prevention are critical. Let’s break down the most common culprits. 👇

1. Ionophores—The Hidden Feed Danger 🧪🥣

Ionophores are additives used in cattle feed to improve weight gain and feed efficiency. But in horses, they are highly toxic and often fatal.

How It Happens:

• ❌ Horse feed is accidentally contaminated during feed production
• 🥣 Horses gain access to cattle feed

Even small amounts can result in:

• 💥 Heart muscle damage
• 🏃‍♂️ Decreased performance
• ⚠️ Sudden death without warning

Always store cattle and horse feed separately and avoid cross-contamination at mills. 🧊

2. Blister Beetles in Alfalfa Hay 🐞🌿

Blister beetles produce a toxin called cantharidin. When horses ingest these beetles in alfalfa hay, the result can be catastrophic.

• 🔥 Causes severe gastrointestinal and heart damage
• 💧 Leads to dehydration, colic, and potentially death
• 💓 Heart muscle is particularly vulnerable in high-dose exposure

Inspect alfalfa hay for beetle carcasses—especially if sourced from areas prone to infestation. ⚠️

3. Ornamental Yew (Taxus) Shrubs 🌲💀

Yew (Taxus spp.) is one of the most toxic plants to horses. Ingesting even small amounts can cause:

• 💥 Sudden heart failure
• 🛑 Death within minutes to hours

All parts of the yew shrub are toxic—even dried trimmings. Keep horses away from landscaping plants near barns or pastures. 🚫🌳

4. Other Toxic Plants That Affect the Heart 🌿⚠️

Horses usually avoid toxic plants—but if dried in hay or if forage is scarce, they may ingest them. Watch out for:

• 🌱 White snakeroot
• 🌺 Rhododendrons and azaleas
• 🍃 Mountain laurels and fetterbush
• 🪻 Milkweed (wildflower and ornamental varieties)

All of these can lead to cardiotoxicity, arrhythmias, weakness, and death. ❌💓

Check Your Hay—Even Good Bales Can Hide Risks 📦🔍

One of the most overlooked routes of exposure is hay contamination with toxic weeds or beetles. Follow these tips:

• 📦 Inspect every bale—look for plant stems, beetles, or unusual texture
• 🧠 Know what toxic plants look like in dried form
• 🔬 If unsure, send samples for identification before feeding

Contaminated hay has been linked to multiple horse deaths—and it’s preventable. 🧰

Signs Your Horse May Be Affected 🐎⚠️

Toxic damage to the heart may appear suddenly or progress over days:

• 📉 Decreased energy or reluctance to work
• 💨 Rapid breathing or heart rate
• 💢 Muscle tremors or weakness
• 🛌 Collapse or sudden death

Always consider a toxic cause if multiple horses on the same feed develop symptoms. 🚨

When to Call Your Vet 🩺📞

Act fast if your horse shows any of the following:

• 💥 Rapid decline or unexplained lethargy
• 💓 Irregular heart rhythm or pulse
• 🤮 Severe colic with no improvement
• ⚠️ Suspected ingestion of an unknown plant or feed

Your vet may administer fluids, activated charcoal, or supportive therapy. Blood tests can help identify organ damage. 🧪

How to Prevent Toxin Exposure 🧼🛡️

• 📦 Buy hay from trusted sources and inspect bales
• 🌲 Remove toxic landscaping plants near barns or paddocks
• 🥣 Never feed cattle rations or feeds with unfamiliar additives
• 🔐 Store feed securely and label all bins

When in doubt, throw it out. Your horse’s heart depends on it. 🐎❤️

Need Help Identifying a Plant or Feed Risk? Ask A Vet 📱🌿

Not sure what’s in your hay or worried about a possible poisoning? Speak to an equine expert now via Ask A Vet.

Download the Ask A Vet app for 24/7 support on toxicology, feed safety, and emergency equine care. 🐴📲

Final Thoughts: One Bite Can Be Deadly 🧠☠️

While rare, heart disease in horses can stem from preventable toxic exposures. With careful feed sourcing, pasture inspection, and vet support, you can keep your horse’s heart strong and safe. 🫀

Need expert help? Visit Ask A Vet for trusted advice and peace of mind. 🐎💙