Signalment: An 8-year-old female mixed-breed pony that had been shown successfully as a hunter pony.
History: The pony was referred for evaluation because of a sudden onset of head tilt and circling. One day prior to the initial evaluation, the pony's head became tilted to the left with the neck turned to the right; the pony was reluctant to move to the left, but remained alert and responsive. Flunixin meglumine (1.1 mg/kg [0.5 mg/lb], IV, once) was administered with no improvement.
Physical examination: Findings of a physical examination were unremarkable.
Neurologic examination
What is the problem? Where is the lesion? What are the most probable causes of this problem? What is your plan to establish a diagnosis? Please turn the page.
Comments: Equine protozoal myeloencephalitis is always strongly suspected when neurologic signs in a horse cannot be explained by a single lesion. In this pony, there was evidence of at least 2 lesions (left region of brainstem and left side of the thoracic portion of the spinal cord). Because the pony was weak in response to a stationary tail-pull on the left side, it is possible that there was an additional lesion in the spinal cord segments that are involved in antigravity reflexes in the left hind limb (ie, L3-S2).
Abnormal test results:
Abnormal laboratory data: Cytologic examination of a CSF sample revealed a mild pleocytosis (7 WBCs/μL; reference range, 0 to 5 WBCs/μL) with an apparently normal differential count and an erythrocyte count of 383 RBCs/μL; total protein concentration was within reference limits (44 mg/dL; reference range, 5 to 65 mg/dL). Results of western blot analysis identified antibodies against S neurona (relative quantity, 35). This result must be interpreted cautiously in light of RBC contamination.1
Imaging procedures: Radiography of the skull and endoscopy of the upper portion of the airway and auditory tube diverticula revealed no abnormalities. Magnetic resonance imaging (T1-weighted, T2-weighted, and fluid-attenuated inversion recovery [FLAIR] imaging and T1-weighted imaging after administration of gadopentetate dimeglumine) was performed. On T2-weighted and FLAIR images, a moderately high signal-intense focus (20 × 17 mm) was detected in the center and left caudal aspects of the brainstem, caudal to the fourth ventricle (Figures 1 and 2). The focus was isointense on T1-weighted images and was not contrast-enhanced after administration of gadopentetate dimeglumine.
Transverse T2-weighted magnetic resonance image of the brain of a pony that had a sudden onset of head tilt and circling. The pony's right side is to the left in the image; dorsal is toward the top of the image. Notice an area of increased signal intensity (arrow) located slightly to the left of midline in the caudal brainstem.
Citation: Journal of the American Veterinary Medical Association 232, 2; 10.2460/javma.232.2.201
Sagittal fluid-attenuated inversion recovery (FLAIR) image of the brain of the pony in Figure 1. Rostral is toward the left and dorsal is toward the top of the image. An ovoid focus of increased signal intensity (arrow) is present in the brainstem.
Citation: Journal of the American Veterinary Medical Association 232, 2; 10.2460/javma.232.2.201
Presumptive diagnosis: A diagnosis of equine protozoal myeloencephalitis was made. Equine protozoal myeloencephalitis is a progressive disease of the CNS in horses caused by a protozoal infection, most often with S neurona.2 The clinical signs are variable; however, multifocal, asymmetric signs predominate. Although the spinal cord is most often affected, signs of brain disease can develop. Onset of clinical signs can be sudden or insidious. In this pony, the location of the lesion was consistent with the neurologic examination interpretation and further suggested that the vestibular signs had developed paradoxically (ie, paradoxic central vestibular syndrome).2,3,4 The characteristics of the lesion in the magnetic resonance images were consistent with an inflammatory process, rather than with a discrete abscess or neoplastic process.5,6,7 In horses, equine protozoal myeloencephalitis is the most likely cause of a discrete inflammatory lesion that is associated with clinical signs similar to those of the pony of this report.2 A diagnosis of equine protozoal myeloencephalitis was supported by the results of western blot analysis and the clinical response to treatment. In this case, magnetic resonance imaging was useful for ruling out neoplasia and abscess formation.
Prognosis with treatment: Fair to good.
Prognosis without treatment: Guarded to poor.
Therapeutic plan: The pony was treated with pyrimethamine (1 mg/kg [0.45 mg/lb], PO, q 24 h), sulfadiazine (20 mg/kg [9.09 mg/lb], PO, q 24 h), and ponazuril (5 mg/kg [2.27 mg/lb], PO, q 24 h) for 1 month before reevaluation.
Outcome: The pony was examined after 1 month of treatment. The cranial nerve VII deficits had improved, but there was a mild left ear droop and muzzle deviation to the right. No head tilt, neck tilt, or circling was evident, and apparently normal physiologic nystagmus could be elicited. When the pony was blindfolded, a mild leftsided head tilt became apparent. It was recommended that the pony be turned out in a small paddock by itself for exercise. Continued administration of the ponazuril, pyrimethamine, and sulfadiazine and monthly evaluations were also recommended. Complete blood counts were performed periodically because of the risk of bone marrow suppression with the administration of pyrimethamine and sulfadiazine.
Comments: Equine protozoal myeloencephalitis should be considered as a differential diagnosis in equids that develop signs of central vestibular disease. Magnetic resonance imaging can be helpful in ruling out other intracranial lesions.
References
- 1.↑
Miller MM, Sweeney CR, Russell GE, et al. Effects of blood contamination of cerebrospinal fluid on western blot analysis for detection of antibodies against Sarcocystis neurona and on albumin quotient and immunoglobin G index in horses. J Am Vet Med Assoc 1999;215:67–71.
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Mayhew IG. Incoordination of the head and limbs: cerebellar diseases. In: Mayhew IG, ed. Large animal neurology. Philadelphia: Lea & Febiger, 1989;227–241.
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de Lahunta A. Cerebellum. In: de Lahunta A, ed. Veterinary neuroanatomy and clinical neurology. Philadelphia: WB Saunders Co, 1983;255–278.
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Ferrell EA, Gavin PR, Tucker RL, et al. Magnetic resonance for evaluation of neurologic disease in 12 horses. Vet Radiol Ultrasound 2002;43:510–516.
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Gray LC, Magdesian KG, Sturges BK, et al. Suspected protozoal myeloencephalitis in a two-month-old colt. Vet Rec 2001;149:267–273.
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Cherubini GB, Mantis P, Martinez TA, et al. Utility of magnetic resonance imaging for distinguishing neoplastic from non-neoplastic brain lesions in dogs and cats. Vet Radiol Ultrasound 2005;46:384–387.
