What Toxins Can Cause Heart Damage or Sudden Death in Horses?

By Dr Duncan Houston

Sudden weakness, collapse or death after opening a new bag of feed is one of the most frightening situations a horse owner can face.

Certain toxins can damage the heart muscle directly. Others interfere with the electrical signals controlling the heartbeat, disturb calcium and potassium concentrations, or cause such severe gastrointestinal injury and shock that the heart becomes affected secondarily.

The difficult part is that a horse may look normal shortly after exposure. With some toxins, serious cardiac abnormalities do not become apparent until hours or days later. This means a known exposure should never be managed by waiting to see whether the horse becomes visibly unwell.

Quick Answer

Yes. Ionophores such as monensin and lasalocid can cause severe myocardial necrosis and delayed cardiomyopathy in horses. Yew, oleander, foxglove and related plants can disrupt cardiac conduction and cause collapse or sudden death. Blister-beetle toxin can cause gastrointestinal and urinary injury, profound electrolyte disturbances, shock and direct myocardial damage.

If exposure is suspected, immediately remove the feed, hay or plant source, stop all exercise, contact a veterinarian and preserve samples, labels and lot numbers. Do not wait for collapse or an obvious irregular heartbeat. (Merck Veterinary Manual)

Which Toxins Are Most Dangerous to a Horse’s Heart?

Toxin or source	How it affects the horse	Common clues
Ionophores, including monensin, lasalocid and salinomycin	Directly damage cardiac and skeletal muscle cells	New or contaminated feed, weakness, sweating, colic, abnormal rhythm, recumbency or sudden death
Yew, Taxus species	Taxine alkaloids disrupt cardiac electrical conduction	Access to hedge clippings, trembling, weakness, breathing difficulty, collapse or sudden death
Oleander, foxglove, yellow oleander and milkweed	Cardiac glycosides interfere with the sodium-potassium pump	Gastrointestinal signs, slow or rapid heart rate, conduction blocks, weakness or collapse
Rhododendron, azalea, mountain laurel and fetterbush	Grayanotoxins disturb sodium channels in nerve and muscle cells	Salivation, colic, weakness, slow or irregular heartbeat and neurological signs
Blister beetles in alfalfa	Cantharidin damages the gastrointestinal and urinary tracts, causes electrolyte abnormalities and may damage the myocardium	Severe colic, frequent drinking or urination, dark gums, diarrhoea, muscle tremors, arrhythmias or shock
White snakeroot and rayless goldenrod	Cause toxic skeletal and cardiac myonecrosis	Weakness, trembling, difficulty swallowing, wide-based stance and recumbency
Hypoglycin A from sycamore or boxelder seeds	Disrupts cellular energy metabolism, causing atypical or seasonal pasture myopathy	Severe weakness, dark urine, rapid breathing, recumbency and cardiac abnormalities
Rattlesnake venom in endemic regions	May cause myocardial injury and arrhythmias	Bite-site swelling, respiratory distress, weakness and abnormal rhythm

These toxins do not all produce the same disease. Some destroy heart-muscle cells, while others primarily alter the heartbeat or cause secondary cardiac injury through shock, endotoxaemia and electrolyte disturbance. (Merck Veterinary Manual)

How Can a Toxin Damage the Heart?

There are three main mechanisms.

Direct Myocardial Injury

Ionophores, white snakeroot, hypoglycin A and some other toxins can cause degeneration or death of cardiac muscle cells.

The damaged heart may become weak, dilated and unable to pump effectively. Damaged areas may also become electrically unstable, producing ventricular premature beats and potentially fatal arrhythmias. Some horses survive the initial poisoning but develop exercise intolerance, heart failure or sudden death weeks or months later. (Merck Veterinary Manual)

Electrical Conduction Disturbance

Yew taxines interfere with cardiac sodium and calcium channels. Cardiac glycosides in oleander, foxglove, milkweed and related plants inhibit the sodium-potassium pump. Grayanotoxins from rhododendron and azalea keep sodium channels abnormally activated.

These effects can produce slow heart rhythms, rapid rhythms, conduction blocks, ventricular arrhythmias or cardiac standstill without requiring extensive visible damage to the heart muscle first. (Merck Veterinary Manual)

Secondary Cardiac Injury

A toxin may initially target another organ but still injure the heart.

Cantharidin causes severe gastrointestinal and urinary tract inflammation, endotoxaemia, dehydration, hypocalcaemia and hypomagnesaemia. These changes increase the risk of circulatory collapse and arrhythmias, while direct myocardial injury has also been documented.

