Understanding Equine Protozoal Myeloencephalitis Symptoms in Horses

Equine Protozoal Myeloencephalitis (EPM) is a serious neurological disease that affects horses. It's caused by a parasite called Sarcocystis neurona.

Symptoms can start to appear anywhere from a few weeks to several months after a horse is first infected with the parasite. Some horses may not show any symptoms at all.

Common symptoms of EPM include loss of coordination and balance, stumbling, and difficulty walking. Horses may also exhibit abnormal behavior, such as pacing or restlessness.

As the disease progresses, horses may experience more severe symptoms, including seizures, paralysis, and even death.

Symptoms of EPM can be quite subtle at first and may resemble other conditions. Over half of all horses in the United States have been exposed to EPM, but very few develop the disease.

The big three symptomatic “As” of EPM include atrophy, ataxia, and asymmetry. Atrophy is the loss of muscle, which may be visible in the rear, across the topline or neck, or even in the facial muscles. Ataxia describes a “drunken” or uncoordinated gait due to decreased muscle control, usually in the hind end.

A horse may develop one, or many of the possible symptoms, and the symptoms can have a slow or sudden onset. They can also stabilize for some time before a more significant relapse. Other potential clinical signs include uncoordinated movement, stiff, stilted movements, and lameness that comes and goes.

Clinical signs can vary from acute to chronic and may involve the brain, brainstem, or spinal cord. A comprehensive neurological examination is often the first step in diagnosing any horse. The examination can reveal a variety of clinical signs, affecting nearly any part of the horse’s body and ranging from mild to severe.

Neurological symptoms include changes in posture, gait, strength, balance, coordination, proprioception, reactions, body condition, and/or mood that suggest the nervous system is not working normally. Possible symptoms affecting the horse’s head include droopy lip, facial twitch, dropping feed, facial paralysis, trouble swallowing, head tilt, and drooping ear.

Here is a list of possible symptoms affecting the horse’s body:

Diagnosing EPM can be tricky because it is the “great pretender” in terms of symptoms. Unfortunately, the only time EPM can be diagnosed definitively is post-mortem (after death). There are a few tests for EPM that can be run if a horse starts to show clinical signs, but other diseases must also be ruled out.

If EPM is suspected, treatment should be started right away to improve the chance of recovery. Treatment usually lasts 1-3 months, depending on the improvement of clinical signs in the horse.

There are three FDA-approved drugs for EPM treatment: Ponazuril, Diclazuril, and Sulfadiazine/pyrimethamine. Ponazuril is considered the "gold standard" and is usually administered orally daily for 1-3 months, costing around $1,000-$1,500 per month.

The duration of treatment can be tricky to determine, so horses should be re-evaluated after one month of treatment. If improvement is seen, another month of treatment may be recommended. If the horse appears clinically normal, treatment can be discontinued.

Here's a summary of the FDA-approved treatments:

It's essential to note that treatment may not repair nervous system damage, and some horses may not recover without targeted support for inflammation control and nervous system regeneration.

There are currently three FDA-approved drugs for the treatment of EPM in horses: Ponazuril, Diclazuril, and Sulfadiazine/pyrimethamine.

Ponazuril is a paste medication administered orally daily for 1-3 months, with a loading dose on the first day to help it reach a therapeutic level in the system. It's usually the most expensive option, costing around $1,000-$1,500 per month.

Diclazuril is an alfalfa-based top feed medication, administered daily for 1-3 months, and typically costs $800-$1,000 per month. It's also used as a form of prevention in some cases.

Sulfadiazine/pyrimethamine is a liquid medication, prescribed for 3-9 months, at a cost of $250-$400 per month. However, prolonged treatment can lead to anemia, so it's essential to monitor the horse's health closely.

The duration of treatment is usually based on the improvement of clinical signs in the horse. Some compounded versions of these medications are available, but they can only be legally prescribed in certain situations.

Here's a comparison of the three FDA-approved treatments:

It's essential to consult with a veterinarian to determine the best treatment plan and dosage regimen for your horse.

Side effects can be a concern during EPM treatment, so it's essential to be aware of what to look out for. Some EPM drugs may cause anemia, requiring periodic checks by your veterinarian.

Horses on Re-Balance (Sulfadiazine and pyrimethamine) may be more prone to adverse reactions, including anorexia, intestinal disturbances, urticaria (hives), and bone marrow suppression. These side effects can be serious, so it's crucial to monitor your horse closely.

Flunixin meglumine (banamine) is often given to horses with moderate to severe symptoms during the first 3-7 days of therapy to help manage inflammation. This can help alleviate some of the discomfort and pain associated with EPM.

Corticosteroid therapy may be prescribed for horses showing severe signs and at risk of becoming recumbent.

