Disorders of the Spinal Column and Cord in Horses
Disorders of the spinal column and cord include birth defects (discussed earlier in this chapter), degenerative diseases, inflammatory and infectious diseases, tumors, nutritional diseases, injury and trauma, toxic disorders, and vascular diseases.
Cervical spondylomyelopathy (also known as cervical stenotic myelopathy, or wobbler syndrome) is the most common noninfectious disease of the spinal cord and occurs in many breeds, with some breeds possibly predisposed. Most horses show signs before they are 3 years old, although signs can show up later, sometimes in association with changes in training and activity. In young horses, diet modification to slow growth rate can help. The mid-neck region is most commonly affected, and incoordination and weakness in all legs may occur. Malformation or instability of the cervical vertebrae exert pressure on the spinal cord, resulting in clinical signs. If not corrected surgically, long-term pressure will cause focal areas of degeneration of the spinal cord. Diagnosis requires imaging with x-rays, often in conjunction with injection of a contrast material into the fluid surrounding the spinal column.
Equine degenerative myeloencephalopathy is a neurologic disorder of horses and zebras. It has been seen in many equine breeds in North America, Australia, and England, and is believed to be related to a vitamin E deficiency and to involve genetic factors. Various parts of the central nervous system degenerate, and signs, which appear during the first year of life, may stabilize or slowly continue to worsen. All 4 legs become weak and uncoordinated, with the hind legs being affected more severely. There is no definitive way to diagnose equine degenerative myeloencephalopathy, although finding low blood levels of vitamin E is supportive evidence. Vitamin E supplements may help horses improve and can be preventive in some cases.
Equine motor neuron disease is a progressive, noninflammatory degeneration of motor neurons in the spinal cord and brain stem of horses. Adult horses of any age and breed can be affected, although Quarter Horses are affected most commonly. This disease is most common in the northeastern United States, but it has been reported in several areas of North and South America, Europe, and Japan. The disease has been reproduced by longterm feeding of diets low in vitamin E.
Signs include weakness, trembling, and muscle wasting all over the body. Horses often stand with their head held low and their feet camped under their body, frequently shifting their weight from one leg to another. Loss of coordination is not seen in this disease. Many horses develop a distinct pigment pattern on their retinas or other abnormalities in their eyes. There is no specific treatment, but some horses improve partially after 2 to 3 months.
Infectious and inflammatory diseases of the spinal column and spinal cord include bacterial, rickettsial, viral, fungal, protozoal, and parasitic infections. Many of these diseases, such as meningitis and encephalitis, can also affect the brain. Some of the more common infectious and inflammatory diseases that affect the spinal column or cord are discussed below.
Vertebral osteomyelitis is inflammation of the vertebra without concurrent disc infection, and discospondylitis is inflammation of the intervertebral disc and adjacent vertebral bodies. Both diseases are usually caused by an infection spread through the blood to the spine. In horses, newly born animals are at greatest risk for these conditions.
Equine infectious anemia usually affects the spinal cord but occasionally infects the brain in horses. Neurologic signs include lack of coordination and weakness in the hind legs. Analysis of the cerebrospinal fluid is required for diagnosis. There is no treatment, and affected horses are usually euthanized to prevent spread of the disease.
Equine herpesvirus-1 (EHV-1) myeloencephalopathy can affect horses of any age; however, horses older than 4 years are most susceptible. Although there is a vaccine for the virus that protects against the respiratory disease it causes, the vaccine does not protect from the neurological disease. Equine herpesvirus-1 infects blood vessels within the central nervous system and causes cell death and bleeding throughout the brain and spinal cord. The neurologic signs may be the first sign of the disease, or they may occur after a respiratory infection or an abortion. Signs begin abruptly but usually do not progress after 48 hours. They include urine dribbling, retention of feces, sensory deficits in the perineum and tail, mild lack of coordination in the hind legs, and possibly paralysis and an inability to rise after lying down. There is no specific treatment, but mildly affected horses often recover with supportive care. Some horses that cannot rise also eventually recover. The antiviral drug acyclovir has been advocated for treatment and prevention of EHV-1 myeloencephalopathy; however, the effectiveness is uncertain and still controversial.
Rabies is caused by a virus that travels through the nervous system, causing inflammation in the spinal cord and brain. It produces multiple areas of inflammation, causing incoordination and progressive paralysis. It is always fatal, typically causing death within 2-7 days of signs of illness.
