A 7-year-old palomino Quarter horse was referred for an acute onset of vestibular dysfunction with facial nerve paralysis. On presentation, the horse was bright, alert, and responsive with normal vital parameters. Vestibular syndrome was characterized by a head tilt to the right and mild horizontal nystagmus with the fast phase to the left. Facial nerve paralysis consisted of right ear and eyelid drooping as well as slight muzzle deviation to the left. Menace and palpebral reflexes were slightly diminished on the right side. No other abnormality was detected on physical or neurologic examination. The results of a complete blood count, serum chemistry profile, and cerebrospinal fluid tap analysis were normal.
What is the most likely diagnosis?
a) Polyneuritis equi
b) Equine herpesvirus myeloencephalopathy
c) Brain abscess
d) Temporohyoid osteoarthropathy
e) Equine protozoal myeloencephalitis