Disorders of the Peripheral Nerves in Horses

Degenerative peripheral nerve diseases happen when nerves slowly wear down over time. Two examples in horses are stringhalt and equine laryngeal paralysis.

Stringhalt causes sudden, jerky lifting of one or both hindlegs when the horse moves. Some horses have only mild cases, whereas others can barely walk.

There are two types of stringhalt: classic and Australian.

• Classic stringhalt usually affects one horse at a time. The cause is unknown. Some horses get better on their own, but others might need surgery.

• Australian stringhalt occurs in outbreaks, affecting many horses at once. This usually happens in late summer or autumn and might be linked to eating certain weeds and possibly to toxic molds that grow on those weeds. Many horses improve after being taken off the affected pasture.

Equine laryngeal paralysis, often called roaring, causes loud breathing and poor athletic performance. It happens when the nerve that controls one side of the voice box (larynx) slowly weakens. It is more common in large, long-necked horses and in males.

Inflammation is the body’s response to injury or infection. One inflammatory peripheral nerve disease in horses is polyneuritis equi. This condition affects nerves near the tail end of the spine.

The exact cause is unknown, but it might be caused by an unusual immune system reaction after a viral infection. Adult horses are most often affected.

Signs of polyneuritis equi include:

• leaking of urine or manure

• weak tail

• less feeling around the anus and genitals

• mild loss of coordination in the hind end

• tail rubbing

The vet might do a rectal exam and take x-rays to make sure the problem is not a broken tailbone. There is no proven treatment for polyneuritis equi, and the outlook for horses with this disease is usually poor.

Some peripheral nerve diseases are caused by exposure to toxins (poisons). Examples include botulism, tetanus, organophosphate poisoning, and grass sickness.

Botulism is caused by a toxin made by bacteria that grows in rotting plants or dead animals. Horses usually get it from contaminated feed, though it can also come from an infected wound. Several horses can get sick from the same feed source.

Botulism causes weakness, paralysis (inability to move), and loss of reflexes. It can also affect the horse's face and swallowing muscles. Botulism is diagnosed by finding the toxin in the horse's feed, blood, or manure. Treatment mainly involves keeping the horse as comfortable and stable as possible while the toxin leaves the body. Horses that can't stand have a poor chance of recovery.

Botulism can often be prevented by giving horses clean, dry feed and keeping feed free from contamination. In some high-risk areas, such as the northeastern United States, a vaccine is available.

Foals can develop a type of botulism called shaker foal syndrome, usually when they are less than 8 months old. The toxin, in this case, is produced inside the foal’s intestines. Signs include weakness, stiff movement, shaking, trouble standing, swallowing problems, constipation, large pupils, frequent urination, and breathing trouble. The foal can die within 1–3 days after symptoms start.

Tetanus is another bacterial toxin. It can invade the body through a wound, such as a healing castration site or a puncture. The horse's breathing muscles and limbs can become paralyzed. Protection from tetanus is provided by a vaccine, which is one of the standard vaccines given to horses.

Organophosphate poisoning happens when a horse is exposed to certain insect-killing chemicals called organophosphates. Severe poisoning can cause drooling, diarrhea, breathing problems, muscle twitching, seizures, and coma.

Eating sorghum-type grasses, such as Sudan grass or johnsongrass, can also damage the spinal cord and peripheral nerves, causing weakness, poor coordination, and urination problems.

Grass sickness, also called dysautonomia, is a fatal disease in horses that usually affects grazing horses 2–7 years old, especially in the spring and especially in Great Britain. The cause is still unknown but is suspected to be a toxin produced by microscopic organisms found in pastures. This toxin affects the brain and then the nerves that control the digestive tract and can paralyze the gut.

Signs of grass sickness include:

• patchy sweating

• muscle twitching

• drooling

• hard manure

• food coming out through the nose

• droopy eyelids or eyelashes

• a tucked-up body shape

Some horses with mild grass sickness can survive with intensive care and diet changes, but severe cases often end in death, so euthanasia might be recommended.

Peripheral nerves of horses are often injured during accidents or other traumas.

Sciatic nerve injury can happen after a hip fracture or an irritating injection near the sciatic nerve, which runs from the lower back to the hindlegs. This injury causes hindleg weakness and loss of feeling below the knee.

Femoral nerve injury can happen to foals during a difficult birth. A foal with and injured femoral nerve (a large nerve in the hindleg) might be unable to support its weight because it can't straighten the affected leg.

Suprascapular nerve injury is damage to a nerve above the shoulder blade (scapula) and often follows shoulder trauma. It causes shoulder muscle loss and makes it hard to move the shoulder.

Facial nerve injury can happen if a horse lies on one side of its face for too long or from halter pressure after anesthesia. Signs of injury include a drooping ear or lip, a twisted muzzle, and weak facial reflexes.

Nerves heal slowly. If a nerve is badly torn or blocked by scar tissue, the horse is less likely to recover. Possible treatments include anti-inflammatory drugs, surgery, physical therapy, joint exercises, and splints or bandages.

• Also see professional content regarding diseases of the peripheral nerves.