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Vetdle Archive

Archive Case #15: Equine Protozoal Myeloencephalitis

Horse veterinary case - Hard - March 18, 2026

Clinical Clues

  1. A 9-year-old Quarter Horse gelding is evaluated for a gradually worsening gait abnormality.
  2. The owner reports intermittent stumbling and poor performance over several weeks.
  3. Asymmetric muscle loss becomes noticeable over the right hindquarter.
  4. Neurological examination reveals asymmetric, multifocal deficits affecting multiple limbs.
  5. Cervical imaging is unremarkable, but CSF testing supports intrathecal antibody production against Sarcocystis neurona.
  6. A protozoal infection of the equine central nervous system associated with opossum-contaminated feed commonly causes this pattern.

Diagnosis

Equine Protozoal Myeloencephalitis

This case is most consistent with Equine Protozoal Myeloencephalitis because of the progressive asymmetric ataxia, focal muscle atrophy, and supportive CSF findings for Sarcocystis neurona. The most important clues are the multifocal neurologic deficits, asymmetry, and negative cervical imaging, which make a compressive cervical lesion less likely.

Learning review

Review the case evidence

Use the paired clues to explain how the evidence in this horse case supports the provided diagnosis, Equine Protozoal Myeloencephalitis, then compare your reasoning with the case explanation.

Evidence checkpoints

Clues 1-2

  1. Clue 1: A 9-year-old Quarter Horse gelding is evaluated for a gradually worsening gait abnormality.
  2. Clue 2: The owner reports intermittent stumbling and poor performance over several weeks.

Clues 3-4

  1. Clue 3: Asymmetric muscle loss becomes noticeable over the right hindquarter.
  2. Clue 4: Neurological examination reveals asymmetric, multifocal deficits affecting multiple limbs.

Clues 5-6

  1. Clue 5: Cervical imaging is unremarkable, but CSF testing supports intrathecal antibody production against Sarcocystis neurona.
  2. Clue 6: A protozoal infection of the equine central nervous system associated with opossum-contaminated feed commonly causes this pattern.

Self-review prompts

  1. After clues 1-2, what evidence from this horse case shaped your initial interpretation?
  2. When you compare clues 3-4 with clues 1-2, what changed in your reasoning about Equine Protozoal Myeloencephalitis?
  3. How do clues 5-6 align with the case explanation for Equine Protozoal Myeloencephalitis?

Educational Use

Vetdle archive cases are educational veterinary games for diagnostic reasoning practice. They do not provide veterinary advice, diagnosis, treatment, or professional medical guidance.