Equine Protozoal Myeloencephalitis: An overview

What are the clinical signs? Clinical signs of EPM may include seizures, depression, muscle atrophy, the inability to coordinate voluntary muscle activity and paralysis. Many of these symptoms are similar to other equine diseases that affect the CNS, thus making EPM difficult to diagnose. In addition, the clinical signs of EPM can be subtle or severe. Clinical signs may progress rapidly, or in other cases progression may be slow and signs may even diminish.

How prevalent is EPM? Studies indicate the prevalence of exposure to S. neurona in some areas to be as high as 50 percent for all horses, with the percentage increasing to 80 percent for older, adult horses in some parts of the United States. Although the chances of being seropositive increase with age, the incidence of clinical disease peaks between 15 months and five years. Stress appears to play an important role in clinical cases and the incubation period is variable. If left untreated, an infection by S. neurona may remain in the horse's system indefinitely. Which horses will display clinical signs and which horses won't is yet unexplained.

How is Sarcocystis neurona transmitted? The organism that causes EPM is mainly transmitted to horse by the opossum. The opossum releases infective sporocysts into the environment through its feces. The horse ingests the sporocysts via contaminated feed or water. How the sporocysts migrate to the CNS to cause disease isn't completely clear. However, a reasonable assumption is that after the sporocysts are ingested, they invade the CNS via the circulatory system. Once the infective organisms reach the CNS, they may proliferate by asexual reproduction and cause disease by their presence and the associated inflammatory response. Insects, birds or small mammals may also transport or ingest the sporocysts, contributing to the cycle of EPM transmission. The horse, however, is considered to be a "dead-end" host, and therefore not infectious to other animals.

Which horses are at greatest risk? Infection is believed to occur along the western coast and from the central states eastward in the United States, most of Mexico, Central America and South America.

Virtually all equine breeds have tested positive in seroprevalence studies. The disease is most common in young (three-year-old) horses, with racehorses (especially standardbreds) and show horses at higher risk than breeding and pleasure horses.

How is EPM diagnosed? Diagnosis of EPM infection is difficult to pinpoint. One notable clue of the disease is that it tends to affect one side or part of the horse more than another. If your veterinarian suspects EPM, a blood and cerebrospinal fluid (CSF) analysis may be conducted. This procedure requires a spinal tap be performed on the horse. If the blood test is positive, this indicates the horse has been exposed to the organism, not that it has attacked the CNS or that it will develop clinical signs.

Another test called the Western blot analysis can also be conducted on the CSF and serum. The greatest value of the Western blot test on CSF is when the result is negative. A negative Western blot result on CSF generally rules out EPM.

Can EPM infection be treated? Several treatment regimens have been developed. They are expensive and can last several months. Treatment consists of anti-protozoal, anti-inflammatory, anti-edema and antioxidant therapy. Immunomodulator treatment and physical therapy may also be attempted. The antiprotozoal treatment is the cornerstone of the treatment programs. The oldest and most commonly used antiprotozoal treatment is a combination of the folate synthesis inhibitors pyrimethamine and a sulfonamide. This combination of drugs is administered orally. Relapses, particularly following stressful situations, are common.

Can EPM infection be prevented? Only now has a vaccine been developed that may aid in the prevention of EPM. The Sarcocystis neurona vaccine stimulates the development of neutralizing antibodies to S. neurona, which may aid in the prevention of disease because of new infections, but efficacy and potency tests are still in progress.

The USDA license is conditional. Considering the prevalence of exposure to S. neurona, the difficulty in diagnosing the disease as well as the expense and time involved in treatment programs, use of the vaccine for its potential to aid in the prevention of new infection may be of utmost importance to some horse owners.

Practical management tools also include proper maintenance of the horses' living and feeding areas. Limiting exposure to opossums, as well as controlling exposure to contaminated feed, will aid in breaking the cycle of transmission.