Large Animals - The Merck Veterinary Manual

Large Animals

Anencephaly means that the brain is largely absent at birth. It is a rare disorder but is seen sporadically in calves; the cause is unknown. Because the pituitary gland may also be absent, prolonged gestation (Prolonged Gestation in Cattle and Sheep: Introduction) of affected calves can occur. Signs include profound lethargy, head pressing, and blindness with normal pupillary reflexes. Cerebral aplasia in calves is usually associated with complete absence of both cerebral hemispheres, and CSF may leak out of a small opening on the midline between the frontal bones.

Exencephaly means that the brain is exposed through a large defect in the skull (cranium bifida). The brain (encephalocele), meninges (meningocele), or both (meningoencephalocele) may protrude through this opening. Encephalocele and meningocele are inherited in pigs.

In hydranencephaly, there is a marked loss of cerebral cortical tissue (primarily the neocortex) within a cranial vault of normal conformation. The resultant cavity communicates with the ventricular system, has an incomplete ependymal cell lining, and is filled with CSF. Hydranencephaly develops as a result of the destruction of developing neural tissues and is sometimes accompanied by cerebellar hypoplasia and arthrogryposis. Hydranencephaly is seen sporadically or as an epidemic in calves, lambs, and less commonly in piglets. Known causes include infection in utero with a number of viruses, including Akabane virus ( Akabane Virus Infection) in ruminants in Australia, Japan, and Israel; bluetongue virus ( Bluetongue: Introduction) in sheep and cattle in North America; Rift Valley fever virus ( Rift Valley Fever: Introduction) and the virus of Wesselsbron disease ( Wesselsbron Disease: Introduction) in sheep and cattle in Africa; the Cache Valley virus in sheep in the USA; and the Chuzan virus in calves in Japan. Rarely, bovine viral diarrhea ( Bovine Viral Diarrhea and Mucosal Disease Complex) and Border disease virus ( Border Disease) produce hydranencephaly in lambs and calves. Hydranencephaly and porencephaly (cystic cavities in the cerebrum) are seen sometimes in lambs with in utero copper deficiency (swayback). It also is seen in a syndrome of prolonged gestation in sheep in Scotland (cause unknown). Clinical signs may include lethargy, propulsive circling, head pressing, and blindness.

Hydrocephalus, foal

Hydrocephalus, an increase in volume of the CSF, can appear similar to hydranencephaly, but in hydrocephalus the ventricles retain a complete ependymal lining. There may be extensive expansion of the lateral ventricles in the frontal lobes. Hydrocephalus is seen sporadically in all large animals, although it is relatively common in calves, in which inheritance and vitamin A deficiency have been implicated.

Cyclopia is characterized by a single orbital fossa. One cause in lambs is ingestion by the gestating dam of plant alkaloids from Veratrum californicum . This malformation also is seen in pigs.

Idiopathic or familial epilepsy has been described in many species. Benign epilepsy is seen in young foals, particularly Arabians, up to 12 mo of age. The foal may present either for seizures, or for head injuries or postictal blindness. Foals usually recover spontaneously within a few months, but anticonvulsant therapy (phenobarbital, 100-500 mg, PO, bid, for a 50-kg foal) is probably advisable for 1-3 mo, followed by withdrawal over 2 wk. Epilepsy beginning by 1 yr of age has been recorded in Brown Swiss and Swedish Red cattle. Seizures are also seen in young Aberdeen Angus calves; if these calves survive, they show cerebellar signs but become clinically normal by 2 yr of age.

Metabolic disorders and lysosomal storage disorders often cause signs of forebrain dysfunction, along with other neurologic deficits, and are discussed further under multifocal disorders (). Cerebral signs seem to be most prominent in citrullinemia.

Citrullinemia is a fatal hereditary metabolic defect of Holstein-Friesian calves (mainly in Australia and New Zealand) associated with cerebral cortical edema. It is due to increased citrulline in plasma, caused by deficiency of the urea cycle enzyme argininosuccinate synthetase. Affected calves appear healthy at birth but die of acute neurologic disease in 1-4 days. Signs are sudden in onset and consist of depression, aimless wandering, blindness, seizures, opisthotonos, and recumbency.

Narcolepsy, a disorder of sleep-wake control (typically characterized by excessive sleepiness or sudden paroxysmal attacks of flaccid paralysis with conservation of consciousness), has been reported in several equine breeds, particularly Shetland ponies. The animal is otherwise healthy. During narcoleptic episodes, rapid eye movements occur, and at the same time, the animal may also show cataplexy or sudden loss of muscle tone with collapse.