Currently, there is no vaccine and prevention is challenging when it comes to equine protozoal myeloencephalitis (EPM). However, good horse keeping can reduce a horse’s risk for coming in contact with the disease. Management is key because even with aggressive treatment, lingering neurologic deficits can remain.
First and foremost to EPM prevention is to manage any possible contamination of horse feed or water from the primary reservoir host, the opossum. Skunks, raccoons, sea otters and even cats also can be protozoal sources. Lock away all feed containers in varmint-proof containers and rooms. When possible, avoid feeding horses from the ground and clean up spilled feed immediately to deter attracting wildlife and rodents. Frequently clean and freshen water sources.
Pastures are another concern as wildlife often has access to areas where horses graze. Keeping wildlife away from horse pastures is important.
Another critical strategy relies on minimizing stress in a horse’s life, since stress tends to suppress a horse’s immune response, making the animal more susceptible to disease.
The most important aspect of treatment is to identify the problem and implement treatment as early as possible. Delays in treatment are associated with poorer outcomes. Treatment also must be administered for the fully recommended course of the drug. Fifty to 80% of horses improve with treatment, and 5-50% recover completely. Horses with Grade 4 (on a scale of 5) neurologic deficits (tendency to buckle, stumble spontaneously, and trip and fall) rarely recover fully.
Three FDA-approved anti-coccidial treatments for EPM are available for horses:
Ponazuril (Marquis by Merial) Bioavailability is improved by 15% when vegetable oil is given concurrently with this medication. The drug is given every seven days and has demonstrated a significant decrease in antibody responses to Sarcocystis neurona (the causative protozoan), although does not eliminate infection in horses already infected.
Diclazuril (Protazil by Merck Animal Health), which is given as a low-dose topdressing. Studies have shown it to significantly reduce the prevalence of the pathogen in blood within a population of treated foals as compared to those not receiving the medication.
Both ponazuril and diclazuril are deemed efficacious when a horse improves at least one grade on neurologic scoring or is negative to antibodies in cerebral spinal fluid (CSF) and blood testing. Efficacy for these medications improves up to 67% of treated horses by at least one grade. Recommendation for treatment is 28 days, although most horses receive the drugs for 6-8 weeks.
Sulfadiazine/Pyrimethamine (ReBalance by PRN Pharmacal) is a folate-inhibiting combination drug that interferes with the parasite’s metabolism. It is usually administered for 3-6 months. Success, measured by clinical improvement in two or more neurologic grades and/or reversion to a negative result from CSF testing, is achieved in 60-70% of horses treated with this combination.
The duration of treatment of any of these drugs is determined by resolution of clinical signs. A horse might experience a relapse once a drug has stopped and/or in instances of stress. Relapse rates are estimated at 10-20%. Horses that seem refractory to one medication might be switched to another or could receive a combination of medications with improved results.
In general, failures in treatment are usually because a horse is not actually infected with EPM, has been infected for a long time before treatment is started, and/or the degree of neurologic damage is beyond recovery. Some horses might benefit from concurrent treatment with non-steroidal anti-inflammatory drugs, corticosteroids or dimethyl sulfoxide (DMSO) during initial anti-coccidial drug treatment. This helps to avoid worsening of signs from inflammation related to protozoal death.
Vitamin E is also used as an anti-oxidant treatment. Immuno-modulators such as levamisole, EqStim or Equimmune can be administered for a few weeks in horses experiencing challenges to their immune systems. However, there are no supporting studies that immuno-modulators are indicated.
The Take-Home Message
Prevention is critical in controlling exposure to EPM. Once a horse contracts the disease and begins to show signs of neurologic deficits, the earlier treatment is implemented, the better the chance for recovery.
By Nancy S. Loving, DVM