Ionophore toxicosis, a severe and often fatal condition in horses, arises from the ingestion of ionophores, antibiotic compounds that are common in feeds for livestock, poultry and other commercially raised animals. [1]

Common ionophores in livestock feeds include monensin, lasalocid and salinomycin. [1] These antibiotics increase growth rates in cattle and poultry, and control coccidiosis, a parasitic disease. [1]

However, ionophores are highly toxic to horses, even in small amounts. Consuming less than 2 grams of monensin can cause death in an average-sized horse. [1]

Accidental feeding of livestock feeds containing ionophores is the most common cause of ionophore toxicosis in horses. That’s why it’s important to source horse feeds from an equine-only, monensin-free manufacturing facility like Mad Barn’s.

Read on to learn everything you need to know about why ionophores are a serious risk to horses and what you can do to protect your herd.

Ionophores in Animal Feed

Ionophores are a group of chemicals that have the ability to transport ions across a lipid membrane. Some ionophores also have antimicrobial properties and are used as antibiotics or growth promoters in animal feeds.

As feed additives for livestock, ionophores help protect against coccidial infections and improve feed efficiency and weight gain. [2] However, their use in animal feed is controversial due to concerns about antibiotic resistance.

Commonly used ionophores and their trade names include: [2]

  • Monensin (Coban, Rumensin)
  • Lasalocid (Avatec, Bovatec)
  • Salinomycin (Bio-cox, Sacox)
  • Narasin (Monteban, Maxiban)
  • Maduramycin (Cygro)
  • Semduramycin (Aviax)
  • Laidlomycin propionate (Cattlyst)

Toxic Effects on Horses

A major concern regarding the use of ionophores in livestock feed is the toxic effects of these compounds on horses, even at low dosages. This poses a significant risk in mixed animal facilities where horses could encounter livestock feeds containing these ingredients.

When horses consume ionophores, these compounds are absorbed from the gastrointestinal tract and form complexes with electrolyte minerals such as sodium, potassium, magnesium, and calcium. [1]

Cells in the heart, muscle, and nervous system absorb these complexes, which disrupts the pH balance within the cell. [1] Ultimately, this results in cell death and the symptoms of ionophore toxicosis.

The LD50 is a standard measurement used to gauge the toxicity of various substances. It represents the dose that is lethal for 50% of animals.

For horses, the LD50 of ionophores ranges between 0.6 mg/kg for salinomycin, to as high as 21.5 mg/kg for lasalocid. Monensin is the most common ionophore causing toxicosis in horses and has a lethal dose of around 2-3 mg/kg. [1]

Feeds with higher levels of oils or fats may result in increased ionophore absorption, as many of these substances are lipophilic (bind to fats). [1]

Toxic Ionophore Doses

The following table provides a summary of toxic doses of different ionophores based on a typical 450 kg (1000 lb) horse: [3][4]

Ionophore LD50 Toxic Amount for Average Horse
Monensin 2-3 mg/kg 0.9 g – 1.4 g
Lasalocid 21.5 mg/kg 9.7 g
Salinomycin 0.6 mg/kg 0.3 g
Narasin 0.8 mg/kg 0.4 g
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Causes of Ionophore Poisoning

Ionophore poisoning in horses usually results from accidental ingestion of feeds intended for other livestock species or contamination of feed during the manufacturing process.

Accidental Exposure

The most common cause of ionophore toxicosis in horses is exposure to feed intended for other livestock species. Species that may have ionophores in their formulated feed rations include: [5]

  • Cattle
  • Pigs
  • Poultry
  • Sheep
  • Goats
  • Rabbits

The feed label should clearly indicate both the drug name and trade name of the ionophore product included in the mixture. If your horse eats a livestock feed and you are unsure whether it contains an ionophore, call your veterinarian.

Contamination

Rarely, ionophore toxicosis can also occur due to contamination of equine feed during production. [1] Some feed production facilities produce feed for multiple different species, and there may be residual ionophore products in the machinery when producing feeds intended for equines.