Severe colic, haemorrhage, shock and systemic inflammation can independently raise cardiac troponin and trigger arrhythmias, even when the original condition is not a primary heart disease. (Merck Veterinary Manual)

Ionophore Poisoning in Horses

Ionophores are among the most important feed-related cardiotoxins affecting horses.

Common ionophores include:

• Monensin, often sold under names such as Rumensin

• Lasalocid, often sold as Bovatec

• Salinomycin

• Narasin

• Maduramicin

They are used in feeds for certain cattle, poultry, sheep, goats and other approved species to improve feed efficiency or control coccidiosis. They are not safe feed ingredients for horses.

Why Are Ionophores So Dangerous?

Ionophores move electrically charged particles across cell membranes and disrupt normal calcium, sodium, potassium and hydrogen gradients.

In cardiac muscle, monensin can cause excessive intracellular calcium accumulation. This damages cell membranes and produces myocardial degeneration, necrosis and electrical instability.

Horses are particularly sensitive. The reported median lethal dose of monensin is approximately 2 to 3 mg/kg in horses, although actual outcomes vary and there is no practical dose that an owner should consider safe. (Merck Veterinary Manual)

How Are Horses Exposed?

Exposure may occur when:

• A horse eats cattle, goat or poultry feed

• A horse accesses a medicated mineral mix

• Feed is incorrectly formulated

• The wrong bag or bulk product is delivered

• Horse feed is produced after ionophore-containing livestock feed without adequate cleanout

• A manufacturing or equipment failure allows medicated feed to carry over into another product

• Feed is stored in unlabelled containers or shared bins

• The same scoop, auger or mixer is used for horse and livestock products

This risk remains real. A 2025 FDA warning followed multiple animal deaths after monensin was detected in non-medicated feed. The investigation identified inadequate control of drug carryover after medicated feed had been manufactured on the same equipment. (U.S. Food and Drug Administration)

What Are the Signs of Ionophore Poisoning?

Acute signs may include:

• Sudden loss of appetite

• Depression

• Colic

• Diarrhoea

• Profuse sweating

• Muscle weakness

• Stiffness

• Unsteady movement

• Rapid heart rate

• Irregular heartbeat

• Breathing difficulty

• Reluctance or inability to stand

• Collapse

• Sudden death

Lower or prolonged exposure may instead cause poor performance, exercise intolerance, weakness, muscle stiffness and gradually developing heart failure. (Merck Veterinary Manual)

Can an Apparently Normal Horse Still Have Heart Damage?

Yes.

Cardiac troponin may not begin increasing until 24 to 72 hours after monensin exposure. In one experimental study, some horses continued to have increased troponin for weeks, while one horse had abnormal ventricular function nine months later. (PubMed)

Delayed abnormalities have also been documented following naturally occurring outbreaks. In one lasalocid incident, horses that appeared clinically normal weeks after exposure still had dysrhythmias or increased cardiac troponin. Some affected horses also developed delayed neurological signs. (PubMed)

This is why every horse that consumed the suspect batch may need assessment, not only the horse that became visibly weak first.

What Is the Long-Term Prognosis?

The outcome is highly variable.

In a study of 76 horses accidentally exposed to monensin-contaminated feed, cardiac abnormalities were detected during acute, subacute and chronic examinations. Among horses with known long-term outcomes, some returned to their previous work, while others remained exercise intolerant, were retired or died. Myocardial thinning, abnormal echogenicity, chamber dilation, reduced contractility and multiple ventricular premature beats were associated with poorer outcomes. (PubMed)

A horse that survives the first few days has not automatically escaped the danger. Long-term rest and repeated cardiac evaluation may be required before return to exercise.

Yew Poisoning in Horses

Yew is one of the most dangerous ornamental plants around horse properties.

It may appear as:

• A clipped evergreen hedge

• An ornamental shrub near a house or stable

• A landscaping plant around an arena or driveway

• Fresh garden clippings placed over a fence

• Dried plant material accidentally mixed with hay or compost

Yew contains taxine alkaloids that interfere with cardiac sodium and calcium channels. Poisoning can produce bradycardia, conduction failure, malignant arrhythmias and cardiac standstill.

Which Parts of Yew Are Toxic?

The needles, bark, branches and seeds are dangerous. The fleshy red portion around the seed is considered less toxic, but the seed itself remains toxic.

From a practical horse-safety perspective, the entire plant should be treated as poisonous. Owners should never assume that a horse will carefully eat only the fleshy part and leave the seed behind. (Merck Veterinary Manual)

How Much Yew Is Dangerous?

