Equine Protozoal Myeloencephalitis (EPM) is a serious and often debilitating neurological disease that affects horses. Caused by the protozoan parasite Sarcocystis neurona (and, less commonly, Neospora hughesi), EPM targets the central nervous system, leading to a wide range of symptoms that can vary in severity. EPM is particularly concerning because of its ability to cause lasting neurological damage, and if left untreated, it can even be fatal.
The parasite that causes EPM is transmitted to horses primarily through the feces of opossums, which act as the definitive hosts. Horses, however, are accidental hosts and do not spread the disease to other horses. Though the disease has been documented across North and South America, it’s most prevalent in areas with high opossum populations. Understanding EPM, including recognizing its symptoms, obtaining an accurate diagnosis, and starting appropriate treatment, is crucial for maintaining the health and well-being of affected horses.
In this post, we’ll dive into the symptoms, diagnosis, and treatment of EPM to provide a comprehensive guide for horse owners and caretakers.
Symptoms of EPM in Horses
The symptoms of EPM can be highly variable, making the disease challenging to diagnose based on clinical signs alone. EPM’s neurological impact often results in asymmetry, meaning one side of the horse’s body is more affected than the other. The range of symptoms can be subtle at first, which may lead to the disease going unnoticed for some time. However, as the condition progresses, the signs become more pronounced and can include a combination of the following:
1. Weakness and Incoordination (Ataxia)
One of the hallmark signs of EPM is incoordination, known as ataxia, which may manifest as stumbling, dragging of the feet, or difficulty maintaining balance. Horses may also experience weakness, particularly on one side, which can be misinterpreted as lameness. Because the protozoa attack the nervous system, the loss of muscle coordination and motor control is a common outcome.
2. Muscle Wasting
EPM can lead to visible muscle atrophy, particularly in the back and hindquarters. This muscle wasting tends to be localized, affecting one side more than the other. In severe cases, the affected muscles may shrink significantly, resulting in a “saw-horse” stance as the horse struggles to maintain normal posture.
3. Difficulty Swallowing or Eating
Some horses with EPM may exhibit problems with swallowing or exhibit a reluctance to eat. This symptom arises when the nerves controlling the muscles involved in swallowing are compromised. Left unaddressed, this difficulty can lead to weight loss and secondary health issues.
4. Abnormal Gait
Gait abnormalities are a common sign of EPM. Affected horses may drag their feet, cross their legs, or stumble during movement. These signs of irregular motor function are due to the nervous system’s impaired ability to control muscles properly.
5. Facial Paralysis or Asymmetry
EPM can cause facial nerve issues, resulting in drooping ears, lips, or eyelids on one side of the horse’s face. Horses may also have trouble blinking or moving their mouths effectively, leading to excessive drooling or difficulty eating.
6. Seizures or Behavioral Changes
In some cases, horses with advanced EPM may experience seizures or significant changes in behavior, such as irritability, depression, or confusion. This typically occurs when the protozoa affect regions of the brain responsible for cognitive and emotional control.
7. Swaying or Leaning
Some horses develop a noticeable sway or lean to one side, which becomes apparent when they’re standing still or walking. This is due to the weakening of muscles on one side of the body, a direct result of the nervous system damage caused by EPM.
Diagnosing EPM
EPM can be difficult to diagnose because its symptoms can mimic other neurological conditions, including wobbler syndrome, arthritis, or traumatic injury. Therefore, an accurate diagnosis requires a combination of clinical evaluation, diagnostic testing, and ruling out other conditions. Veterinarians typically use a multi-step approach to diagnose EPM:
1. Neurological Examination
A thorough neurological examination is the first step in diagnosing EPM. Veterinarians assess the horse’s gait, reflexes, coordination, and overall motor function. They look for asymmetrical weakness, incoordination, and other signs indicative of nervous system involvement. While this exam may suggest EPM, it cannot confirm the diagnosis without further testing.
2. Blood Tests
Blood tests, such as serology, are commonly used to detect antibodies to Sarcocystis neurona or Neospora hughesi. A positive test indicates that the horse has been exposed to the parasite, but it doesn’t necessarily confirm active infection. Many horses may have been exposed to the organism without ever developing EPM, so further testing is usually required.
