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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 |
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]
- Exertional rhabdomyolysis (tying up syndrome)
- Cardiotoxic plant ingestion
- Calcium-containing plant ingestion
- Blister beetle intoxication
- Vitamin E/selenium deficiency
- Other causes of appetite loss and colic
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]
- Weakness
- Uncoordinated gait
- Weight loss
- Exercise intolerance
Cardiac Problems
Cardiac evaluation of affected horses typically shows evidence of previous damage to the heart. Findings can include: [12]