Toxicity varies by species, plant material and horse. Ingestion of approximately 0.1% of body weight in yew leaves has been estimated to be potentially lethal to a mature horse.

That is a relatively small quantity for a large animal. Every suspected ingestion should therefore be treated as an emergency rather than measured against an internet dose calculation. (University of Minnesota Extension)

What Does Yew Poisoning Look Like?

Signs can include:

• Nervousness

• Trembling

• Weakness

• Incoordination

• Breathing difficulty

• Slow or irregular heart rate

• Collapse

• Seizures

• Sudden death

Some horses are simply found dead near recently dumped hedge clippings. Once obvious clinical signs develop, treatment is frequently unsuccessful because the cardiovascular effects progress so rapidly. (Merck Veterinary Manual)

Oleander, Foxglove and Other Cardiac Glycoside Plants

Several plants contain toxins with effects similar to digoxin.

Important examples include:

• Oleander

• Yellow oleander

• Foxglove

• Milkweed

• Lily of the valley

• Star of Bethlehem

• Desert rose

• Some kalanchoe species

These plants inhibit the sodium-potassium ATPase pump that is essential for normal cardiac electrical and mechanical function. This increases intracellular calcium and disrupts potassium and sodium concentrations.

Clinical signs may begin within hours and include gastrointestinal discomfort, weakness, diarrhoea, slow heart rate, rapid heart rate, irregular rhythm, conduction block, hyperkalaemia, collapse and death. (Merck Veterinary Manual)

Why Are Garden Clippings Particularly Dangerous?

Some poisonous landscaping plants are not especially attractive while growing normally. Once cut, wilted or mixed with other vegetation, they may become easier for horses to consume.

Never dispose of hedge trimmings, flower-bed waste or lawn cuttings in or beside a horse paddock. A well-meaning neighbour tossing garden clippings over the fence can create a fatal emergency before anyone realises what has happened. (PubMed)

Can These Poisonings Be Treated?

Treatment is highly specialised and depends on the rhythm disturbance, electrolyte changes and time since ingestion.

Veterinary treatment may include gastrointestinal decontamination, intravenous fluids, continuous ECG monitoring, correction of electrolyte abnormalities and antiarrhythmic medication. Digoxin-specific antibody fragments may be used for severe cardiac-glycoside poisoning when available.

Even with aggressive treatment, fatalities have been reported after oleander and foxglove exposure. (Merck Veterinary Manual)

Rhododendron, Azalea and Mountain Laurel

Rhododendrons, azaleas, mountain laurels, fetterbush and Japanese pieris contain grayanotoxins.

These toxins bind to sodium channels and leave excitable nerve and muscle cells abnormally depolarised. Cardiac muscle is commonly affected.

Signs may include:

• Profuse salivation

• Tearing

• Abdominal discomfort

• Diarrhoea

• Weakness

• Breathing difficulty

• Slow or irregular heart rate

• Tremors

• Convulsions

• Coma

• Death

All parts of the plant, including pollen and nectar, can contain toxin. Signs usually begin within several hours, although onset may be delayed. (Merck Veterinary Manual)

Blister Beetles and Cantharidin Poisoning

Blister beetles may become trapped and crushed during alfalfa harvesting. The toxin inside them, cantharidin, remains active after the beetles die and is not neutralised by drying or normal hay storage.

Cantharidin may contaminate:

• Alfalfa hay

• Alfalfa cubes

• Alfalfa pellets

• Alfalfa-containing treats

• Other processed alfalfa products

Visible whole beetles do not need to be present. Toxic dried fluids and tiny insect fragments may remain in the product. (Merck Veterinary Manual)

Is It Really Four to Six Beetles?

No. This is a commonly repeated but potentially dangerous misunderstanding.

The amount of cantharidin in an individual beetle varies substantially according to species, sex and reproductive status. Some beetles contain less than 1 mg, while others may contain more than 5 mg.

The veterinary reference is that approximately 4 to 6 grams of dried blister beetles may be fatal to a horse. That could represent a very different number of insects depending on the beetle species and toxin concentration. Counting visible beetles cannot establish whether hay is safe. (Merck Veterinary Manual)

How Does Cantharidin Affect the Heart?

Cantharidin is a potent blistering and tissue-irritating compound.

It damages:

• The mouth and oesophagus

• The stomach and intestines

• The kidneys and urinary bladder

• Blood-vessel endothelium

• The myocardium

Severe gastrointestinal injury can cause endotoxaemia, dehydration and circulatory shock. Profound hypocalcaemia and hypomagnesaemia may cause muscle tremors, synchronous diaphragmatic flutter and cardiac rhythm abnormalities. Direct myocardial damage and cardiac haemorrhage have also been documented. (Merck Veterinary Manual)

What Are the Warning Signs?

