EPM, Equine Protozoal Myeloencephalitis, come from the poop of the possum. Basically the disease is caused by one of two protozoa, Sarcocystis neurona or Neospora hughesi. Almost all infections are caused by S. neurona, and both infections are treated with the same drug protocols.
The life cycle of S. neurona is dependant upon the opossum. This makes the disease naturally occurring only where the opossum lives, North and
South America. A horse becomes infected by ingesting the protozoa in feed, hay, pasture or water contaminated with opossum feces.
The reason some horses go from exposure to active infection are unknown, but a weakened immune system and stress have been sited as two possible causes. After a horse has ingested the protozoa, it travels through the digestive tract, and enters the bloodstream. If the horse’s immune system does not clear the protozoa from the blood, it can cross the blood brain barrier. Researchers do not know with certainty how the protozoa crosses; however, theories include its ability enter leukocytes, and cross the barrier inside of these cells. Another theory questions if the blood-brain barrier has been damaged by disease or drugs, which allows the protozoa to cross.
The protozoa live within cells in the CNS, reproducing very slowly. The horse’s immune system is not able to detect protozoa while it is inside of other cells. A stressful event is thought to trigger rapid reproduction and the protozoa begin to break out of the host cells. They move along the CNS, and enter other cells. There the reproduction starts again.
In the process of exiting the cell, the protozoa kill the host cell. Areas of killed cells are lesions in the CNS. As the lesions grow, they impede the transmission of signals from the brain to muscles, and from muscles to the brain. Lesions in the brain can cause behavior changes in the horse. The infection also causes inflammation, or swelling, of the CNS. Inflammation of the CNS can be just as destructive to nerves as the infection.
As the damaged areas of the CNS increase, the horse generally finds ways of compensating for the loss of feeling in a muscle. The immune system may fight the disease for months, with the only outward sign being a slight tiredness or occasional stumble. The outward signs become apparent to humans when the horse can no longer compensate, and the neurological symptoms appear.
The infection in the CNS requires remedies that can cross the blood-brain barrier to combat the protozoa. Currently there are four drugs listed under treatment which have this ability. Other drugs or remedies may be able to kill the protozoa in the blood, but leave the active infection raging in the CNS. From my reading most people use a combination of drug treatments to cleanse the horse of all life cycles of the parasite.
Horses are dead-end carriers, they cannot infect other horses. Not every horse exposed develops an active infection. Now we are moving forward to help Vince and bring to more people this debilitating and often hard to diagnose disease. Tomorrow, more on blood tests, spinal taps and other diagnostic tests.
For more information, check out: EPMHorse.org.