Poor Performance in The Sport Horse
Dr. Melissa Mazan
Equine Protozoal Myeloencephalitis (EPM)
EPM is caused by a protozoal parasite that invades the neural tissues of the horse. The spinal cord is most frequently affected, but the brain may also be involved.
The parasite, Sarcocystis neurona, is thought to be passed to horses when they accidentally ingest the feces of opossums. Horses do not pass on the infection to other horses or the environment. That's because S. neurona cannot complete its life cycle in the horse, thus the horse is an accidental (incomplete), "dead end" host, in technical terms.
EPM causes a wide array of neurologic symptoms. The most commonly seen abnormalities include ataxia (lack of coordination) and muscle atrophy, especially of specific muscles in the head and tongue, as well as the gluteal and quadriceps muscles.
EPM often initially appears to be a lameness that can't be localized, which is when it becomes a cause of poor performance. With time, the disease worsens, and it is recognized as a major neurological dysfunction. Most cases of EPM are bad enough that they don't allow competition, so the symptoms are picked up at rest, but some are much more subtle. In these latter cases, it can be difficult to implicate EPM as the cause, since there is much lameness related to the muscles and bones that are more likely, and yet equally frustrating to pinpoint. Keep an open mind.
Cervical Vertebral Myelopathy (CVM or Wobblers)
CVM causes neurologic symptoms due to problems in the neck. The bones in the neck have a deformity or instability that pinches the cord. This can happen either slowly over time due to unequal growth of the spinal cord and surrounding bones (vertebrae), or suddenly due to an up and down movement of a neck, which has unstable connections, that pinches the cord while slipping from its stable position.
Alternatively, osteochondrosis dessicans (OCD) can affect the joints in the neck bones, just as it can any joint such as a knee or hock. In this case, the OCD causes inflammation and huge calluses to build up around the bones, which later get so big that they actually pinch the spinal cord.
Horses with CVM have difficulty transmitting signals from the brain to the nerves controlling the legs, since they need to travel through the cord within the neck, which is narrowed or pinching it off.
Horses may show general incoordination, especially in the hind limbs. They may drag or catch their toes, stumble, completely knuckle, misplace their feet, sway from side to side unknowingly, or pitch out (circumduct) a leg when circled. They may also have a very stiff gait, and the owner or veterinarian may note that the toes are chipped and worn due to the abnormally stiff gait.
Although owners often report an initial traumatic incident, such as falling in the field, the fall is usually due to CVM, and the subtle signs become severe enough to be clearly recognized.
CVM is a developmental disease, and is usually recognized in young horses, especially large, rapidly growing male horses. It gets worse with development since the spinal cord is growing, but the bony canal is deformed.
Many of us know what gastric ulcers feel like. Gastric ulceration is the pitting or deep erosion of the stomach lining. It's painful, and some horses don't perform well when they have ulcers.
In nature, horses were meant to eat relatively poor quality roughage continually. Under domestication, horses are generally fed large amounts of high quality concentrates at infrequent intervals. For a horse, even 4 feedings a day constitutes infrequent intervals, unless he always has hay in front of him.
Because horses are by nature continual eaters, they also secrete gastric acid continually. When they stop eating, the acidity in their stomachs rises dramatically. It is important to note that horses that are turned out to pasture and have continual access to roughage, do not develop gastric ulceration.
Although the discovery of the bacterium, Helicobacter pylori, has been a great advance in the understanding and treatment of gastric ulceration in humans, no evidence of this bacterium has been found in horses.
Horses with gastric ulceration may show varying signs, such as poor appetite, chronic colic, poor performance, 'crabby attitude', and teeth grinding.