Neurology

In headshaking, the horse involuntarily and often violently flicks or jerks its head, mainly vertically, in response to a painful hypersensitivity of the trigeminal nerve (facial nerve). A common form is light-triggered (photic headshaking) and is worse seasonally in summer and in sunlight. The syndrome is not a vice but a neuropathic process that can severely impair quality of life.

In wobbler syndrome, a malformation or instability of the cervical vertebrae narrows the spinal canal (stenosis) and thus compresses the spinal cord. The result is a usually progressive ataxia (incoordination), often in young, fast-growing large-breed horses. SAFETY NOTE: ataxic horses are unpredictable in movement and handling and can fall — riding and close handling carry a considerable injury risk for human and animal.

EPM is an inflammation of brain and spinal cord caused by protozoa (usually Sarcocystis neurona, less often Neospora hughesi) that can produce very varied neurological deficits. GEOGRAPHIC NOTE: EPM occurs mainly in North, Central and South America, because its definitive host — the opossum — is native there; in Europe and the German-speaking region it appears practically only in imported horses or returning travellers that were previously in the Americas. Horses become infected via feed or water contaminated with opossum faeces, not horse-to-horse.

In facial nerve paralysis, the facial nerve (nervus facialis) is impaired on one or both sides, so that the mimic muscles of lip, nostril and ear no longer work properly. Common causes are pressure or trauma, e.g. from a too-tight halter, prolonged lying on hard ground or unfavourable positioning during anaesthesia. The typical picture is a drooping lip and a drawn-in nostril on the affected side.

Stringhalt is a movement disorder in which one or both hind legs are jerked up exaggeratedly high during protraction, sometimes towards the belly. It is a neuromuscular disorder; besides a classic, often one-sided form, there is a pasture-associated (Australian) variant linked to the intake of certain plants — mainly flatweed (Hypochaeris radicata), less often vetches or Lathyrus species — that can affect several horses of a herd.

Shivering (shivers) is a usually slowly progressive neuromuscular movement disorder that mainly affects large horse breeds. Typical are involuntary twitching and an overextended, trembling posture of the hindquarters, especially when backing up and lifting the hind legs (e.g. for hoof picking or shoeing). The exact cause is not conclusively clarified; changes in the cerebellum (Purkinje cells) are discussed.

Hereditary disease of the cerebellum in the Arabian, in which specialised nerve cells degenerate prematurely. CA is inherited autosomal-recessive (marker in the TOE1 gene, regulatory effect on the neighbouring MUTYH); only homozygous animals are affected. Typical are coordination and balance disturbances that usually become apparent in foalhood and persist.

Severe hereditary neurological disease of the newborn Arabian foal (also Coat Color Dilution Lethal), caused by a mutation in the MYO5A gene and inherited autosomal-recessive. Affected foals show the most severe neurological disturbances directly after birth and are not viable; often a diluted (lavender/silvery-pink) coat colour is noticeable at the same time.

Hereditary developmental disorder of the Friesian with pathological accumulation of cerebrospinal fluid and a strongly enlarged skull of the foal, caused by a mutation in the B3GALNT2 gene and inherited autosomal-recessive. Affected foals are mostly not viable (often stillbirth); the enlarged head additionally means a considerable risk for a difficult birth (dystocia) and thus a danger for the mare.