Equine Neurologic Diseases

Equine Protozoal Myeloencephalitis (EPM)

Disease Background
Sarcocystis neurona is the causative organism of Equine Protozoal Myeloencephalitis (EPM). EPM has a complicated life cycle but involves the common opossum and various intermediate hosts. The horse is an aberrant host to the EPM organism. A large percentage of normal horses have evidence of exposure to S. neurona (EPM antibodies present in serum); a smaller population of horses are clinically effected by EPM. Clinical signs result from S. neurona damaging the central nervous system resulting in neurologic deficits.

The organism cannot be passed from horse to horse, so quarantine of affected animals is not necessary.

Clinical Signs
Common clinical signs include:

  • in-coordination
  • abnormal gait
  • lameness
  • muscle atrophy
  • paralysis of facial muscles
  • head tilt

Upon arriving at the Veterinary Medical Center the equine patient will undergo a general physical exam and the clinician or student will take the patient's history. A complete neurologic exam will be performed to evaluate the central nervous system. If EPM is highly suspected various diagnostics are available to obtain a diagnosis.

Diagnostics for detecting S. neurona include:

  • Detection of S. neurona antibodies in serum. (This test is only of value if negative as most horses are serum positive.)
  • Detection of S. neurona antibodies in the cerebrospinal fluid (spinal tap)
  • Presumptive diagnosis based on clinical signs and physical/neurological exam.

Response to treatment is partially dependent on the severity of disease. It is important to remember that this is a disease of the central nervous system and permanent damage can occur in some cases. Unfortunately we are unable to determine how your horse may respond to treatment.

Some of the available treatments include:

  • Licensed EPM medication (Ponazuril[Marquis], Nitazoxanide[Navigator])
  • Vitamin E
  • Anti-inflammatory drugs

Early diagnosis and treatment will provide your horse with the best odds in defeating this devastating disease. You can reduce the chances of your horse contracting EPM by following these guidelines.

  • Keep feed rooms and containers closed and sealed
  • Use feeders which minimize spillage, don't feed on the ground, clean up spilled grain
  • Regularly clean water troughs
  • Properly dispose of any animal carcasses on the premise
  • Trap, relocate, or eliminate opossum populations from your farm
  • Keep your horse on a good nutritional program, vaccinate and deworm as prescribed by your veterinarian

Wobbler's Disease (Cervical Vertebral Stenoic Myelopathy)

Disease Background
Wobbler's is produced by a compression of the spinal cord. A key symptom is gait instability - which is how the disease got it's name. The horse may seem "wobbly" when walking or exercising.

Wobbler's is a devastating disease that has few treatment options. There are some techniques that are known to prolong life, but none that are considered complete successes.

Below are some of the more common symptoms of Wobbler's.

  • Gait Instability
  • Stiff Neck
  • May appear weak or "lazy"
  • Stumbling
  • Missteps
  • Reluctant to rise
  • Fall easily

Upon arriving at the Veterinary Medical Center our board certified clinicians will perform a neurologic exam on a patient who is suspected of Wobbler's. During this examination they will attempt to determine the location of the lesion in the cervical canal.

Normally they will ask for radiographic images of the cervical spine, and in some cases they will ask for a myelogram. A myelogram consists of injecting a high contrast fluid into the cervical spine and taking radiographic images of the area to see the dimensions of the cervical canal. The fluid will only expand to the width of the canal, allowing our veterinarians to look for any narrowing of the cervical spine. This procedure is performed under general anesthesia.

Sometimes it is necessary to draw Cerebral Spinal Fluid (CSF) for testing. This is essential because of the spinal nature of Wobbler's. The CSF will also be tested for the presence of EPM (see above)

There are very few long term treatments for this disease. Many patients who are treated with the short term alternatives have relapses. If Wobbler's goes untreated the patient's neurological system will degrade dramatically. The horse could then hurt itself or others especially when struggling rise. Many patients with this devastating disease are euthanized.

The causes of Wobbler's have not yet been identified. Some research shows that the problem may be genetic and can be passed from mother to foal. The foals of pregnant mares who are tall and have long necks may be predisposed to Wobbler's. These pregnant mares should be given balanced rations during their pregnancy to help combat passing this disease to the foal.