Watch for:

• Sudden colic

• Restlessness

• Depression

• Refusal to eat

• Sweating

• Dark, congested gums

• Rapid breathing

• Rapid heart rate

• Diarrhoea or bloody faeces

• Salivation or oral lesions

• Repeatedly placing the muzzle in water

• Frequent, painful attempts to urinate

• Blood in the urine

• Muscle tremors

• Synchronous diaphragmatic flutter

• Weakness or collapse

A horse repeatedly approaching water but only taking small drinks, or placing the muzzle into the trough without drinking normally, is a particularly useful clinical clue when contaminated alfalfa is possible. (Merck Veterinary Manual)

Can Mineral Oil Be Given?

Do not give mineral oil unless a veterinarian has specifically directed it.

Current toxicology guidance states that mineral oil is not effective for cantharidin poisoning and may worsen the situation. It can also reduce the ability of activated charcoal to bind the toxin when the two are given close together.

Veterinary treatment may include aggressive intravenous fluids, calcium and magnesium replacement, pain relief and carefully timed activated charcoal. There is no specific antidote. (Merck Veterinary Manual)

Other Toxins That Can Injure the Equine Heart

White Snakeroot and Rayless Goldenrod

These plants contain benzofuran ketones that interfere with cellular energy metabolism and can cause severe skeletal and cardiac muscle necrosis.

Affected horses may develop:

• Weakness

• Muscle trembling

• Difficulty swallowing

• A wide-based stance

• Low head carriage

• Inability to stand

• Death within one or two days after signs begin

The toxins may remain active in dried plant material and contaminated hay. (University of Minnesota Extension)

Atypical or Seasonal Pasture Myopathy

Hypoglycin A in sycamore seeds, seedlings and related boxelder material can cause a highly fatal toxic myopathy.

Although skeletal muscle signs dominate, affected horses frequently develop increased cardiac troponin, ventricular premature beats, prolonged electrical repolarisation and abnormal myocardial wall motion. Some survivors continue to have cardiac abnormalities weeks later. (University of Minnesota Extension)

Rattlesnake Envenomation

In a prospective study of 20 horses with rattlesnake bites, 70% developed an arrhythmia and 40% had evidence of myocardial injury based on increased cardiac troponin.

This means that a snake-bitten horse may require cardiac monitoring even when the most obvious problem is swelling or respiratory difficulty. (PubMed)

Avocado

Avocado leaves, stems, seeds and fruit contain persin and have been associated with myocardial injury in horses and other animals.

Possible signs include breathing difficulty, lethargy, exercise intolerance, oedema and cardiac failure. The risk varies between avocado varieties and the amount ingested, but horses should not be deliberately fed avocado material or allowed access to pruned branches. (Merck Veterinary Manual)

Bracken Fern and Horsetail

Long-term consumption of bracken fern or horsetail can cause a thiamine-deficiency-like neurocardiac syndrome.

Horses may show weight loss, incoordination, muscle trembling, seizures and, in severe cases, tachycardia or arrhythmias. Unlike most cardiotoxins discussed here, early thiamine treatment may be highly effective. (Merck Veterinary Manual)

What Signs Suggest Toxin-Related Heart Damage?

Possible cardiac or circulatory signs include:

• Unexpected exercise intolerance

• Sudden weakness

• Collapse or fainting

• Rapid, slow or irregular heartbeat

• Weak or irregular facial pulse

• Breathing difficulty

• Persistent tachycardia

• Jugular-vein distension

• Swelling under the abdomen or in the limbs

• Coughing or fluid accumulation

• Cold extremities

• Pale, dark or blue-tinged gums

• Recumbency

• Sudden death

Additional clues that make poisoning more likely include:

• Several horses becoming unwell

• Signs beginning after a new feed delivery

• Access to cattle or poultry feed

• Recent garden or hedge trimming

• A damaged feed-storage area

• Beetle fragments in alfalfa

• Unusual plants or leaves in hay

• Sudden illness after moving paddocks

• A mixture of colic, muscle weakness and cardiac signs

Heart disease can produce low-output signs such as weakness, tiring and collapse, or congestive signs such as respiratory difficulty and fluid accumulation. However, toxins are only one possible cause. (Merck Veterinary Manual)

How Urgent Is the Situation?

Possible Exposure, No Clinical Signs

The horse appears normal but has eaten a suspect feed, plant or alfalfa product.

