Dr. David Ramey is a veterinarian in Los Angeles, and in an article in Veterinary Practice News he talks about the three most common neurological diseases in horses. Most everyone with a horse has heard of a disease that affects the horse's nervous system that is transmitted by opossums; some people even call it 'that opossum disease.' Although the protozoal organism that causes the disease is transmitted in opossum feces, the opossum itself does not cause the disease. The correct name of the disease is equine protozoal myelitis, or EPM for short, and it is the most commonly diagnosed and misdiagnosed neurological disease in horses. Lots of horses are diagnosed with EPM that probably do not have the disease because of the difficulty in making a diagnosis. Symptoms of EPM include typical nervous system signs such as wobbling when walking and atrophy of muscles, but many other signs can also occur.
Another neurological disease called wobbler is a syndrome in which the vertebrae in the neck narrow, thus pinching off the horse's spinal cord. It causes similar symptoms to EPM.
The third nervous system disease that is a real concern is infection with herpes virus 1 because it is contagious to other horses. Usually these horses initially become weak and paralyzed in their back legs and then the disease progresses. This disease tends to affect horses at shows; I suspect that's due to stress, and because it's contagious, it can cause a lot of horses to be quarantined.
Two nervous system diseases of concern in Texas that Dr. Ramey did not mention are rabies and West Nile virus infection. Any nervous system disease in a horse is serious and your veterinarian should be called as soon as symptoms are noted.