FAQ: Ionophore and Monensin Contamination in Horse Feeds

Ionophores are compounds commonly used as additives in feeds for cattle, poultry, and other commercially raised animals. While these compounds are beneficial for certain animals, horses metabolize ionophores differently, making them susceptible to negative effects if exposed to toxic doses.

Recently, concerns about ionophore contamination in equine feeds have risen after a tragic incident at an Oklahoma rodeo facility, where up to 70 horses were poisoned. This feed, sourced from a private feed mill, contained levels of the ionophore monensin that were toxic not only to horses but also to cattle, highlighting the severity of the contamination.

While this incident has understandably alarmed many horse owners, it’s important to note that such cases are extremely rare. In fact, this is the only reported incident of its scale, and it resulted from a significant feed mixing error. Despite this, the incident serves as a reminder of the importance of sourcing equine feeds and supplements from reputable mills with robust quality control measures.

To minimize your horse’s risk of ionophore toxicosis, choose feeds and supplements made in equine-only facilities to eliminate the potential for cross-contamination with medicated feed ingredients. Always buy from reputable companies that adhere to strict quality standards and testing protocols to guarantee safety.

In this article, we highlight how Mad Barn ensures the safety of your horse’s feed by addressing common questions about ionophores and our manufacturing processes.

1. What are Ionophores and Monensin?

Ionophores are a group of chemicals that livestock producers use as growth promoters in animal feeds. ^[1] They can also help prevent coccidial infections in livestock species. ^[1]

The most common ionophores found in livestock feeds are: ^[1]

• Monensin
• Lasalocid
• Salinomycin

While trace amounts of ionophores are not harmful to horses, the risk arises when they are exposed to higher concentrations. Most cases of ionophore toxicosis in horses result from exposure to large doses (greater than 1 gram) of monensin, far beyond trace contamination levels. ^[1]

2. How Can Horses Become Exposed to Ionophores?

The most common cause of ionophore exposure in horses is accidental ingestion of feed meant for other livestock species. ^[1]

Other potential causes include: ^[1][2]

• Consuming feed manufactured in a feed mill that also prepares medicated livestock feeds without appropriate cleanout procedures
• Consuming feed transported in a truck that also transports medicated livestock feeds without appropriate cleanout procedures
• Contamination between feed products on a farm feeding multiple types of animals
• Consuming water contaminated by medicated livestock manure

• Clearly labeling bags or feed bins
• Using separate storage or feed rooms
• Separating horses from other livestock at feeding time

Additionally, train all personnel involved in feeding on the risk of mixing up or mis-feeding products and why extra care should be taken to avoid cross-contamination.

3. What Regulations Exist to Prevent Ionophore Contamination in Horse Feed?

In North America, the Canadian Food Inspection Agency (CFIA) and the Food and Drug Administration (FDA) are the main regulatory bodies involved in ensuring the safety of livestock feed production. Both organizations have recommended practices surrounding the use of medications in feed mills. ^[8][9]

CFIA regulations are very strict, with specific requirements for ionophore levels after the “clean-out” process, which involves thoroughly cleaning feed mills before producing non-medicated feeds. These standards guarantee that any residual ionophores are reduced to safe levels to prevent adverse effects.

To ensure safe feed products, feed mills that manufacture medicated and non-medicated feeds must show that their cleanout practices reduce ionophore levels to less than 1.0 ppm (1,000 ppb) for monensin. ^[9] You can learn more about validating cleanout procedures on the CFIA’s website.

4. What is the Toxic Dose of Ionophores for Horses?

The amount required to cause toxic effects varies based on the body weight of the horse and the specific type of ionophore. Scientists typically describe toxic doses in terms of LD50, or the dose that was lethal to 50% of the animals exposed. ^[1]

The LD50s for common ionophores in horses are: ^[1][3][4][5]

• Monensin: 2 – 3 mg/kg of horse’s body weight. The toxic dose for a 500 kg (1,100 lb) horse is estimated at around 1000 mg, or 1 gram.
• Lasalocid: 21.5 mg/kg of horse’s body weight. The toxic dose for a 500 kg (1,100 lb) horse is around 10 grams.
• Salinomycin: 0.6 mg/kg of horse’s body weight. The toxic dose for a 500 kg (1,100 lb) horse is around 300 mg, or 0.3 grams.

The toxic dose of ionophores may be reduced if the contaminated feed is fed alongside oils. ^[1] A general rule is that feeds containing more than 125 parts per million (125,000 parts per billion) in monensin would likely be toxic to horses. ^[1] Previously reported ionophore toxicosis events showed ionophore concentrations around 1,000 ppm (1,000,000 ppb). ^[6]

5. What Happens when Horses Consume Ionophores?

Ionophores disrupt normal muscle function, particularly in the cardiac (heart) muscle. ^[1] Consumption of a toxic dose of ionophores can rapidly cause tissue death and heart failure in affected horses. ^[1]

Symptoms of ionophore toxicosis can begin shortly after consumption of a toxic dose. The initial symptoms include reduced appetite and lethargy. ^[1] Intestinal symptoms such as colic and diarrhea can develop within an hour after ingestion. ^[1] These symptoms may be related to ionophores’ antibiotic effects on the intestinal flora. ^[1]