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Neurological symptoms in horses can be caused by pathways that route information from the brain to the body, which can lead to a loss of control over the body. This can manifest in various ways, depending on the specific pathways affected.

The pathways that route information from the body back to the brain, especially about the position of the legs and feet in space, can also be disrupted, leading to proprioceptive deficits. This means the horse may not be aware of its own body position.

Nerve fibers that run through the musculature can be damaged, causing muscle wasting and weakness. This is because the muscles rely on these nerve fibers to function properly.

Specific structures in the brain that control balance, vision, hearing, and mood can also be affected, leading to a range of cognitive and sensory symptoms.

The severity of the symptoms will depend on the extent of the damage caused by the protozoa. The more protozoa present, the more damage will occur, and the more support the horse will need for recovery.

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Exposure to EPM can occur through various means, including ingestion of contaminated water or feed.

Without treatment, EPM will progress to severe paresis and possibly recumbency, which can occur over a matter of hours or years.

Treatment can help stabilize the condition of infected horses, but it's not a guarantee of complete recovery.

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Exposure means that the horse has ingested the protozoa and the immune system is making antibodies, but the horse does not have neurological symptoms because the protozoa are not present in the brain and spinal cord.

This distinction is crucial in determining the prognosis for a horse. Exposure indicates that the horse has been exposed to the protozoa, but it does not necessarily mean that the horse will develop the disease.

The presence of antibodies is a sign that the horse's immune system is actively fighting the protozoa, but it does not guarantee that the horse will recover.

If your horse is diagnosed with EPM, it's essential to understand the prognosis. Without treatment, EPM can progress rapidly, leading to severe paresis and recumbency in a matter of hours or years.

The severity of symptoms can vary widely, and some horses may appear normal, bright, and alert, while others may show more obvious signs of illness. Neurological examination can reveal a range of clinical signs, from mild to severe, affecting nearly any part of the horse's body.

With treatment, approximately half of infected horses will stabilize in their current state, while the other half will improve about one neurologic grade. This means that a horse with mild signs of EPM may return to full and safe athletic use, but a horse with severe signs may never be safe to ride.

Some horses may experience a gradual progression in severity of symptoms, while others may rapidly decline or stabilize only to relapse days or weeks later. It's also possible for horses to recover on their own over time, but many cannot recover without targeted support for inflammation control and nervous system regeneration.

Here are some possible symptoms of EPM, grouped by body part:

Neurological symptoms of equine protozoal myeloencephalitis (EPM) can be quite subtle at first and may resemble signs of other conditions.

Neurological symptoms include changes in your horse's posture, gait, strength, balance, coordination, proprioception, reactions, body condition, and/or mood that suggest the nervous system is not working normally.

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Owner observations about changes in mood, posture, gait, and body condition are important contributions to understanding possible neurological symptoms. A thorough neurological exam can reveal a variety of clinical signs, affecting nearly any part of the horse's body and ranging from mild to severe.

Some possible neurological symptoms include Ataxia: a "drunken" or uncoordinated gait due to decreased muscle control, usually in the hind end.Asymmetry: muscle atrophy and ataxia/lameness in affected horses may present more significantly on one side of the body.Uncoordinated movement (ataxia) of the rear feet, worse on one side.Stiff, stilted movements.

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The causes of neurological symptoms in horses with EPM are quite complex, but they all point to damage to the nervous system.

The damage is caused by the protozoa S. neurona, which can infect the nervous system and disrupt the normal functioning of the brain and spinal cord.

This can happen in several ways: through pathways that route information from the brain to the body, pathways that route information from the body back to the brain, nerve fibers that run through the musculature, and specific structures in the brain that control balance, vision, hearing, and mood.

The more protozoa present in the nervous system, the more damage will occur, and the more severe the symptoms will be.

It's impossible to know exactly how many protozoa are present, but titers (measurements of the immune system's response) can give us an idea of the level of infection.

Here are some key points to keep in mind:

Neurological symptoms in horses can be quite subtle, and it's essential to recognize the signs to get your horse the proper care.

Changes in your horse's posture, gait, strength, balance, coordination, proprioception, reactions, body condition, and/or mood can indicate neurological symptoms. Your veterinarian can perform a formal neurological exam to assess these changes.

Owner observations about changes in mood, posture, gait, and body condition are crucial in understanding possible neurological symptoms. These changes can be a result of damage to the nervous system, which can be caused by various factors, including pathways that route information from the brain to the body, pathways that route information from the body back to the brain, nerve fibers that run through the musculature, and specific structures in the brain that control balance, vision, hearing, and mood.

The more protozoa present in the nervous system, the more damage will occur, leading to symptoms. Unfortunately, it's impossible to know how many protozoa are present, and titers only reflect the activity of the immune system in response to the protozoa.

Here are some possible symptoms affecting the horse's head:

And here are some symptoms that can affect the rest of the body: