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EQUINE LAMENESS

Equine Protozoal Myeloencephalitis (EPM)
Definition and Cause

Equine Protozoal Myeloencephalitis (EPM) is a disease that affects the horse's spinal cord and brain. It is caused by a protozoa that until recently was called Sarcocystis neurona. Researchers have discovered that this organism is virtually identical to a protozoa called Sarcocystis falcatula, which is a parasite that infests several bird species, including cowbirds and grackles. Opossums are the normal host animals, although the parasite must pass through a bird before completing its lifecycle in the opossum.

How is it spread?
Presumably, the parasite is spread to the horse in opossum droppings on the pasture. The horse is the wrong host--the parasite cannot complete its lifecycle in the horse's body. It also cannot be spread from horse to horse, or from the horse to other animals (or people).

Once the parasite enters the horse's body, it travels to the spinal cord and brain. There it enters the nerve cells and eventually destroys them. The horse's immune response to the parasite causes inflammation and further damage to the nervous system tissues. These destructive changes can occur anywhere in the CNS, often at multiple sites.

Which kinds of horses are affected?
The disease has been reported in virtually all breeds of horses. It is seen in most states in the U.S., although it appears to be more common in the northeastern and northern Midwest states. All age groups of horses can be affected, but the disease is most often seen in young adult horses. The amount of stress the horse is under and its immune status probably have much to do with the incidence of this disease. Young horses in training are often under stress, and they are exposed to many infectious organisms that continually challenge their immune systems. Mature horses that are ill are more prone to developing EPM because their immune systems are compromised. Pregnant mares may be more prone to developing EPM in the second half of the pregnancy because of the demands on their systems.

Are epidemics common?
In most cases, only one or two horses in a group are affected at any one time. Outbreaks of EPM are uncommon. Many people feel that this disease is on the increase. However, the apparent increase in the incidence of EPM is probably due to an increased awareness of its existence and improved diagnostic techniques.

Signs of EPM

The type, severity, and progression of the neurologic abnormalities vary widely among horses. This is because the parasite can cause damage anywhere within the CNS, and usually at more than one site. The signs may begin subtly and progress gradually, or they may appear suddenly and worsen rapidly. The most common signs are:

• ataxia and weakness
• muscle atrophy
• cranial nerve deficits

These signs are discussed in the earlier section on Neurologic Examination.

Ataxia and weakness—typical signs of spinal cord disease—are common findings in horses with EPM. The signs are usually asymmetrical. In many horses the only initial sign may be slight ataxia or weakness in one leg. This is often mistaken for low-grade lameness. Although the signs may at first be mild, it is common for them to worsen if left untreated. In severe cases the horse may be so unsteady that it falls and is unable to get up.

Photo Courtesy of Dr. Rebecca McConnico, Oklahoma State University

The muscle atrophy that is commonly seen with EPM is also asymmetrical. It is usually most obvious over the hindquarters. Muscle atrophy takes a couple of weeks to become obvious, so this sign may not be apparent at first.

Horses with EPM may also show evidence of brain involvement, in particular, cranial nerve deficits. These signs include facial paralysis, drooping ear, lazy eyelid, and difficulty eating and swallowing. As with the other signs, cranial nerve deficits are usually asymmetrical and highly variable. In some cases the signs reflect cerebral or cerebellar disease. Horses with EPM usually do not have a fever, depression, or loss of appetite, even though they may have difficulty eating. (Cerebral, cerebellar, and cranial nerve signs are listed in Figure 12-44.)

Diagnosis of EPM

In many cases the veterinarian suspects EPM simply on the signs, although there are several other neurologic diseases that can cause similar signs. In a horse with very mild signs, musculoskeletal causes of vague lameness or gait abnormalities should be ruled out before concentrating on the nervous system. Nuclear scintigraphy (see Chapter 2) often helps to pinpoint the site(s) of low-grade lameness when physical examination and other tests cannot.

What tests are available to diagnose EPM?
At present the only way to confirm the diagnosis of EPM in the live horse is by taking a sample of cerebrospinal fluid. This is usually done with a lumbosacral tap. The CSF sample is tested for antibodies against the parasite (serology), or for protozoal DNA using PCR. The serology test, called Western blot immunoassay, is used more often than PCR.

If the veterinarian cannot collect CSF from the horse, he or she may treat the horse based on the signs. This approach is reasonable; however, without a definite diagnosis it is impossible to know whether the horse actually has EPM. Treating the horse for EPM when it has another neurologic condition delays appropriate treatment. Also, the treatment for EPM can be expensive. Wherever possible the diagnosis should be confirmed.

Interpreting the Test Results

According to recent research, up to 50% of all horses (both normal horses and those with neurologic signs) have been exposed to this Sarcocystis and so have detectable antibodies in their bloodstreams. But detecting antibodies in the bloodstream only proves that the horse has been exposed to the parasite in the past. It does not mean that the parasite caused, or will cause disease, or even is still present in the body. Therefore, testing for antibodies in a blood sample is usually a waste of time and can even be misleading if it is the only test performed. (This is why requesting a blood test for EPM during a prepurchase examination is discouraged by most veterinarians, especially when there are no suspicions of neurologic disease.)

Treating a horse for EPM because it had a positive blood test is an even bigger waste of time and money in most cases. Nevertheless, in attempting to diagnose EPM, it is often useful for the veterinarian to submit both blood and CSF samples to the laboratory, especially if the CSF sample was contaminated with blood. Some veterinarians use the blood test as a "screen." If the blood test is negative, it is unlikely that the horse has EPM. However, if the blood test is positive, the CSF should be tested to determine whether the horse has the disease.

To read about the Treatment, Prognosis, and Prevention of EPM, be sure to order your copy of
Equine Lameness!

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