Cryptococcus neoformans is the most common fungus to cause a central nervous system infection in dogs and cats, and is seen occasionally in horses. Other fungal organisms may also invade the central nervous system. Infections often affect other organs, such as the lungs, eyes, skin, or bones. Signs of spinal cord infection include partial or total paralysis and spinal pain. Blood or cerebrospinal fluid tests are necessary to diagnose an infection and identify the organism.
Treatment and the outlook for recovery depend on the specific fungus involved. The drug fluconazole is often effective for Cryptococcus infections. Infections with Blastomyces or Histoplasma fungi are difficult to treat, and the outlook for recovery in horses infected with these fungi is uncertain.
Equine protozoal myeloencephalitis causes signs of spinal cord disease and encephalitis (brain inflammation) in horses. It results from an infection with Sarcocystis neurona, which is carried by opossums. In a few cases it is caused by infection with other protozoa.. Any age horse can be affected, although signs are very rare in horses less than 12 months old. Signs vary depending on the location of the infection. A loss of motor control and partial paralysis of the legs are common. Other potential signs include weakness and wasting of leg muscles and cranial nerve dysfunction. Diagnosis is based on signs, analysis of cerebrospinal fluid, and response to drug treatment. Many horses recover with treatment, but permanent neurologic damage is possible. Prevention is difficult but involves keeping opossums away from the horse and its feed and water.
Verminous myelitis and encephalitis are inflammatory conditions of the spinal cord and brain, respectively, caused by a parasite. The most common such parasites in horses are Setaria species, Halicephalobus deletrix, and Strongylus vulgaris. Signs of central nervous system inflammation strike suddenly, often affecting one side of the body more than the other, and may worsen over time. This condition is difficult to diagnosis, but may be suspected on the basis of cerebrospinal fluid analysis. Drug treatment can be beneficial, but a full recovery is uncertain.
Acute spinal cord injuries can lead to spinal fracture or dislocation. In horses, falls are a common cause of these injuries. Damage to the spinal cord is not only caused by the fall but also as a result of secondary changes such as fluid accumulation or bleeding into the space surrounding the spinal cord, causing increased pressure. Diagnosis may require imaging. Anti-inflammatory medications are often used to reduce complications due to inflammation. Animals with mild neurologic deficits may recover with 4 to 6 weeks of stall rest and pain mediations, but the prognosis is guarded for horses that cannot stand.
Delayed organophosphate intoxication can be seen after ingestion or skin contact with insecticides or pesticides that contain organophosphates. In addition to the signs of severe exposure to organophosphates, delayed paralysis can develop 1 to 4 weeks after exposure. Partial paralysis of the hind legs worsens progressively and occasionally all 4 legs become paralyzed. A veterinarian will need a history of the horse’s possible chemical exposure to make the correct diagnosis. The outlook for recovery is poor for animals with severe signs.
Sorghum subspecies, such as Sorghum, Sudan, and Johnson grass, can cause degeneration of the spinal cord in horses. This may be caused by the high levels of hydrocyanide in these grasses. Signs include lack of coordination, weakness of the hind legs, and incontinence or urine retention. Diagnosis is based on signs and a history of exposure. Signs may improve once the feed is removed.
Tetanus is caused by toxins produced by Clostridium tetani bacteria that usually are produced at the site of a wound. Signs usually develop within 5 to 10 days of infection and include muscle stiffness and rigid leg extension, inability to swallow, protruding eyelids, and locking of the jowl and facial muscles. In severe cases, the animal may be unable to stand as a result of muscle spasms. Treatment consists of wound care, antibiotics to kill any remaining organisms, and tetanus antitoxin. In mild cases, a horse may recover completely with early treatment. In severe cases, death may occur due to respiratory paralysis.
Postanesthetic hemorrhagic myelopathy in horses is a rare complication seen in horses positioned on their backs under general anesthesia. The cause may be impaired drainage of blood away from the spinal cord due to compression of large abdominal veins under the weight of the intestines and other abdominal organs. Horses with this damage will be paralyzed immediately after recovery from anesthesia, and the prognosis is poor. Changing the horse's position during surgery may help prevent this complication.
Also see professional content regarding diseases of the spinal cord.