Action: Treat this as urgent. Remove the source, stop exercise and contact a veterinarian immediately. Do not wait for abnormal heart sounds, weakness or collapse.

This is particularly important for ionophores, yew, oleander and known blister-beetle contamination.

Early Clinical Signs

The horse has mild depression, reduced appetite, sweating, intermittent colic, weakness or an unexpectedly rapid heart rate.

Action: This is an emergency. Arrange immediate examination and prepare for referral or hospitalisation.

Early signs can progress rapidly and may not reveal how severely the myocardium has already been injured.

Multiple Horses Affected

More than one horse has become weak, colicky, ataxic or recumbent after consuming the same feed or hay.

Action: Treat this as a herd poisoning emergency. Stop the product for every horse, preserve all feed and packaging, and have all exposed horses assessed.

Some exposed horses may have cardiac abnormalities without obvious signs. (PubMed)

Critical

The horse has collapsed, cannot stand, has severe breathing difficulty, a markedly irregular rhythm, rapidly worsening weakness, seizures or signs of circulatory shock.

Action: Obtain immediate emergency veterinary assistance. Minimise movement and protect people from a falling or neurologically abnormal horse.

When Is This an Emergency?

Contact a veterinarian immediately after any known or suspected ingestion of:

• Ionophore-containing livestock feed

• Yew

• Oleander or foxglove

• Blister-beetle-contaminated alfalfa

• A poisonous plant associated with sudden cardiac signs

• An unknown feed that has affected several horses

Do not wait for the horse to look sick.

Emergency signs include:

• Collapse

• Sudden inability to stand

• Rapid or laboured breathing

• An obviously irregular or very rapid heartbeat

• Profuse sweating

• Severe weakness

• Persistent colic

• Dark, pale or blue gums

• Muscle trembling

• Incoordination

• Seizures

• Very frequent urination

• Bloody urine or faeces

• Sudden death of another horse sharing the same feed

With yew, death may occur before there is enough time for effective treatment. With ionophores, a normal-looking horse may still be developing myocardial damage. (Merck Veterinary Manual)

What Should You Do Right Now?

1. Stop the Suspected Source

Remove the feed, hay, mineral product, supplement, water source or plant immediately.

Prevent every horse from accessing it. Close the feed room, block the trough or move horses to a clean area as appropriate.

2. Stop All Exercise

Do not ride, lunge, trot up or “test” an exposed horse.

A horse with myocardial injury may appear relatively normal at rest but develop dangerous arrhythmias or collapse when cardiac demand increases.

3. Call Your Veterinarian

Provide:

• The suspected product or plant

• The approximate time of exposure

• How much may have been eaten

• How many horses had access

• Which horses are showing signs

• The first sign observed

• Any recent feed or management change

• The horse’s current heart rate, temperature and breathing rate when safe to obtain

A regional animal poison service or veterinary toxicologist may also be contacted through the veterinarian.

4. Preserve Evidence

Keep:

• The original bag

• The label

• The feed lot or batch number

• Purchase invoices

• Delivery records

• Unopened bags from the same batch

• Samples from the feeder

• Samples from the storage bin

• Suspicious hay or plant material

• Photographs of the plant and surrounding area

Place separate feed samples in clean, labelled containers. Do not mix samples from different bags or bins.

5. Photograph Plants Before Removing Them

Take clear photographs of:

• The whole plant

• Leaves

• Stems and bark

• Flowers

• Fruit or seeds

• The location where it was found

Wear gloves when handling unknown plant material. Blister beetles and cantharidin-contaminated debris can cause painful skin blistering.

6. Do Not Give Home Treatments

Do not:

• Attempt to make the horse vomit

• Administer mineral oil

• Give activated charcoal without veterinary direction

• Give human heart medication

• Administer electrolytes blindly

• Give several anti-inflammatory drugs together

• Force-feed the horse

• exercise the horse to assess severity

Some toxins require charcoal while others require specialised electrolyte management or antiarrhythmic treatment. Incorrect home treatment can delay care or make the horse worse.

7. Monitor Every Exposed Horse

Record:

• Appetite

• Water intake

• Manure and urination

• Heart rate and regularity

• Breathing rate

• Sweating

• Gait

• Weakness

• Muscle stiffness

• Urine colour

• Ability to stand

• Any colic behaviour

A written list helps ensure that a quieter horse with early signs is not missed while attention is focused on the most obviously affected horse.

How Do Veterinarians Diagnose Toxin-Related Heart Injury?

Diagnosis usually combines exposure history, examination, cardiac testing and toxicology.

