What is EPM?
EPM is Equine Protozoal Myeloencephalitis, it is a neurologic disease of the horse caused by two kinds of parasites. These parasites are Sarcocystis neurona and Neospora hughesii. Many horses are exposed to the parasite, but it is estimated that only approximately 1% of horses are effected by EPM.
What does EPM look like?
EPM is called the great imitator, meaning that it can mimic lameness and other neurologic diseases. Often, a horse with EPM will look like a horse with a lameness that can not be tracked down. Other times, it is a horse that has had a decrease in its‘ performance or is tripping or falling when performing. Sometimes, muscle atrophy will be noticed. The muscle atrophy can be any muscle, but often is the gluteal muscles. EPM can also affect cranial nerves and cause signs such as facial paralysis and loss of sensation along face, neck or body.
How does my horse get EPM?
The opossum is the definitive host of these protozoal parasites. A definitive host is the host in which a parasite reaches maturity. There are many intermediate hosts including raccoons, skunks and armadillos. The intermediate hosts harbors the parasite for a short period of maturation. The horse is an aberrant host, which means it is not the normal host for the organism and the organism can not develop normally and complete its’ development. The parasite develops into a sarcocyst in the skeletal muscle of the intermediate host and is then ingested by the definitive host of the opossum. In the digestive tract of the opossum the sarcocyst develops into a sporocyst. The sporocyst is the phase that is infective to the horse. The sporocyst is found in the opossum feces. In order for the horse to become infected with EPM, it ingests the opossum feces. Typically from eating hay, but it can be from the pasture or grain as well. After the horse eats the sporocyst, the sporocyst then travels to the horses’ central nervous system and causes lesion or damage to the spinal cord. The damage to the spinal cord is what causes the signs that you, as an owner see in your horse.
The spinal cord and the nerves that come off the spinal cord control the motor function or muscle movement and muscle tone in your horse. When the spinal cord is damaged in an area, it is unable to correctly send the signals to your horses’ muscles to move the muscles that are innervated by the nerves that connect to the damaged portion of the spinal cord.
How do I find out if my horse has EPM?
FIrst, a complete neurologic exam by your veterinarian needs to be performed. Next, your horse can be tested for EPM. EPM tests can be done on cerebral spinal fluid or on blood. Obtaining spinal fluid in the horse can be difficult and has risks associated with it, so a highly sensitive and specific blood test is desired. All of the tests are antibody tests. The oldest test is the Western Blot test. Currently there are more sensitive and specific tests available, so the Western Blot is no longer favored. ELISA (enzyme-linked immunosorbent assay) is another type of test. This ELISA test is used to test for antibodies to one of many surface antigens that are expressed on the organism. The third test available is the IFAT (indirect fluorescent antibody titer). This test gives a quantitative value and provides a chart that gives the likelihood that the horse is truly infected with the organism. The IFAT test has a higher sensitivity and specificity than the Western Blot test. The ELISA tests are promising, but currently are best used in combination with the IFAT test.
What do I do if my horse has EPM?
Currently, there are several treatments for EPM as well as constant research to form better treatments for EPM. EPM is caused by a protozoa and treatment with antibiotics alone is not appropriate and has a high incidence of relapse of the disease. There are only 4 FDA approved treatments for EPM at this time.
- Trimethoprim sulfadiazine plus pyrimethamine: This treatment has been around for many years. This drug currently is not made by any major manufacturers and must be compounded. The initial cost of this drug is fairly low, however, you often have to treat your horse daily for 3-7 months. In addition, this medication has an increased relapse rate of 10-28%.
- Marquis or Ponazuril: This treatment is currently the gold standard. It is a 28 day treatment and is a very safe medication with minimal side effects.
- Protazil or Diclazuril: This treatment has a comparable cost to Marquis and is a 30 day treatment. Recently, the labeled dose is less than previously recommended and may result in a higher relapse rate than previously reported with the higher dose.
- Navigator or Nitazoxanide: This treatment is no longer available.
A new treatment that is currently in the process of receiving FDA approval is Oroquin 10. This new medication combines two older drugs, levamisole and decoquinate. This medication is given for 10 days.
EPM can be a complicated and scary disease to horse owners. By working closely with your veterinarian, you can make a plan that best suites you and your horse.