West Nile Virus

Disease Background
In 1999 West Nile Virus (WNV) appeared in New York for the first time ever in the Western Hemisphere and caused disease and some deaths in humans and animals, including horses. Previously WNV had only been reported in Africa, Asia, Europe, and the Middle East.

WNV is a mosquito-transmitted viral disease that affects humans and animals. Humans, horses, and other animals contract the virus when bitten by an infected mosquito. Scientists do not believe horse to horse, or horse to human transmission can occur because the levels of the virus in the horse's blood are not sufficient to transmit the disease.

In horses, the mortality rates for those individuals who show clinical signs of infection are higher than in humans, but most horses that are exposed to the virus do not become ill. For those individuals that do become ill, they suffer from neurological disease. The severity and nature of clinical signs depend on the area of the nervous system affected by the virus.

The clinical signs in horses vary considerably but the most common symptoms include:

  • Ataxia (lack of coordination, stumbling, weak limbs)
  • Partial paralysis
  • Muscle twitching - especially around the nose and lips
  • Hypersensitivity to touch or sound
  • Head drooping
  • Drooping lips, smacking, chewing, grinding teeth
  • Falling asleep inappropriately (while eating for example)
  • Transient fever before clinical signs develop
  • Lethargy
  • Recumbency (inability to rise)
  • Unresponsiveness
  • In severe cases: high fever, seizures, coma, and death

The best prevention is to reduce your horse's exposure to mosquitoes. Reducing the number of mosquitoes on your farm by eliminating their breeding grounds. Standing water is a hotbed for mosquito breeding. Eliminate all unnecessary standing water on your farm. Aerate ponds and consider adding fish to water tanks to consume mosquito larve.

You can reduce your horse's exposure to mosquitoes by stabling them during the active mosquito feeding times (dawn/dusk) by utilizing fans, barrier cloth, flysheets, repellents, and insecticide misting systems to control the mosquito population in the stables.

Consider getting your horse vaccinated. The vaccine requires two doses, three weeks apart. The vaccination should be completed a minimum of two weeks before the onset of mosquito season for proper performance. Ask your veterinarian about the advantages of vaccinating your horse against WNV.

Encephalitis (Eastern/Western)

Disease Background
There are two forms of encephalitis that affect equine patients in the Central United States, Eastern and Western. Western encephalitis is most common in the central and western states, and is the least lethal of all encephalitis. Eastern encephalitis is more rare in the Midwest, and is more commonly seen on the east coast of the United States. Eastern encephalitis is also more deadly than western encephalitis.

Encephalitis is carried by birds and mosquitoes, much like the West Nile Virus. Encephalitis involves swelling of the brain and the effects of this swelling present themselves in the symptoms listed below.

If your horse has contracted encephalitis they may seem weak or lethargic. They may also seem depressed. Other symptoms may include fever, refusal to eat, walking into objects, or walking in circles.

There are a few other diseases that present themselves in a way similar to encephalitis. Make sure to have your veterinarian check your horse for encephalitis if you suspect that they have been infected.

There are no anti-viral drugs that are effective in treating encephalitis, so the use of vaccines is the only protection that can be provided to horses. Treatment of a horse who has contracted the disease consists of providing the proper nutrition and anti-inflammatory drugs to control the swelling in the brain.

Horses who have contracted encephalitis and have successfully recovered are considered unfit to ride and may be poor mothers. This is due to the disease's permanent affects on the central nervous system.

First and foremost horse owners should have their horses vaccinated. Vaccines are highly effective in stopping contraction of this disease. These vaccines are short lived, and horses who have been vaccinated prior to May 1 and have had heavy exposure to mosquitoes should consider a second vaccination in August or September. Today we have a bi-valent vaccine that will combat both Eastern and Western encephalitis in one shot, where as in the past there were two separate shots.

Secondly any attempts to control the mosquito population in and around your stable can help reduce the number of possibly infected mosquitoes that are near your horses. Eliminate all unnecessary standing water as it is a prime breeding ground for mosquitoes.