Equine Performance Veterinary Practice
A positive EPM blood test can be alarming, but it does not automatically mean that a horse has active neurologic disease or needs treatment. Most commonly used tests detect antibodies, the immune system’s response to exposure. The central question is whether EPM is causing the horse’s clinical signs. Answering that requires a neurologic examination and careful interpretation of the laboratory results. [1]
What EPM does in the body
Equine protozoal myeloencephalitis, or EPM, is a disease in which protozoal parasites affect the brain or spinal cord. Most cases involve Sarcocystis neurona; a smaller number involve Neospora hughesi. Horses can encounter S. neurona through feed or water contaminated with opossum feces. Exposure is common in many areas, while clinical disease develops in only a small proportion of exposed horses. [2]
Damage in different parts of the nervous system produces different signs. Affected horses may show incoordination, weakness, abnormal limb placement, or muscle loss. Unequal involvement of the two sides of the body can increase suspicion, although it is not diagnostic. Some horses develop facial changes or difficulty swallowing. [1]
Why the examination comes before the label
Stumbling, reduced performance, and an unusual gait have many possible explanations. Lameness, spinal cord compression, vitamin E associated neurologic disease, and other infections can resemble parts of the EPM picture. A veterinarian first determines whether the problem is neurologic and where the dysfunction appears to be located. Testing can then address the most likely causes. [2]
Stop riding a horse that appears weak or uncoordinated and arrange prompt veterinary assessment. Do not attempt balance challenges or neurologic maneuvers yourself. If there is fever, rapid deterioration, difficulty swallowing, or more than one affected horse, report that immediately. These details can change the urgency and the precautions needed while other causes are investigated.
What a positive blood test means
A positive antibody result means the horse has mounted an immune response to the organism being tested. It does not establish that the organism has reached the nervous system. Even a high blood antibody level cannot, by itself, confirm EPM or measure how much neurologic damage has occurred. Routine screening of clinically normal horses can therefore produce positive results that are difficult to act on. [1]
Think of two horses with positive blood tests: one is coordinated and performing normally; the other has progressive neurologic deficits. The laboratory word “positive” is the same, but the clinical meaning is different. In the second horse, the result may support further investigation for EPM. In the first, it is not an automatic reason to prescribe an antiprotozoal medication. [3]
Some reports give a titer, which reflects how far a sample can be diluted while antibodies remain detectable. A larger titer is not a parasite count or a severity score. Tests differ in their methods and interpretation, so numbers from different laboratories or assays should not be compared as though they were interchangeable. [3]
Why spinal fluid can add useful information
Cerebrospinal fluid, or CSF, surrounds the brain and spinal cord. Comparing a CSF sample with a blood sample helps determine whether antibodies are being produced within the central nervous system. The laboratory may calculate a serum to CSF ratio or an antibody index. This can provide stronger evidence than blood testing alone when the horse’s examination also fits EPM. [4]
A positive CSF result still needs interpretation. Antibodies can move from blood into CSF, and blood contamination during collection can affect results. The veterinarian considers sample quality, the particular assay, and alternative diagnoses. There is no single numerical cutoff that should be applied to every type of EPM test. [3, 4]
A negative result generally lowers suspicion, but an early infection may precede a detectable antibody response. If the examination remains concerning, repeat or additional testing may be appropriate. The aim is to explain the neurologic signs, not simply to obtain a positive result. [3]
Treatment and recovery
Veterinary treatment options include ponazuril, diclazuril, and sulfadiazine with pyrimethamine. The choice and duration depend on the horse and its response. Some patients also need supportive care and a carefully staged rehabilitation plan. Controlling infection does not instantly repair damaged nervous tissue, so recovery can continue beyond the medication course and may be incomplete. [2, 4]
Progress is judged primarily through neurologic reassessment and function. Improvement during treatment is useful information but does not prove the original diagnosis. Antibodies may persist, so a blood test becoming negative is not the standard goal for deciding that treatment has succeeded. Worsening or recurrent signs deserve reassessment rather than an automatic repeat prescription. [3, 4]
Practical steps for owners
Reduce opportunities for wildlife to contaminate feed and water: secure stored feed, clean spills, and inspect feeding areas. EPM caused by S. neurona is not ordinarily spread directly from horse to horse. However, a newly neurologic horse should not be assumed to have EPM before other causes are considered. [2]
Bring the complete laboratory report and a timeline of changes to your veterinary appointment. Contact Equine Performance Veterinary Practice for help interpreting a positive result in the context of your horse’s examination, history, and daily function.
Sources and further reading
[1] AAEP — EPM Disease Guidelines 2021
[2] UC Davis Center for Equine Health — Equine Protozoal Myeloencephalitis
[3] Cornell Animal Health Diagnostic Center — EPM Testing
[4] Reed and colleagues 2016 — Updated EPM Consensus Statement
This information does not replace veterinary advice for your individual horse.