Neoplasia of the Nervous System in Large Animals

Intracranial Neoplasia in Horses

Horses with neurological disease localized to the intracranial space may rarely have intracranial neoplasia. Reported intracranial neoplasms include the following:

• macroadenomas of the pituitary gland associated with pars pituitary intermedia dysfunction (PPID)

• cholesterol granulomas of the choroid plexus

• other primary neoplasms: oligodendroglioma, astrocytoma, ependymoma, gangliocytoma, olfactory neuroblastoma, ganglioglioma, medulloblastoma, meningioma

• secondary (metastatic) neoplasms: lymphoma, pineoblastoma, melanoma, hemangiosarcoma, adenocarcinoma

Clinical signs associated with these neoplasms are referable to their specific intracranial location or the primary neoplasm site in the case of metastasis.

Many of these neoplasms can be diagnosed with advanced imaging (CT or MRI); however, this diagnostic investigation is unlikely to change treatment options or prognosis. CSF evaluation of horses with intracranial neoplasms is generally unrewarding and is either normal or demonstrates nonspecific pleocytosis with increased protein. CSF collection is contraindicated if increased intracranial pressure is suspected.

Macroadenomas of the pituitary gland are by far the most common intracranial neoplasm of aged horses. Based on the location of the neoplasms, horses may develop blindness, loss of pupillary light reflex, mentation abnormalities, and seizures, in addition to clinical signs of PPID.  

A presumptive diagnosis of a pituitary macroadenoma is based on patient history and clinical diagnosis of PPID, along with neurological disease localized to the optic chiasm and forebrain. A definitive diagnosis is achieved through advanced imaging (CT or MRI) or necropsy.

For horses with pituitary macroadenomas, treatment for PPID is recommended; however, treatment may not decrease neoplasm size or improve neurological function. Additional therapy aimed at reducing intracranial pressure, edema, and inflammation can be considered; however, the prognosis is poor. Hypertonic saline solution (7.2%, 2–4 mL/kg, IV over 15 minutes) and dexamethasone (0.05–0.1 mg/kg, IV) are generally used.

Cholesterol granulomas are generally incidental neoplasms of the choroid plexus found in up to 20% of aged horses. Cerebral disease is possible if these neoplasms restrict flow of CSF or compress cerebral tissue.

When present, clinical signs are often intermittent. Cholesterol granulomas are more likely to be clinically important when present in the lateral ventricle (versus the fourth ventricle).

The prognosis for horses with cholesterol granulomas is variable and based on severity of clinical signs. When clinical signs are present, treatments aimed at reducing intracranial pressure, edema, and inflammation are recommended (see above).

Spinal and Peripheral Nerve Neoplasia in Horses

Primary or secondary neurological neoplasms affecting the spinal cord or peripheral nerves are rare in horses and include melanoma (most common), lymphoma, adenocarcinoma, fibrosarcoma, myeloma, squamous cell carcinoma, and hemangiosarcoma. Peripheral nerve sheath tumors have been reported in horses.

Clinical signs associated with these neoplasms are referable to their location along the spinal cord, peripheral nerves, or the primary neoplasm site in the case of metastasis.

Neoplasms affecting the spinal cord can be challenging to diagnose antemortem. Advanced imaging of the spinal cord is not readily available beyond the cervical vertebrae in full-sized adult horses. As with intracranial neoplasia, CSF evaluation in horses with spinal neoplasia is generally unrewarding and either is normal or demonstrates nonspecific pleocytosis with increased protein.

The prognosis for clinically important neoplasms affecting the spinal cord or peripheral nerves is poor. Peripheral nerve sheath neoplasms are likely to recur in horses, as they do in small animal species.

Intracranial Neoplasia of Cattle

Intracranial neoplasms in cattle are rare. As in horses, these neoplasms may be primary or secondary (metastatic).

Metastatic lymphoma is the most common CNS neoplasm in cattle and is often associated with bovine leukosis (infection with bovine leukemia virus [BLV]). Typically, BLV-associated lymphoma of the CNS affects the spinal cord; however, intracranial disease is also possible.

Other intracranial neoplasms of cattle include thymic lymphosarcoma, ependymoma, oligodendroglioma, astrocytoma, glioma, meningioma, carcinoma, ganglioneuroma, and medulloblastoma. Medulloblastoma and ganglioma are reported as congenital neoplasms of neonatal calves.

Based on practical challenges and the individual value of affected animals, definitive antemortem diagnosis of intracranial neoplasia in cattle is rare, and the prognosis is poor.

As with horses, when treatment is considered, it is aimed at reducing intracranial pressure, inflammation, and edema.

Spinal and Peripheral Neoplasia of Cattle

Lymphoma associated with BLV infection is the most common clinically important neurological neoplasm of cattle. Although lymphoma can occur anywhere along the CNS, it most often affects the spinal cord, and specifically, the lumbosacral spinal segments.

Affected cattle develop pelvic limb paresis or paralysis and variable deficits in perineal tone, sensation, and urinary continence. Spinal rhabdomyosarcoma and melanoma, and peripheral nerve sheath neoplasms, have also been reported in cattle.

Peripheral nerve sheath neoplasms may be more common in cattle than in other domestic species but are typically only identified during slaughter. These neoplasms most often affect the celiac and thoracic ganglia.

A correlation between bovine spongiform encephalopathy (BSE) and peripheral nerve sheath neoplasms has been postulated. In a study of cattle in Germany, 1 of 4 (25%) analyzable celiac ganglia among 20 cattle with BSE had neurofibroma, compared with 6 of 66 (9.09%) analyzable celiac ganglia among 103 cattle in the control group (1).

As with horses, atypical cells are rarely identified in the CSF of cattle with CNS neoplasia. In these animals, CSF is usually normal or demonstrates a mild pleocytosis with increased protein.

The prognosis for cattle with any neoplasm affecting the nervous system is poor. As with other neurological neoplasms, treatment aimed at reducing intracranial pressure, inflammation, and edema can be considered.