After 12 – 36 hours, horses often develop muscular symptoms such as weakness, collapse, and exercise intolerance. ^[1] Sudden death can occur in these horses due to irregular heartbeat rhythms. ^[1]

6. How is Ionophore Toxicosis Diagnosed?

There are several diagnostic tests that can help confirm a diagnosis of ionophore toxicosis. These include: ^[1]

• Blood tests: Measuring blood concentrations of cardiac troponin, a molecule that indicates heart damage, can indicate ionophore toxicosis. These tests are performed on live horses shortly after the consumption event, as cardiac troponin levels typically drop to normal levels several days after ingestion.
• Echocardiography: Ultrasound of the heart to show thin heart walls indicative of heart failure.
• Feed testing: Chemical analysis of a feed sample to determine whether ionophores are present at a toxic concentration.
• Necropsy: Autopsy of the horse will typically show characteristic findings indicating heart failure.

7. How do I Interpret a Feed Test for Ionophores?

Interpretation of an ionophore test starts with determining the detection limit of the test. The detection limit tells you the minimum amount of ionophore the test can detect.

These diagnostic tests are extremely sensitive and can detect ionophore presence in concentrations as low as 0.0000001%. Diagnosticians use parts per billion (ppb) and parts per million (ppm) to describe these very small concentrations of a chemical. ^[7]

These sensitivity levels are thousands of times lower than the threshold for toxicity in horses, providing an extra layer of safety by identifying even the smallest traces of ionophores well below the concentration that would pose a risk to your horse’s health.

A conversion chart for these terms is below: ^[7]

For example, a detection limit of 1 part per billion means that the test can detect the presence of ionophores in concentrations higher than 0.0000001%. Keep in mind that the CFIA’s cleanout validation target for monensin is 1 ppm (1,000 ppb), below which levels are considered safe for preventing harmful exposure to horses.

After determining the detection limit, then look at the result of the test. Some tests use terminology such as below detectable limit (BDL) to indicate that their test was unable to identify any contamination. If they detect contamination, there will be a number value listed along with a unit. Most tests return their results in ppm or ppb. Remember that parts per billion (ppb) is 1,000 times smaller than parts per million (ppm).

To determine the quantity of ionophore the horse consumed, you need to know the amount of feed provided and the ppm or ppb of the test result. Convert the ppm or ppb into a percentage, as shown above. We will use two examples to show these calculations:

• Example 1: A test returning a value of 50 ppb would have 0.000005% ionophore.
• Example 2: A test showing 125 ppm would have 0.0125% ionophore.

Then multiply this percentage by the amount of feed fed. Examples:

• Example 1: Feeding 1 kg of this feed would provide 0.000000005 kg of ionophore. You can convert this to milligrams by multiplying by 1,000,000. In this example, the total number of milligrams fed would be 0.005 mg.
• Example 2: Feeding 1 kg of this feed would provide 0.000125 kg of ionophore, or 125 mg.

Rough conversions between ppm and ppb and milligrams per kilogram are available in the previous table.

Once you know how much ionophore the horse received, you can compare this with the toxic doses indicated above. Examples:

• Example 1: The horse received only 0.05 mg of ionophore per kilogram of feed, which is 20,000 times below the toxic dose for all types of ionophores.
• Example 2: The horse received 125 mg of ionophore per 1 kg of feed. If the horse ate 3 kg of this feed and received 375 mg, it would be over the toxic dose for salinomycin at 300 mg, but under the toxic dosage for other ionophores like monensin and lascalocid.

These calculations underscore the importance of reading the label and knowing which ionophore was used in the feed.

8. Does Mad Barn Produce any Medicated Feed or Livestock Feed?

Mad Barn does not manufacture any medicated feeds or livestock feeds on our premises.

Some of our nutrition team provide consultations for dairy cattle barns as well as horses, but the formulations used by dairy facilities are manufactured at third-party facilities and do not use the manufacturing lines within our production facilities.

To further minimize the risk of ionophore exposure, Mad Barn products are made exclusively in equine-only, medication-free facilities, ensuring there is no risk of cross-contamination from other animal feeds. We also conduct rigorous testing of incoming raw materials to detect contamination before production. Moreover, Mad Barn follows strict quality control protocols, regularly inspecting equipment and production lines to maintain the highest safety standards.

Our manufacturing processes are regulated by the CFIA and FDA, ensuring compliance with all safety and regulatory requirements to protect your horse from any potential ionophore exposure.

Summary

The best way to protect your horses from ionophore contamination is to source feeds and supplements from reputable manufacturers that operate equine-only, medication-free facilities.

• Ionophores, such as monensin, are commonly used in livestock feed, but they can be toxic to horses in high doses due to differences in metabolism.
• A recent incident at an Oklahoma rodeo barn led to the poisoning of up to 70 horses after they consumed feed with monensin levels so high that they would have been toxic even to cattle, not just horses.
• Regulations from the CFIA require feed mills to adhere to strict clean-out procedures to reduce ionophore levels below 1 ppm in non-medicated feeds, further ensuring equine feed safety.
• Mad Barn ensures the highest safety standards by producing all products in ionophore-free facilities, rigorously vetting suppliers, and conducting thorough testing of both raw materials and finished products for contamination.