Final answer:
Equine Degenerative Myeloencephalopathy (EDM) is the most likely cause of ataxia, weakness, and spasticity in a young horse, especially with symptoms predominantly in the hind limbs, suggesting a UMN lesion associated with this particular disorder.
Step-by-step explanation:
Based on the signs of symmetric ataxia, weakness, spasticity, and toe dragging in a 1.5-year-old Quarter Horse gelding, the most likely cause of these clinical signs is Equine Degenerative Myeloencephalopathy (EDM). EDM is a progressive neurodegenerative disorder most commonly affecting young horses, characterized by ataxia and spasticity, particularly worse in the hind limbs. This disease is associated with lesions in the spinal cord and the brain stem that affect the upper motor neurons (UMN), leading to both spastic and flaccid paralysis.
The clinical signs described, which are worse in the hind limbs and include interference between the legs during walking, suggest a neuromuscular issue affecting the horse's spinal cord. Considering the age and symptoms of the horse, EDM, which is related to a vitamin E deficiency and genetic predisposition, should be considered as the underlying cause. The diagnosis can be supported by a thorough neurological examination, blood tests, and potentially advanced imaging like MRI, to exclude other possible causes such as Equine Protozoal Myeloencephalitis (EPM) or Cauda Equina Syndrome.