The Food and Drug Administration (FDA) has strict protocols regarding the use of medications in feed mills to reduce the risk of feed contamination. [6] Feed mills must be licensed to produce feed containing ionophores and must follow regulations aimed at preventing contamination of non-medicated feed. [6]

The required equipment cleanout procedures involve one of the following measures: [7]

  • Physical cleaning such as vacuuming, sweeping, or washing
  • Flushing the machinery by running a grain, such as corn, through the production system to remove any residual feed
  • Sequential production of feeds such that several non-medicated feeds for livestock are produced after a medicated feed, before producing feed designated for horses

Symptoms

Symptoms of ionophore toxicosis in horses can vary, depending on which organ system is most affected. [1]

Cardiac Symptoms

Most horses develop cardiac disease from ionophore toxicosis, with symptoms such as: [1]

  • Collapse
  • Exercise intolerance
  • Rapid heart rate
  • Profuse sweating
  • Sudden death

Horses typically show cardiac symptoms within 12 hours of ionophore ingestion. [1]

Chronic Toxicosis

Horses that ingest a sublethal dose of ionophores also develop heart damage, although the symptoms may not be readily apparent. [1] Horses that consume a sublethal dose are described as having chronic toxicosis.

Symptoms of chronic toxicosis include: [8]

  • Exercise intolerance
  • Poor performance
  • Poor coat and body condition
  • Distended jugular veins
  • Swelling of the lower abdomen
  • Increased urination

Gastrointestinal Symptoms

Some horses may develop gastrointestinal symptoms, likely due to the effect of ionophores on the intestinal flora. Symptoms include: [1]

Neurologic Symptoms

Rarely, some horses develop neurologic symptoms that affect their motor control and muscle function. Neurologic symptoms of ionophore poisoning include: [1]

  • Uncoordinated gait
  • Weakness
  • Muscle tremors
  • Muscle atrophy

Diagnosis

Ionophore toxicosis is usually diagnosed following a known history of exposure to ionophores, often after the owner identifies that the horse was fed the incorrect feed. Given this information, treatment can begin without need for further diagnostics. [1]

Horses that develop signs of ionophore toxicosis require a cardiac work-up to identify the extent of heart damage. Diagnostic tests may include: [1][9]

  • Measuring cardiac troponin on bloodwork, a compound that identifies heart damage
  • Ultrasound of the heart to identify structural changes
  • Electrocardiogram to identify arrhythmias

Although these tests do not alter the treatment plan, they can help determine the prognosis for the affected horses.

Feed Analysis

Feed analysis is typically performed if there is a suspicion of ionophore contamination in an equine feed product. Analysis may return negative results even if there is a strong suspicion for ionophore toxicosis. [9]

The main challenge is that the distribution of ionophores may be inconsistent. [9] For example, ionophores trapped in one section of machinery may only release when filling a particular feed bag or bulk delivery truck. This could lead to contamination of one feed sample in a batch of hundreds. [9]

Additionally, within that sample, only a small portion may have high enough ionophore levels to cause toxicosis. [9] Due to this limitation, it is possible that the affected horse consumed the only contaminated feed present in a feed sample. [9]

Sampling multiple sources from several batches may help confirm the presence of ionophores. However, in some cases, confirming the diagnosis through feed analysis is impossible. [9]

Post-mortem Examination

Unfortunately, some horses die suddenly from ionophore toxicosis. These horses usually require a post-mortem examination to identify or confirm the cause of death. [1]

Common post-mortem findings include: [3][9]

  • Enlarged heart with pale heart muscle
  • Fluid accumulation in the lungs and chest cavity
  • Fluid accumulation within the pericardium, the sac protecting the heart
  • Pale muscles throughout the body

Some horses die so quickly after ingestion of ionophores that they show no changes at post-mortem. [3] To confirm the diagnosis in these cases, the examiner usually collects a sample of feed from the horse’s stomach and submits it for ionophore testing. [3] Heart tissue samples can also be tested for ionophores. [9]

Differential Diagnoses

For cases without a known history of exposure, the horse’s clinical signs may lead the veterinarian to suspect ionophore toxicosis. However, other toxins and diseases can cause similar symptoms and should be considered.

