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Equine Protozoal Myeloencephalitis

By: Dr. Philip Johnson

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The blood test for EPM is not very useful since approximately 50 percent of horses in endemic areas have a positive blood test. A negative blood test provides evidence that the horse may not have been exposed to EPM (but it is not a guarantee). A positive blood test result does not confirm a diagnosis of EPM.

It is necessary to make a thorough effort to rule out other causes of neurological disease in horses, in order to support a diagnosis of EPM (especially to rule out cervical vertebral malformation in young male horses - the "wobbler" syndrome).

The best test currently available requires the collection of a cerebrospinal fluid (CSF) sample that is tested for the presence of antibodies against Sarcocystis neurona; this test is called the western immunoblot, and relies on the fact that, during active EPM disease, antibodies are being produced against S neurona within the spinal cord and brain and can be detected in the CSF. However, an effort must be made to prove that the spinal fluid has not been contaminated with blood. It only takes a tiny quantity of blood contamination to turn a negative CSF test in to a positive ("false positive").

A positive CSF western immunoblot test result is strong evidence that EPM may be the problem if the tested horse if truly demonstrating signs of neurological dysfunction. A positive CSF western immunoblot test result is NOT evidence that EPM is present in horses in which neurological dysfunction cannot be demonstrated. Therefore, it is not recommended that "normal" horses be tested using the western blot on CSF.

All too commonly, neurological symptoms are contrived in order to support the diagnosis of EPM in the face of a positive spinal test result. Although EPM is common in specific geographical locations, the diagnosis must be supported by proper interpretation of the horse's clinical situation and the diagnostic tests that are currently available. There are no logical reasons at the current time to test CSF for EPM in horses that are not demonstrating any signs of neurological dysfunction. A positive CSF test in a non-neurological horse is meaningless.

Collection of CSF can be done at two locations: the lumbo-sacral tap is done in the standing horse at a point just behind where the saddle is located. This procedure requires a specific technique for which veterinarians have been trained. Alternatively, the horse can be anesthetized, and a CSF sample obtained from the cisterna magna, close to the head. The CSF must be stored carefully and submitted promptly to the laboratory. The CSF must arrive in the laboratory in the beginning of the week when it can be processed fresh; it should not be mail-delayed, as the sample might deteriorate.

Both serum and spinal fluid can be frozen to help in submitting the sample to the laboratory.

It is also possible to test CSF for the DNA (genetic material) of S neurona using a PCR test. A positive PCR test is very strong evidence of EPM. However, a negative PCR test does not rule-out EPM. Unfortunately, most cases of EPM tested using PCR are negative.

Another important aspect to establishing the diagnosis includes ruling out other possible causes of similar neurological signs. It should also be remembered that EPM might occur simultaneously with other neurological diseases. The response to treatment (for EPM) might lend further support to the diagnosis of EPM.

Ultimately, an accurate diagnosis of EPM can often be made after an affected horse has died. The veterinary pathologist is able to examine the spinal cord and the brain under the microscope and identify either the protozoal parasites or the characteristic effects of parasite damage. In some cases following death, it is even possible to culture the Sarcocystis neurona parasite from the CNS tissues but this method is not routinely available.


There are three treatment strategies when faced with horses that are likely affected with EPM:

  • Drugs intended to inhibit or kill Sarcocystis neurona. The most common anti-protozoal drugs used in the treatment of EPM are sulfadiazine and pyrimethamine. Other anti-protozoal drugs that, although not so readily available, are currently being investigated for the treatment of EPM include diclazuril, toltrazuril, ponazuril, and nitazoxanide.

  • Anti-inflammatory drugs such as glucocorticoids and dimethyl sulfoxide. These drugs must always be used in conjunction with an anti-protozoal drug. Anti-inflammatory treatment is used to alleviate the body's reaction to the invading parasites. As noted above, a substantial contributor to the neurological dysfunction is the CNS inflammation that attends the parasite's destructive activity. Short-term improvement in clinical signs following onset of treatment is usually attributed to reduction in the inflammatory process.

  • Other miscellaneous treatments. These are based on individual horse requirements. It is necessary to use a sling to support some severely compromised horses. Other drug strategies include the use of immune stimulating substances and vitamins that confer protection for damaged nerves. The length of time needed for treatment depends on the individual horse, the drugs that are selected and results of on-going evaluation of the horse's progress. In most instances, it is necessary to treat affected horses for several months.

    It should be noted that approximately 10 percent of EPM-affected horses become more severely affected immediately following initiation of treatment. This is attributed to the death of parasites and an increase in the immune reaction to dead parasites. This worsening phase, known as a treatment crisis, is usually transient. The veterinarian might use specific anti-inflammatory drugs at this time to minimize the worsening phase.

    Currently, there is no evidence that one anti-protozoal drug strategy is better than any other. Certainly, if one strategy has failed to cause any improvement over the course of four weeks, the strategy might be changed or the diagnosis should be re-considered.

    Horses should be treated for at least six months. In our experience, the western immunoblot test rarely becomes negative within eight months. If the signs of neurological dysfunction disappear, it is recommended that treatment be continued for at least another four weeks. The treatment may not eliminate the Sarcocystis neurona parasite; it might only constrain the organism until the body is able to neutralize it. Some horses are not able to eliminate the organism, and their problems probably improve because the parasite is inhibited, but they are at risk for further problems if their immune system is compromised again in the future.

    Some horses are so severely affected with EPM that they are unable to stand up, even with treatment. Many of these horses are clearly suffering and it is unlikely that, even if they could eventually stand up, they would be able to recover sufficiently to ever be ridden again. It is not possible to treat some horses because the disease has affected the horse's ability to swallow. For those reasons, euthanasia may be considered for those horses.

    Note of Caution

    It should be strongly cautioned that there are numerous treatments for EPM, many tried unsuccessfully, which have not been tested and have gained popularity simply because they sound "logical." The period that elapses during these unsuccessful treatments allow time for the parasite to do more damage. There is no strong evidence that any one treatment is more effective than another, although one treatment might be better than another in a specific horse. If one treatment selection is ineffective, another drug might be selected by the veterinarian who is managing and monitoring the case.

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