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Epitope screening: on the way to a vaccine for Equine Protozoal Myeloencephalitis (EPM)


To identify immune molecules that may be protective against S. neurona infection as a first step towards developing a vaccine against EPM.


EPM is a devastating equine neurological disease. EPM can lead to long-term, permanent deficits in horses who develop disease. In some rare cases, affected horses may recover without treatment. However, many horses that develop disease have permanent neurologic changes, and may be limited in what they can do. Affected horses, if they continue to get worse and treatment proves ineffective, can develop progressive disease to the point that they may need to be euthanized.

Our goal in this study is to identify potential immune targets that can protect horses against infection with S. neurona, the primary organism that causes EPM. Identifying these immune targets is a first step towards developing a vaccine against EPM.


  • All enrolled horses must be aged 3+ years, sound, neurologically normal, and with a normal gait.
  • Horses should remain in the area, with no planned rehoming for 1-2 years, because we may be collecting blood at a few timepoints. 

Two groups of horses will be included in this study:

  1. Group 1: S. neurona resistant horses. Horses that are neurologically normal and have strong serum antibody titers against S. neurona. A titer of>1:250 indicates these horses have been exposed to S. neurona, but because they are neurologically normal, they have developed a protective immune response.
  2. Group 2: Control horses. Sound, healthy horses with no history of EPM exposure. These horses have a titer of <1:250 to S. neurona, indicating they haven’t been exposed.

Exclusion Criteria

  • Any uncontrolled medical condition that may disrupt study intent and objectives
  • Systemic illness within the past 30 days
  • Horse is pregnant or a female who is planned to be bred within the next 1-2 years

Study Design

We will perform neurologic exams on each horse at the screening exam. If the horse is neurologically normal, the horse will be enrolled and we will collect a blood sample, approximately 10mL, for testing for antibodies to S. neurona. If the horse has been exposed to S. neurona but remains neurologically normal, the horse is assumed to have made a protective immune response against the disease. Based on these antibody results, we will classify the horse in either the resistant or control population. We will then collect blood, approximately 40-60mL, for immune cell function tests to see if we can identify molecules that may have the ability to stimulate a protective immune response. For some enrolled horses, blood collections may occur up to four times over a two-year period.


There are no costs to you for your horse to participate in the study. The study does not provide any compensation; however, the results of this study may benefit future animals who are at risk of developing EPM.


Dr. Sharon Witonsky, Associate Professor, Equine Field Service
Office Phone: 540-231-9042

If your query is urgent, please call the Large Animal Hospital on 540-231-9042 and ask for Dr. Witonsky to be paged.