Understanding Equine Protozoal Myeloencephalitis in Horses

Equine protozoal myeloencephalitis, or EPM, is a serious disease that affects horses worldwide. It's caused by a parasite that's typically spread through the bite of an infected fly.

The parasite, Neospora hughesi, is found in the feces of opossums and other wildlife, and can be ingested by horses through contaminated feed or water. This parasite is particularly problematic because it can survive in a dormant state within the horse's brain tissue.

Symptoms of EPM can be subtle at first, but can progress to more severe neurological problems, including loss of coordination, weakness, and paralysis. The parasite can also cause seizures, blindness, and even death.

Equine protozoal myeloencephalitis (EPM) is caused by the parasite Sarcocystis neurona. This parasite requires two hosts to complete its lifecycle: a definitive host and an intermediate host.

The definitive host is the opossum, specifically the Virginia opossum (Didelphis virginiana) in North America. Other South American opossums can also act as the definitive host.

A number of mammals can serve as intermediate hosts, including raccoons, cats, armadillos, skunks, and sea otters. These animals can become infected by eating sarcocyst-containing muscle tissue from an infected intermediate host.

Horses contract EPM from contaminated feed or water, but they cannot pass the disease among themselves. This means that one horse cannot contract the disease from another infected horse.

The horse is a dead-end host of the parasite, meaning that infectious sarcocysts are rarely formed in equine muscle tissue. As a result, clinical disease in the horse is usually caused by schizonts and merozoites within equine nervous system tissues.

Six subspecies of S. neurona can be identified by surface antigens (SAG), with Equine EPM being caused by parasites that exhibit SAG1, SAG5, and SAG6.

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Signs of illness in horses with equine protozoal myeloencephalitis (EPM) can be quite varied and may mimic other neurologic diseases. Many signs can come and go, making it challenging to diagnose.

Incoordination, stiff, stilted movements, and abnormal gait or lameness are common signs. Incoordination and weakness are also frequent symptoms.

Muscle atrophy can occur, leading to a horse's face, ears, or lips drooping. Difficulty swallowing is another potential sign, which can be a concern for horse owners who have to manage their horse's diet.

Seizures or collapse can also occur in affected horses, making it crucial to monitor their behavior closely. Abnormal sweating is another possible sign, which can be a sign of underlying neurological issues.

Loss of feeling along the face, neck, or body can also occur, which can be detected by a veterinarian during a neurologic examination. Head tilt with poor balance is another sign that may indicate EPM.

Here is a list of potential signs of EPM:

Diagnosing equine protozoal myeloencephalitis (EPM) can be challenging due to its variable signs and symptoms that can mimic other central nervous system (CNS) diseases.

A postmortem examination is the only definitive way to confirm EPM, but it's not feasible for live horses. To diagnose EPM, veterinarians rely on a combination of clinical signs, laboratory tests, and exclusion of other differential diagnoses.

A thorough neurological examination is essential in assessing the horse's CNS function. This examination can help identify horses with EPM, but it's not foolproof as many other neurological diseases can cause similar signs.

Cervical radiographs can be used to identify compression in the spinal cord, which can cause coordination loss in horses. However, this test is not specific to EPM and can be used to rule out other conditions.

Blood and spinal fluid tests can be used to confirm exposure to opossum feces, which is the primary source of EPM. However, these tests can yield false positives and negatives, making them unreliable on their own.

Immunodiagnostic testing on spinal fluid can evaluate the production of antibodies against the parasite, but it's not a guarantee of an active infection.

The serum:CSF antibody titer ratio is a more reliable indicator of EPM, but it requires a paired sample of blood and spinal fluid. A ratio of < 1:100 is highly supportive of the diagnosis, while a ratio of ≤ 64 is suggestive of intrathecal antibody production.

In some cases, CSF analysis may reveal abnormalities such as mononuclear pleocytosis and high protein concentration, but these values can also be within normal limits.

Diagnostic procedures that can aid in ruling out differential diagnoses include CSF cytological evaluation, imaging, additional infectious disease testing, and clinicopathological investigation.

Here are some common differential diagnoses to consider:

Treatment for equine protozoal myeloencephalitis (EPM) is highly dependent on the severity of the disease, and may require a combination of medications and supportive care.

Early treatment is key to a successful outcome, with about 60 to 70 percent of horses showing significant improvement or complete recovery.

The exact treatment plan may include antiprotozoal drugs, such as ponazuril or diclazuril, which are FDA-approved for 28-day administration, but may require multiple rounds of treatment.

Non-steroidal anti-inflammatories (NSAIDs) may also be necessary to alleviate EPM signs and prevent reactions to parasite die-off caused by the treatment.

In some cases, steroids or dimethyl sulfoxide may be used to reduce brain inflammation, and vitamin E may be prescribed to support nervous tissue healing.

Rechecks with your veterinarian will be necessary to monitor your horse's neurological progress and check for anemia, which is a potential treatment side effect.

Here is a summary of the approved treatments for EPM:

It's essential to work with your veterinarian to develop a treatment plan tailored to your horse's specific needs, and to be prepared for the possibility of relapse within two years.

There is no proven preventive measure for equine protozoal myeloencephalitis (EPM), but several approaches can help minimize the risk of infection.

One way to prevent EPM is to reduce exposure to opossum feces, which contain the infective sporocysts. To do this, it's essential to prevent opossums from accessing horse-feeding areas. This can be achieved by storing feed in closed containers and keeping garbage and open feed bags in galvanized metal containers.

You can also reduce the risk of EPM by keeping your horse's living area clean and free of food debris. This includes thoroughly cleaning the horse's water source often and prompt disposal of animal carcasses.

Research has shown that administering antiprotozoal drugs, such as diclazuril, can prevent EPM in foals. In one study, administering diclazuril every 24 hours prevented foals from seroconverting against S neurona and N hughesi.

Here are some practical tips to help prevent EPM:

Nonsteroidal anti-inflammatory drugs like flunixin meglumine are often given to horses with equine protozoal myeloencephalitis to help reduce inflammation and prevent worsening of neurologic deficits.

In the first 3-7 days of antiprotozoal treatment, these medications can be given daily to help manage symptoms.

Horses that are in danger of falling down or show signs of brain involvement may also benefit from a short course of corticosteroids, such as dexamethasone, and dimethyl sulfoxide.

Vitamin E is sometimes used as an adjunct antioxidant treatment to help protect the damaged CNS from oxidant injury, but its effectiveness remains unproven.

Giving vitamin E at a dose of 20 IU/kg daily orally may be a common practice, but more research is needed to confirm its benefits.