Equine protozoal myeloencephalitis, or EPM, is a disease cause by a protozoa

Protozoa

Protozoa are a diverse group of single-cells eukaryotic organisms, many of which are motile. Throughout history, protozoa have been defined as single-cell protists with animal-like behavior, e.g., movement...

l infection of the central nervous system

Central nervous system

The central nervous system is the part of the nervous system that integrates the information that it receives from, and coordinates the activity of, all parts of the bodies of bilaterian animals—that is, all multicellular animals except sponges and radially symmetric animals such as jellyfish...

 of horse

Horse

The horse is one of two extant subspecies of Equus ferus, or the wild horse. It is a single-hooved mammal belonging to the taxonomic family Equidae. The horse has evolved over the past 45 to 55 million years from a small multi-toed creature into the large, single-toed animal of today...

s.

History

EPM was discovered in the 1960s by Dr. Jim Rooney. The disease is considered rare, though recently, an increasing number of cases have been reported. Research at the University of Kentucky

University of Kentucky

The University of Kentucky, also known as UK, is a public co-educational university and is one of the state's two land-grant universities, located in Lexington, Kentucky...

 has labeled the opossum as the definitive host

Host (biology)

In biology, a host is an organism that harbors a parasite, or a mutual or commensal symbiont, typically providing nourishment and shelter. In botany, a host plant is one that supplies food resources and substrate for certain insects or other fauna...

 of the disease.

Causes

EPM is caused by the parasite Sarcocystis

Sarcocystis

Sarcocystis is a genus of protozoa. Species in this genus infect reptiles, birds and mammals. The name is dervived from Greek: sarx = flesh and kystis = bladder.There are about 130 recognised species in this genus...

 neurona. In order to complete its life cycle this parasite needs two hosts, a definitive and an intermediate. In the laboratory, raccoon

Raccoon

Procyon is a genus of nocturnal mammals, comprising three species commonly known as raccoons, in the family Procyonidae. The most familiar species, the common raccoon , is often known simply as "the" raccoon, as the two other raccoon species in the genus are native only to the tropics and are...

s, cat

Cat

The cat , also known as the domestic cat or housecat to distinguish it from other felids and felines, is a small, usually furry, domesticated, carnivorous mammal that is valued by humans for its companionship and for its ability to hunt vermin and household pests...

s, armadillo

Armadillo

Armadillos are New World placental mammals, known for having a leathery armor shell. Dasypodidae is the only surviving family in the order Cingulata, part of the superorder Xenarthra along with the anteaters and sloths. The word armadillo is Spanish for "little armored one"...

s, skunk

Skunk

Skunks are mammals best known for their ability to secrete a liquid with a strong, foul odor. General appearance varies from species to species, from black-and-white to brown or cream colored. Skunks belong to the family Mephitidae and to the order Carnivora...

s, and sea otter

Sea Otter

The sea otter is a marine mammal native to the coasts of the northern and eastern North Pacific Ocean. Adult sea otters typically weigh between 14 and 45 kg , making them the heaviest members of the weasel family, but among the smallest marine mammals...

s have been shown to be intermediate hosts. The opossum is the definitive host of the disease. Horses most commonly contract EPM from grazing or watering in areas where an opossum has recently defecated. However, horses cannot pass the disease among themselves. That is, one horse cannot contract the disease from another infected horse. The horse is the dead-end, or aberrant, host of the disease.

Symptoms

The neurologic signs that EPM causes are most commonly asymmetric incoordination (ataxia

Ataxia

Ataxia is a neurological sign and symptom that consists of gross lack of coordination of muscle movements. Ataxia is a non-specific clinical manifestation implying dysfunction of the parts of the nervous system that coordinate movement, such as the cerebellum...

), weakness and spasticity

Spasticity

Spasticity is a feature of altered skeletal muscle performance in muscle tone involving hypertonia, which is also referred to as an unusual "tightness" of muscles...

, although they may mimic almost any neurologic disorder. Clinical signs among horses with EPM include a wide array of symptoms that may result from primary or secondary problems. Some of the signs cannot be distinguished from other problems, such as lameness, which can be attributed to many different causes. Airway abnormalities, such as laryngeal hemiplegia (paralyzed flaps), dorsal displacement of the soft palate

Soft palate

The soft palate is the soft tissue constituting the back of the roof of the mouth. The soft palate is distinguished from the hard palate at the front of the mouth in that it does not contain bone....

 (snoring), or airway noise of undetermined origin may result from protozoa infecting the nerves which innervate the throat. Apparent lameness, particularly atypical lameness or slight gait asymmetry of the rear limbs are commonly caused by EPM. Focal muscle atrophy

Muscle atrophy

Muscle atrophy, or disuse atrophy, is defined as a decrease in the mass of the muscle; it can be a partial or complete wasting away of muscle. When a muscle atrophies, this leads to muscle weakness, since the ability to exert force is related to mass...

, or even generalized muscle atrophy or loss of condition may result. Secondary signs also occur with neurologic disease. Upward fixation of the patella (locking up of the stifle joint

Stifle joint

The stifle joint is a complex joint in the hind limbs of quadruped mammals such as the sheep, horse or dog. It is the equivalent joint to the human knee...

) is among the most common findings among horses with neurologic disease. Another common side effect of EPM is back soreness, which can be severe. The actual method by which the Sarcocystis neurona infects a horse is still unknown, however it is thought to preferentially infect leukocytes (white blood cells) in order to cross the blood brain barrier.

Treatment and prevention

This disease is curable if caught soon enough and treated with antiprotozoal drugs. There are currently three antiprotozoal treatments available: potentiated sulfonamide medications such as ReBalance, Marquis (ponazuril

Ponazuril

Ponazuril , sold by the Bayer Corporation under the trade name Marquis, is a drug currently approved for the treatment of equine protozoal myeloencephalitis in horses, caused by Sarcocystis neurona. More recently, veterinarians have been preparing a formulary version of the medication for use in...

), and Navigator.

Control of this disease includes a recently released vaccine

Vaccine

A vaccine is a biological preparation that improves immunity to a particular disease. A vaccine typically contains an agent that resembles a disease-causing microorganism, and is often made from weakened or killed forms of the microbe or its toxins...

 against the parasite and control of opossums in an area. The vaccine, however, has only been conditionally approved by the USDA until efficacy tests are available.