Normal behavior, coordinated  movement and muscular strength in the horse are all integrated through a normal central nervous system (brain and spinal cord). Damage to this coordinated system may lead to signs of  ataxia (incoordination), spastic movements, exaggerated movements and weakness. Changes in mental status may also occur with inflammation or damage to the CNS (central nervous system). The spinal cord is a direct continuation of the brain and runs through and is protected by the spinal or vertebral canal which is divided into the cervical (7), thoracic (18), lumbar (6), sacral (5) and caudal (15-21) vertebrae. The brain and spinal cord are also surrounded by 3 membranes, called the meninges and is bathed by cerebrospinal fluid that nourishes and protects it. At each space between vertebrae (intervertebral spaces) a spinal nerve exits (one on the right and one on the left). These spinal nerves travel down and innervate the limbs, organs, etc. Sensory information is transmitted from the periphery up to the CNS, intergrated, and then appropriate motor information is transmitted back down the spinal nerve to the limbs. This is what allows the horse to know where its limbs are in space and to move appropriately. (see picture below)

 (Abnormal positioning of the limbs as a sign of neurologic disease)

Signs of neurologic disease then, whatever the cause, generally include some degree of incoordination, weakness, spasticity and exaggerated limb  movements. These changes may be very mild and expressed as only minor gait or lameness issues or they may be severe, causing the horse to be unable to stand up or walk normally, presenting a danger to itself or to the associated humans. Years ago horses showing these neurologic signs were all called "wobblers" because of their incoordinated gaits. Research of the past 15-20 years has found quite a few separate, distinct diseases that have these similar signs. Differentiation between these is based on the age, sex and breed of the horse, manner of presentation,progression and severity of signs, laboratory evaluation (blood and/or cerebrospinal fluid analysis) and possibly imaging (myelogram, MRI, etc.).

The most common of the neurologic diseases is EPM, or equine protozoal myelitis. This disease is caused by the ingestion of a protozoal parasite called Sarcocystis neurona. The organism is aquired through the ingestion of contaminated feed material with the feces of opposums which are the natural hosts of this parasite. The horse is not a normal host for the Sarcocystis and in the horse it damages the spinal cord leading to the neurologic signs discussed above. Severity of signs is extremely variable and may also include facial paralysis, atrophy of large muscle masses and changes in behavior or attitude. The definitive diagnosis of EPM is based on the clinical signs, blood tests and ultimately a cerebrospinal fluid evaluation. Treatment is based on the administration of one of a number of oral anti-protozoal drugs.

Other conditons that may present with similar neurologic signs include equine cervical stenotic myelopathy, which is caused by compression of the spinal cord by abnormal development of adjoining cervical vertebrae. Definitve diagnosis is based on radiological imaging (myelogram, MRI) and the only definitive treatment is surgical stabilization of the involved vertebrae. Equine herpesvirus (EHV1,4; rhinopneumonitis) infection may also present with neurologic signs, often within 2 weeks of an upper respiratory outbreak, as the most common presentation of "rhino" is an upper respiratory infection with signs including fever, cough and nasal discharge. The neurologic form may also cause bladder paralysis and therefore constant dribbling of urine along with incoordiantion, weakness, etc. Diagnosis is based on history, presentation, blood testing and possible nasal swabs. Treatment is limited to nursing care (sometimes steroids as well). Equine degenerative myeloencephalopathy and equine motor neuron disease, which is similar to human Lou Gehrig's disease are 2 fairly rare neurologic conditions with unknown causes and no consistently successful treatments. (see picture below) 

The stance of a horse with motor neuron disease.

Certain infectious diseases such as EEE and WEE (eastern and western equine encephalomyelitis), rabies and West Nile Virus may present with neurologic signs as may trauma to and congenital malformations of the CNS.