3. Cerebrospinal Fluid (CSF) Analysis
For a more definitive diagnosis, veterinarians may collect a sample of cerebrospinal fluid (CSF) via a spinal tap. Testing the CSF for antibodies against the protozoa provides stronger evidence of active infection, as it indicates that the organism has breached the blood-brain barrier and is affecting the nervous system.
4. Response to Treatment
In some cases, a trial treatment approach may be used. If the horse shows significant improvement after starting EPM treatment, it can be a strong indicator that the diagnosis was correct. However, this method should be used cautiously, as other neurological conditions may also improve with anti-inflammatory treatments.
5. Ruling Out Other Conditions
Because many other conditions can mimic the symptoms of EPM, it’s important for the veterinarian to rule out issues such as cervical vertebral malformation (wobbler syndrome), arthritis, or infections of the brain or spinal cord. Diagnostic imaging, such as X-rays or MRIs, may be used to help distinguish EPM from other diseases.
Treatment Options for EPM
The good news is that EPM is treatable, especially if diagnosed early. A range of medications is available to combat the protozoal infection and alleviate the horse’s symptoms. Treatment typically involves a combination of antiprotozoal drugs, anti-inflammatory medications, and supportive care.
1. Antiprotozoal Drugs
- Ponazuril (Marquis): Ponazuril is one of the most commonly prescribed antiprotozoal medications for EPM. It works by inhibiting the protozoa’s ability to reproduce, thereby reducing the infection’s severity. Ponazuril is given orally and is usually administered for a period of 28 days. Horses receiving this treatment often show improvement within a few weeks.
- Toltrazuril: Similar to Ponazuril, Toltrazuril targets the protozoa that cause EPM. It’s also known for its ability to disrupt the protozoa’s reproductive cycle, reducing their numbers and allowing the horse’s immune system to regain control. While not as commonly used as Ponazuril, Toltrazuril has shown effectiveness in treating EPM.
- Diclazuril (Protazil): Diclazuril is another antiprotozoal drug used in EPM treatment. Like Ponazuril, it prevents the protozoa from replicating and is administered as a feed additive. This medication is convenient for horses that may resist oral dosing.
2. Anti-Inflammatory Medications
In addition to antiprotozoal drugs, anti-inflammatory medications such as non-steroidal anti-inflammatory drugs (NSAIDs) may be prescribed to reduce inflammation in the nervous system. Reducing inflammation helps alleviate symptoms such as pain, muscle stiffness, and coordination issues.
3. Supportive Care
Supportive care plays a critical role in a horse’s recovery from EPM. Horses may require physical therapy, nutritional support, and special accommodations (such as stall rest or reduced exercise) during their treatment and recovery. Ensuring the horse has a safe and supportive environment can improve their chances of recovery.
4. Long-Term Management
Even after successful treatment, some horses may experience residual neurological deficits. Long-term management strategies, including regular monitoring and physical therapy, can help horses regain as much strength and coordination as possible. Horses that recover from EPM may return to their normal activities, though some may require ongoing support.
Conclusion
Equine Protozoal Myeloencephalitis is a serious and complex disease that can have devastating effects on horses if not caught and treated early. Recognizing the symptoms of EPM, such as weakness, incoordination, and muscle wasting, is crucial for early detection. Veterinarians rely on a combination of clinical evaluations and diagnostic tests, such as blood analysis and cerebrospinal fluid testing, to confirm the disease.
The good news is that with proper treatment, many horses recover from EPM. Antiprotozoal medications like Ponazuril, Toltrazuril, and Diclazuril are effective in controlling the infection, while anti-inflammatory drugs help manage symptoms. Supportive care is equally important, ensuring that horses have the best chance at a full recovery.
Ultimately, early detection and prompt treatment are key to preventing long-term damage and ensuring the best possible outcome for horses with EPM. By staying vigilant and working closely with a veterinarian, horse owners can protect their animals from the serious consequences of this disease.