Physical Examination

The veterinarian will assess:

• Heart rate and rhythm

• Pulse quality

• Heart sounds

• Mucous membranes

• Capillary refill

• Jugular veins

• Breathing

• Hydration

• Muscle pain and weakness

• Neurological function

• Gastrointestinal activity

• Urine production

• Evidence of oedema or heart failure

Electrocardiography

An ECG identifies the type of rhythm disturbance.

Depending on the situation, monitoring may include:

• A short resting ECG

• Continuous hospital monitoring

• A portable Holter recording

• Repeat ECGs over several days

• Exercise ECG only after the horse is clinically stable and the cardiologist considers it safe

A single normal rhythm tracing does not always exclude intermittent ventricular arrhythmias.

Cardiac Troponin

Cardiac troponin I or T is used as a biomarker of myocardial-cell injury.

Marked increases support myocardial damage, but the result does not identify the toxin by itself. Serial testing is often more useful than one sample because concentrations may rise after the initial examination. Hard exercise, severe colic, haemorrhage and other conditions can also increase troponin, so results must be interpreted in context. (PubMed)

Echocardiography

Ultrasound of the heart can assess:

• Ventricular contractility

• Chamber dilation

• Wall thickness

• Abnormal wall movement

• Myocardial appearance

• Valve function

• Pericardial fluid

Following ionophore exposure, myocardial heterogeneity, thinning, dilation and reduced contractility can provide important prognostic information. (PubMed)

Other Blood and Urine Tests

Testing may include:

• Creatine kinase and AST for muscle injury

• Calcium and magnesium

• Sodium and potassium

• Kidney and liver values

• Packed cell volume

• Blood gases and acid-base balance

• Lactate

• Urinalysis

• Myoglobin

• Inflammatory markers

Profound hypocalcaemia and hypomagnesaemia strongly support cantharidin poisoning when the clinical history includes alfalfa. (Merck Veterinary Manual)

Toxicology Testing

Depending on the suspected toxin, laboratories may test:

• Feed

• Hay

• Mineral mix

• Water

• Plant material

• Blood

• Serum

• Urine

• Stomach or intestinal contents

• Liver

• Heart tissue

For monensin exposure, blood, urine, liver, heart and gastrointestinal contents may confirm exposure. Heart tissue can be particularly useful after death, but the measured tissue concentration does not reliably predict how severely a living horse will be affected. (PubMed)

Cantharidin is most usefully tested in urine or gastrointestinal contents, and samples should be collected early. (Merck Veterinary Manual)

How Is Cardiotoxic Poisoning Treated?

There is no single treatment because the toxins work differently.

Treatment may include:

• Removing or binding unabsorbed toxin

• Intravenous fluid therapy

• Electrolyte correction

• Continuous ECG monitoring

• Antiarrhythmic medication

• Treatment of heart failure

• Oxygen

• Pain relief

• Management of endotoxaemia

• Renal support

• Gastrointestinal protection

• Strict rest

• Repeated troponin testing

• Follow-up echocardiography

Ionophores

There is no specific antidote for ionophore poisoning.

Treatment is supportive and directed towards maintaining circulation, managing arrhythmias and heart failure, controlling pain and monitoring progressive myocardial injury. The prognosis becomes guarded to grave when severe ventricular dysfunction is present. (Merck Veterinary Manual)

Yew

Treatment is symptomatic, but the speed of cardiovascular collapse means it is often unsuccessful once signs are present.

This makes prevention and immediate intervention after witnessed ingestion particularly important. (Merck Veterinary Manual)

Cardiac Glycoside Plants

Treatment may include activated charcoal, intravenous fluids, electrolyte management, atropine or other antiarrhythmics depending on the rhythm, and digoxin-specific antibody fragments in specialist settings.

The horse requires close ECG and potassium monitoring. (Merck Veterinary Manual)

Cantharidin

There is no antidote.

Treatment focuses on aggressive fluid therapy, calcium and magnesium correction, analgesia, circulatory support and elimination of toxin. Activated charcoal may be useful when administered appropriately, while mineral oil is contraindicated. (Merck Veterinary Manual)

Bracken Fern

High-dose thiamine treatment can be highly effective when started early, although other supportive care may also be required. (University of Minnesota Extension)

Can a Horse Recover and Return to Exercise?

Recovery depends on:

• The toxin involved

• The amount ingested

• How quickly treatment began

• Whether arrhythmias developed

• The degree of myocardial necrosis

• Echocardiographic findings

• Whether other organs were injured

• Whether the horse remains clinically normal during follow-up

A horse should not return to work simply because appetite and energy have improved.

