It’s been said in equine medicine, albeit with tongue in cheek and a good bit of frustration, “Neurology is just a euphemism for necropsy.”
While it’s true that some horses suffering from neurologic problems cannot be helped, tossing all neurologic cases into a dustbin of pessimism does neither the horse nor owner any good. In fact, a thorough physical exam and appropriate testing can help veterinarians achieve a diagnosis in many cases, which, of course, is essential for proper treatment. Some horses with neurologic conditions can certainly be helped.
A thorough physical examination is critical in evaluating neurologic patients. For example, routinely obtained physical data such as body temperature can help a practitioner differentiate between acute infectious causes of neurologic disease. Similarly, evaluation of the horse’s limbs may help a practitioner distinguish between gait abnormalities due to soreness and those caused by proprioceptive deficits.
Horses should be evaluated for behavioral abnormalities as such changes commonly occur with neurological problems, especially acute infectious ones. Once the physical examination has been completed, a neurologic assessment should follow.
Neurologic exams commonly begin with evaluation of the cranial nerves. Moving a hand rapidly toward the eye typically elicits a blink—the menace response. Pupillary light reflexes and the pupil position should be evaluated.
Facial sensation is tested up the nostril and along the eyelid and ear. The facial muscle tone should be evaluated, as well as facial muscle symmetry. A head tilt is a common sign of a cranial nerve problem. A horse’s ability to chew and swallow also should be checked.
The first signs of neurologic problems are often proprioceptive deficits—that is, evaluation of the horse’s awareness of where he is in space.
A gait evaluation is critical because horses with neurologic disease usually demonstrate some degree of ataxia, although some are ataxic due to musculoskeletal problems. Signs of ataxia in affected horses include toe dragging, stumbling or an outward turning of the rear limbs. Commonly used neurologic tests include:
- Tail pull. A horse typically will resist being pulled to one side by his tail.
- Crossing the hind feet. When a horse’s hind feet are crossed when standing, he typically will move the feet quickly to a normal stance.
- Tight circles. Horses with normal proprioception are agile and cross their hind limbs when spinning around the examiner without stumbling or hitting the hind feet or legs together.
- Walking a serpentine over a curb. Horses normally negotiate a curb without stumbling or hitting the curb, whereas affected horses may miss the curb with a front or hind foot, or stumble.
The loss of deep-pain sensation is the last function lost in neurologic conditions, and it typically carries a grave prognosis.
Cervical stenotic myelopathy (CVM)
Also known as Wobbler syndrome, narrowing of the cervical vertebral canal is the most common noninfectious cause of neurologic disease in the horse. CVM can be a developmental problem in young horses, often first recognized when they start training, or it can be an acquired problem in aged horses that develop osteoarthritis of the vertebral facets. The genetic influences resulting in CVM are not clear, but the condition seems to have a higher prevalence in Thoroughbreds than in any other breed.
Accurate diagnosis of CVM is usually made with radiography and perhaps with myelography. In young horses, cervical vertebral abnormalities may be accompanied by abnormalities of the distal limbs (osteochondrosis).
Even if CVM is identified, mildly affected horses may still be useful riding animals.
Equine protozoal myelitis (EPM)
EPM has been the subject of confusion since it was first recognized in the 20th century. The definitive host of the disease is the opossum, which eats an intermediate host containing encysted stages of the protozoan parasite
Sarcocystis neurona. The opossum defecates oocysts, and if the feces is deposited on horse feed, the horse can ingest the oocysts and parasites can develop in the nervous system. Typical signs of EPM include ataxia and asymmetric muscle atrophy.
Diagnosis of EPM may not be easy. Blood tests indicate only that a horse has been exposed to the protozoan organism, but most exposed horses do not also develop clinical disease. Thus, blood tests usually can rule out EPM but should not be used to make a positive diagnosis.
Accurate diagnosis of EPM requires evaluation of cerebrospinal fluid, but even slight blood contamination can elicit a false positive test. As with blood tests, false negatives are rare. Still, due to problems with accurate testing, other possible causes of the clinical signs should be ruled out before settling on a diagnosis of EPM.
The most highly publicized equine neurologic infection is undoubtedly equine herpesvirus-1 (EHV-1). Herpesviruses are arguably the most successful viruses on the planet, existing in just about every species. In horses, EHV-1 can cause respiratory disease—it’s known as rhinopneumonitis—but the notoriety comes from a neurologic form that is acute and typically progressive.
Signs of EHV-1 can be highly variable. Acute infections commonly cause fever, which may help differentiate it from other neurologic conditions, cough and nasal discharge, followed by neurologic indications that typically begin within five days of the initial signs.
Neurologic signs include symmetric ataxia, which typically progresses from the rear of the horse forward. It starts with urinary incontinence and poor tone of the anus and tail before moving to incoordination, paralysis and recumbency. The disease is contagious, and vaccination affords no protection.
Older horses seem to be at higher risk of infection, and stress may play a role in the onset of clinical signs.
A suspect EHV-1 case is confirmed by virus isolation or PCR detection, or a fourfold change in titer on the serum neutralization test using paired sera.
Other important neurologic diseases include arboviruses (arthropod borne viruses), which cause Eastern, Western, and Venezuelan encephalitis, as well as West Nile virus and, of course, rabies, which is in the family rhabdovirus.
These diseases are commonly of acute nature, and clinical signs are often behavioral. Affected horses may appear drunk or unresponsive, but they also may shake or have muscle tremors. Head pressing may be seen in severe cases.
When confronted with suspected neurologic infections in horses, treating veterinarians should request assistance from colleagues.
Dr. David W. Ramey is an author, lecturer and Southern California equine practitioner.
Originally published in the November 2016 issue of Veterinary Practice News. Did you enjoy this article? Then subscribe today!