Examples of other diagnoses to rule out include: [8]

Distinguishing between these conditions and ionophore toxicosis can be difficult. Additional diagnostic tests that may be helpful include: [8]

  • Routine bloodwork
  • Investigation of the plants in the pasture or hay provided to the horse
  • Finding blister beetles in the hay
  • Vitamin and mineral level testing on a liver biopsy

Treatment

In the first 10 hours after feed ingestion, gastric lavage can help remove any unabsorbed ionophores remaining within the stomach. [1]

Administration of activated charcoal or magnesium sulfate can also bind any remaining toxin and prevent its absorption. [1] Both of these procedures should only be performed by a licensed veterinarian.

Once horses develop symptoms, these treatments are no longer effective. There is no specific antidote for ionophore toxicosis, so treatment focuses on supportive care. [1] Most horses showing symptoms require hospitalization for appropriate management. [1]

Supportive care provided to horses includes: [1][9][10]

  • Intravenous fluids, including electrolytes
  • Administration of selenium and vitamin E for muscle cell and cardiac support
  • Medications to treat any arrhythmias present
  • Stall rest to prevent additional cardiac stress from exercise

Prognosis

Many horses that develop symptoms from ionophore toxicity die rapidly, even with treatment. [1]

Horses that survive the initial ingestion event usually have long-term complications due to heart or nervous system damage, requiring ongoing monitoring. [1]

Many veterinarians recommend not riding an affected horse for 2-3 months after the ingestion event. [11] Before returning to athletic work, a complete work-up of the heart should be performed by a veterinary specialist. [11]

Long-term Complications

Around 20% of horses may have long-term symptoms of cardiac disease from ionophore ingestion. [12] Common long-term symptoms include: [12]

Cardiac Problems

Cardiac evaluation of affected horses typically shows evidence of previous damage to the heart. Findings can include: [12]

  • Enlarged heart with thin heart walls
  • Scarring within the heart muscle
  • Cardiac arrhythmias

Around 40% of horses with evidence of heart damage show no obvious symptoms at rest, and only exhibit symptoms when exercising. [12] For this reason, a thorough evaluation of horses previously exposed to ionophores is critical before returning the horse to under saddle work, for the safety of the rider. [12]

In one study, approximately 10% of the horses exposed to ionophores died or were euthanized due to a poor prognosis by 10 months after the ingestion event. [12]

Return to Exercise

Despite cardiac damage occurring after ionophore ingestion, some horses can return to athletic performance after thorough evaluation. [12][13]

A study evaluating the long-term effects of ionophore exposure showed that cardiac function 16 months after exposure was not significantly correlated with performance capacity. This outcome implies that even horses with cardiac damage can have a successful athletic career. [12]

In this study, around 50% of affected horses returned to their previous performance level, with approximately 20% of horses experiencing continued exercise intolerance that prevented further athletic pursuits. [12]

Another study reported similar results, with around 50% of horses returning to athletic competition. [13]

Veterinary Evaluation

Determining whether a horse can return to performance requires a thorough veterinary investigation into cardiac function, including: [14]

  • Ultrasound of the heart before and after exercising
  • Electrocardiogram, including 24-hour ECG monitoring and exercising ECG

In general, horses with known cardiac damage should not be ridden by children or used as a lesson horse, as there is a risk of sudden death due to heart disease. [14] Informed adults can continue to use these horses for athletic events. [14]

Horses that develop exercise intolerance should not be used for riding until they are re-evaluated by a veterinary specialist. [14]

Prevention

Ionophore toxicosis most commonly occurs on farms with multiple livestock species, including horses. Prevention strategies can include: [1]