Following significant cardiotoxic exposure, clearance may require:

• Normal or stable serial cardiac troponin results

• A normal resting ECG

• No important intermittent arrhythmias

• Acceptable echocardiographic function

• A period of strict rest

• A gradual return-to-work plan

• Exercise ECG or post-exercise cardiac assessment in selected horses

Some ionophore-exposed horses return to previous athletic use. Others remain permanently exercise intolerant or develop delayed heart failure. (PubMed)

Could the Signs Be Caused by Something Other Than Poisoning?

Yes.

Weakness, collapse, an irregular rhythm or increased cardiac troponin may also result from:

• Atrial fibrillation

• Valvular disease

• Myocarditis

• Congenital heart disease

• Severe colic and endotoxaemia

• Acute haemorrhage

• Electrolyte disturbance

• Heatstroke

• Severe dehydration

• Sepsis

• Respiratory failure

• Anaemia

• Neurological disease

• Atypical myopathy

• Snakebite

• Severe pain or shock

Horses can also have benign rhythm variations that do not indicate heart disease. A veterinarian must determine whether an abnormal rhythm is physiological, secondary to another illness or evidence of primary myocardial disease. (Merck Veterinary Manual)

The exposure history is therefore critical. A horse collapsing after strenuous exercise requires a different investigation from several horses becoming weak after opening the same bag of feed.

How Can Ionophore Exposure Be Prevented?

Never Feed Livestock Products to Horses

Do not allow horses access to:

• Cattle feed

• Goat feed

• Poultry feed

• Medicated mineral mixes

• Cattle supplements

• Unlabelled bulk feed

• Spilled livestock feed

• Livestock lick tubs

Read every label and look for ionophore names, including monensin, lasalocid, salinomycin and narasin.

Separate Storage and Equipment

Use clearly labelled:

• Feed bins

• Scoops

• Buckets

• Augers

• Mixing equipment

• Delivery areas

A small amount of medicated livestock product carried into a horse ration can have serious consequences.

Choose Feed Manufacturers Carefully

Purchasing from an equine-only mill can reduce one source of risk, but it is not the only acceptable system.

A reputable mixed-species manufacturer should have documented procedures for:

• Drug reconciliation

• Batch sequencing

• Equipment cleanout

• Carryover testing

• Ingredient identification

• Preventive controls

• Lot traceability

• Recall management

Recent FDA investigations demonstrate that inadequate cleanout and carryover controls can contaminate products that were not intended to contain monensin. (U.S. Food and Drug Administration)

Keep the Bag and Lot Number

Do not throw out the packaging immediately after opening a new feed.

Keep the label and lot details until the product has been consumed without incident. This may substantially speed up an investigation and help identify other horses that received the same batch.

Stop Feeding an Abnormal Product

Stop and investigate when feed:

• Has an unusual smell

• Looks different from the previous batch

• Contains unexpected pellets or particles

• Has a different colour

• Is refused by several horses

• Is followed by signs in more than one horse

• Arrives in damaged or mislabelled packaging

Do not blend the suspect feed with a new product and continue feeding it more slowly.

How Can Poisonous Plant Exposure Be Prevented?

Survey the Entire Property

Inspect:

• Pastures

• Fence lines

• Stable entrances

• Gardens

• Driveways

• Compost areas

• Wooded paddocks

• Hay fields

• Areas accessible after storms

Plant identification should ideally be confirmed by a veterinarian, toxicologist, agricultural extension service or botanist. A phone application can help generate possibilities but should not be the sole basis for declaring a plant safe.

Remove Dangerous Ornamentals

Do not plant yew, oleander, foxglove, rhododendron, azalea or related cardiotoxic plants within a horse’s reach.

Remember that horses may reach through, over or under fencing farther than expected.

Never Dump Garden Waste Near Horses

Clippings remain one of the most preventable causes of severe plant poisoning.

Tell:

• Family members

• Gardeners

• Landscapers

• Neighbours

• Stable staff

• Contractors

that no plant material is to be placed in or beside horse enclosures.

Remember That Dried Plants May Remain Toxic

Drying does not make every poisonous plant safe.

Yew, white snakeroot, cardiac glycoside plants and cantharidin-contaminated beetles may remain dangerous when dried or incorporated into hay. (Merck Veterinary Manual)

Avoid Overgrazed Pastures

Horses are more likely to sample undesirable plants when safe forage is limited.

Provide suitable supplementary forage during drought, late autumn, severe overgrazing or other periods when pasture availability falls.

How Can Blister-Beetle Risk Be Reduced?

