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as stables. Easy access to grain, horse feed, pet food and trash will attract opossums. Since opossums are also nocturnal, their presence or the feces they leave behind might not be noticed. The sporocysts which are shed in the opossum's feces are viable for several months.

  • Unexplained lameness
  • Stumbling
  • Stiffness or stilted movements
  • Gait abnormalities
  • Weakness that worsens when going up or down slopes
  • Drooping eyelids, ears or lips
  • Difficulty chewing or swallowing
  • Head tilting or shaking
  • Poor balance or leaning against a wall for support
  • Inability to stand or falling down
  • Recumbency or the inability to rise



Diagnosing EPM is difficult. Not only does it mimic other neurological disease and can exhibit a wide range of variable symptoms, but currently there is no definitive test to diagnose the disease. However, there are tests that veterinarians can perform to support a diagnosis of EPM and to rule out other neurological problems.

Veterinarians can use the Mayhew neurological scale to evaluate the severity of clinical symptoms. It is based upon a visual exam of the horse's behavior, posture, sensitivity and movement. The Mayhem scale ranges from zero to five, with zero being normal and five indicating recumbency.

A blood sample can indicate the horse has been exposed to the parasite. But due to the large number of horses that test positive for the S. neurona antibody, this type of test does not provide a basis for the diagnosis of an active infection. Tests that analyze spinal fluid can indicate the presence of the parasite in the central nervous system. Acquisition of spinal fluid requires a spinal tap, a procedure in which a veterinarian inserts a needle into the horse's spinal canal. There are potential risks associated with this procedure. Veterinarians will base a diagnosis of EPM on the clinical symptoms that are present, blood and spinal fluid test results and by eliminating other possible explanations for the presence of clinical symptoms.

Currently, there are two medication that are FDA-approved for the treatment of EPM. Marquis®, manufactured by Bayer Animal Health, is an antiprotozoal oral paste. Protozil® is an alfalfa-based pellet that can be top dressed on the horse's daily ration. Treatment length for both drugs is 28 days. Skipping doses or under-medicating can affect these drugs' ability to kill the infecting parasites.

Early diagnosis and treatment of EPM is essential in preventing further damage to the horse's central nervous system. Even so, damage that has already occurred can be permanent. Statistics show that less than 25 to 30 percent of equine EPM cases will make a full recovery. However, 60 to 70 percent do show improvement with treatment. Relapse of the disease within 90 days does occur in approximately 20 percent of treated horses. Researchers speculate this may be due to the reemergence of hibernating parasites, incomplete annihilation of the parasitic infection within the horse or re-exposure to the parasite.

Currently, the best way to prevent EPM is through management practices aimed at eliminating sources of parasitic infection. Cleaning up and properly disposing of road kill and animal carcasses as well as preventing contamination of hay and grain stored inside the stable by infected opossums will help curb the spread of the S. neurona. Likewise, owners can protect their horses from consuming sporocysts while grazing by locating pastures away from wooded areas, providing an uncontaminated source of water and avoid feeding grains and concentrates outside of the stable or on the ground where opossums can easily scavenge leftovers at night.

Finally, boost your horse's immunological health and reduce stress with a balanced diet, regular exercise, and by following a routine vaccination and deworming schedule. Until further research brings advances in prevention, testing and treatment options for EPM, deterring both the exposure to the parasite and preventing the onset of the disease are our best defenses.

Equine Protozoal Myeloencephalitis

Asymmetrical symptoms are those that affect one side of the body more than the other, while atrophy is the loss of muscle. Horses with EPM often exhibit unilateral symptoms, with either the right or left side showing a higher level of incoordination, paralysis or muscle loss.

Depression, abnormal sweating, loss of sensation to a part of the body, blindness and seizures can also occur. Horses typically do not exhibit an elevated temperature with EPM, nor is it transmitted from horse to horse. In all, the symptoms of the disease can vary widely in individual horses and become progressively worse over time.

In 1995, the opossum was determined to be the definitive host for the protozoan parasite, S. neurona. The definitive or primary host is the one in which the parasite reaches maturity, often undergoing sexual reproduction, before being shed as an infectious stage. The transmission of S. neurona occurs when horses consume food or water that has been contaminated by the feces of infected opossums.

Experts have speculated that equines are not the intended intermediate host for this protozoan as the encysted stage of the parasite or sacrocysts are not present in the muscle tissue of horses. The species that are believed to be intermediate hosts are striped skunks, raccoons, domestic cats, nine-banded armadillos, sea otters and Pacific harbor seals. Like the horse, these hosts ingest the protozoan from food or water sources contaminated with opossum feces.

The life cycle of S. neurona is completed when opossums consume the sarcocyts or encysted parasites in the muscles of infected intermediate hosts. As a scavenger and an opportunistic feeder, the opossum is an ideal host to receive and distribute this parasite. The opossum's diet consists of carrion and road kill as well as food sources found near and around dwellings, such 

Your horse could be harboring the threat for a potentially fatal neurological disease and you might not even know it. Although researchers can only make an estimate, they believe that somewhere between 30 to 80 percent of horses in the United States have been exposed to the parasite responsible for Equine Protozoal Myeloencephalitis or EPM. This statistic is based upon blood analysis that measures the antibody that is produced when horses are infected with Sarcocystis neurona, the protozoan responsible for most cases of EPM.

Fortunately, studies also indicate that less than one percent of all horses that test positive for these antibodies will develop clinical symptoms of the disease. Researchers have yet to uncover why some horses succumb to EPM and others do not. Although stress and other unrelated health events may contribute to the onset of clinical symptoms, little is known about the incubation period and progression of the parasite once it enters the horse's body.

Equine Protozoal Myeloencephalitis is not a new disease. The symptoms of EPM have been identified in horses since the 1960's. At that time the disease was described as segmental myelitis. It took ten years before the disease was linked to some type of protozoan and it wasn't until the early 1990's that Sarcocystis neurona was identified as the leading cause of EPM. Since that time, another closely-related protozoan, Neospora hughesi, has also been identified. The symptoms from N. hughesi are clinically identical to those caused by S. neurona.

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Opossums are nocturnal so their presence or the feces they leave behind might not be noticed.

Once the sporocysts are ingested, the immature protozoans they contain gain access to the horse's bloodstream. It is not known how or under what conditions the developing protozoans are able to cross the blood/brain barrier and generate the neurological disease that is EPM. This process and the onset of symptoms may depend upon the number of protozoans that were ingested, the horse's immunological health, environmental stress and the use of certain drugs that can hinder the horse's ability to fight infection. Since a sizable percentage of horses exhibit seroposititivity for the antigen but never exhibit the symptoms of the EPM, it is believed these horses are immunologically able to control the parasite.

If protozoans gain access to the horse's central nervous system, the onset of symptoms can progress slowly or be acute. The exact symptoms can vary depending upon where in the CNS the parasites lodges. Locations can include the cerebrum, cerebellum, brain stem, cranial nerves and spinal cord. These variables lead to differing symptoms which makes diagnosing EPM difficult. In addition, the symptoms of EPM can mimic other neurological disorders such as West Nile, Eastern and Western encephalitis, rabies and wobbler syndrome.

In general, the symptoms of EPM fall in the categories of ataxia, asymmetry, and atrophy. Ataxia refers to incoordination and the inability to move normally. Symptoms of EPM can range from mild gait abnormalities to recumbency or the inability to stand. Typical signs of EPM associated ataxia include:

The information contained within this article does not constitute medical advice. Please consult your horse's veterinarian for further information, diagnosis and treatment of equine protozoal myeloencephalitis.

Master of Disguise