  • Storing feed in clearly labelled containers, and marking any ionophore-containing feed with a warning
  • Keeping feed storage areas secure to prevent inadvertent animal access
  • Training all care personnel on the severity of ionophore toxicosis
  • Not using the same buckets, tubs, or feed carts for both horse feed and ionophore-containing feed
  • Avoiding using ionophore-containing feed or mineral products in pastures shared by horses and cattle

To prevent ionophore exposure due to contamination of equine feed products, owners should follow these practices: [1]

  • Choose feed brands produced in an ionophore-free or equine-only facility
  • Ensure that bulk feed is not transported in trucks also used for ionophore-containing feeds
  • Carefully monitor horses for symptoms, particularly when a new batch of feed is introduced
  • Use feeds specifically marketed towards horses, rather than all-species products

Horses can be very sensitive to changes in their feed, and some may refuse to eat a new batch of feed due to contaminants. [1] If horses are refusing to eat a new batch of feed, discontinue feeding and contact the supplier.

Horse owners should also keep a record of the production lot numbers of any feed products they use, in case there is a contamination event. This allows prompt disposal of any potentially contaminated feed, as well as identification of horses who may have been exposed to the contaminant.

Mad Barn is committed to maintaining the highest standards of quality in our equine-only, ionophore-free manufacturing facility. If you are unsure about where your feeds are sourced, consider working with a qualified nutritionist to get a comprehensive diet evaluation so you can rest assured your herd is healthy and safe.

Summary

  • Ionophores are antibiotic compounds found in many livestock feeds, but they are highly toxic to horses
  • Ingestion of ionophores in horses cause serious heart damage and may result in sudden death
  • There is no specific antidote for ionophore toxicosis, but affected horses may benefit from supportive care
  • Horses that survive the initial ingestion event can have successful athletic careers with careful monitoring of heart function

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References

  1. Hovda. L. R. et al., Blackwell’s Five-Minute Veterinary Consult Clinical Companion: Equine Toxicology. Wiley Blackwell, 2022.
  2. Constable. P. D. et al., Practical Antimicrobial Therapeutics, in Veterinary Medicine, 11th ed.Elsevier, 2017, pp. 153–174.
  3. Hall. J. O., Toxic Feed Constituents in the Horse. Veterinary Clinics of North America: Equine Practice. 2001. View Summary
  4. Ekinci. ?. B. et al., Ionophore Toxicity in Animals: A Review of Clinical and Molecular Aspects. Int J Mol Sci. 2023.
  5. Mercer. M. A., Ionophores Use in Animals – Pharmacology, Merck Veterinary Manual, 2022.
  6. Medicated Feeds, United States Food and Drug Administration, Sep. 20, 2023.
  7. 21 CFR Part 225 — Current Good Manufacturing Practice for Medicated Feeds, Code of Federal Regulations.
  8. Blomme. E. a. G. et al., Ionophore Toxicity in Horses. Equine Veterinary Education. 1999.
  9. Fritz. S. A. and Hall. J. O., Ionophores. Veterinary Clinics of North America: Equine Practice. 2023.  View Summary
  10. Reed. S. M. et al., Equine internal medicine, 3rd ed. St. Louis, Mo: Saunders Elsevier, 2010.
  11. Marr. C. M. and Bowen. I. M., Eds., Cardiology of the horse, 2nd ed. Edinburgh New York: Saunders, 2010.
  12. Gy. C. et al., Acute, Subacute and Chronic Sequelae of Horses Accidentally Exposed to Monensin-Contaminated Feed. Equine Veterinary Journal. 2020. View Summary
  13. Hughes. K. J. et al., Long-Term Assessment of Horses and Ponies Post Exposure to Monensin Sodium in Commercial Feed. Equine Veterinary Journal. 2009.
  14. Reef. V. b. et al., Recommendations for Management of Equine Athletes with Cardiovascular Abnormalities. Journal of Veterinary Internal Medicine. 2014. View Summary