Risk is greatest in alfalfa-producing areas where blister beetles are present, but contaminated hay may be transported far from the region where it was grown.

Ask the grower or supplier:

• Where the alfalfa was grown

• Which cutting it came from

• Whether blister beetles were present

• Whether the field was scouted before cutting

• Whether the hay was conditioned or crimped

• Whether alfalfa was cut before peak bloom

• Whether bales from heavily affected field edges were separated

Blister beetles are attracted to alfalfa blossoms. Harvesting before peak bloom and allowing cut forage time to dry before raking can allow mobile beetles to leave. Crushing or crimping can trap and kill beetles in the hay, while spraying insecticide may leave toxic dead beetles in the crop. (SDSU Extension)

Visual inspection of a few flakes cannot guarantee that an entire batch is free from toxin. Beetles may cluster in one small section of a bale, leaving other sections apparently normal.

Common Mistakes Owners Make

Waiting for Heart Signs

The first sign may be mild depression, colic or weakness rather than an obvious arrhythmia.

With some toxins, sudden death is the first visible indication.

Exercising the Horse to See Whether They Are Affected

Exercise increases cardiac demand and may expose an unstable rhythm. A known or possibly exposed horse should remain rested until cleared.

Treating Only the First Sick Horse

When feed, hay or water is involved, every horse with access may be at risk.

Assuming Four Beetles Are Fatal

The reference is approximately 4 to 6 grams of dried beetles. The amount of toxin per insect varies widely, making beetle counting unreliable.

Assuming No Visible Beetles Means the Hay Is Safe

Cantharidin remains in insect fragments and dried fluids that may be too small to see.

Giving Mineral Oil for Suspected Blister-Beetle Poisoning

Mineral oil is not considered effective and may interfere with activated charcoal or worsen management of cantharidin toxicosis.

Throwing Away the Feed Bag

The lot number, label and manufacturing details may be essential for toxicology, recall tracing and assessing other exposed horses.

Assuming Every Collapse Is Poisoning

Primary cardiac disease, severe colic, haemorrhage, heatstroke and neurological disease can cause similar signs. Exposure history and veterinary diagnostics determine the cause.

Frequently Asked Questions

How quickly do cardiotoxic poisoning signs develop in horses?

Yew and cardiac glycoside plants may cause signs or death within minutes to hours. Cantharidin signs commonly develop within hours. Ionophore effects may begin within hours or days, while delayed cardiac abnormalities can remain detectable for weeks or months.

Can a horse look normal after eating monensin?

Yes. A horse may have early myocardial injury without obvious outward signs. Cardiac troponin may not rise until 24 to 72 hours after exposure, and rhythm or echocardiographic abnormalities can appear later. (PubMed)

Can one mouthful of yew kill a horse?

A relatively small quantity can be fatal, but the exact risk depends on the yew species, plant material and amount swallowed. Because progression can be extremely rapid, every suspected ingestion should be treated as an emergency rather than waiting to calculate a dose.

Can horse feed be tested for monensin?

Yes. Feed, blood, urine and tissue samples can be analysed for ionophores. A positive result confirms exposure, but the measured concentration does not reliably predict the severity or outcome for an individual horse. (PubMed)

Are blister beetles only a risk in loose alfalfa hay?

No. Cantharidin may contaminate alfalfa cubes, pellets, treats and other processed products. The toxin is not destroyed by drying or normal storage. (Merck Veterinary Manual)

Final Thoughts

Toxic heart disease in horses is uncommon, but when it occurs the consequences can be devastating.

The most important feed-related danger is ionophore exposure. Yew and cardiac glycoside plants can cause rapid electrical failure of the heart. Blister beetles cause a wider systemic poisoning that may involve severe gastrointestinal injury, electrolyte disturbances, shock and myocardial damage.

The safest approach is based on speed:

1. Stop the suspected source.

2. Prevent every horse from accessing it.

3. Do not exercise exposed horses.

4. Preserve feed, packaging and plant samples.

5. Contact a veterinarian immediately.

6. Evaluate every exposed horse, even those that appear normal.

7. Continue cardiac monitoring for as long as the veterinarian recommends.

The mistake that causes the greatest harm is waiting for the horse to prove that the toxin was dangerous. By the time collapse or obvious heart failure develops, the opportunity for successful treatment may already be disappearing.

---

ASK A VET™ can help you organise feed lot numbers, plant photographs, symptom timelines and test results while you work with your local veterinarian. Suspected cardiotoxic poisoning cannot be managed safely through online advice alone and requires immediate hands-on veterinary assessment.