Equine Neurologic Disease

Horses of all ages and breeds can be affected with disease involving the nervous system. Horses with neurologic disease may exhibit changes in behavior or mental status, seizures, inability to eat or drink normally, altered head position, changes in locomotion, weight loss, or other signs, including sudden death. These diseases may be acute or chronic, and can be the result of congenital, developmental, traumatic, degenerative, infectious/inflammatory, neoplastic, and other processes. When death ensues or the condition necessitates euthanasia, a necropsy examination is often sought to establish a diagnosis or confirm the clinical diagnosis.

Horses that undergo a neurological necropsy examination should be submitted to the laboratory as soon after death as possible, since post-mortem decomposition of the nervous tissue proceeds rapidly. A complete history is important, as well as a detailed clinical neurologic examination prior to death. This examination greatly assists the pathologist by localizing the lesion to an area of the nervous system, or by indicating diffuse or widespread involvement. Additionally, certain non-neurological diseases can appear clinically to be neurologic in origin, and a complete examination can help differentiate these. Rabies can cause a range of clinical signs in horses and must be considered a possibility in most cases of equine neurologic disease of less than 10 to 12 days' duration.

Neurologic necropsy examinations are difficult and time consuming. This is due to the difficulty of removal of the brain and spinal cord. Special training and equipment are required in order to perform central nervous system removal. Certain viral diseases are zoonotic, and proper procedures are required to minimize risk of transmission to necropsy room workers. Brain and spinal cord removal are usually accomplished by a combination of sawing and disarticulation of the skull and vertebrae. The vertebrae and skull must be handled in such a way that detailed examination of these bones can also be undertaken, since fractures or bone abnormalities can cause nervous system damage and signs of disease. Once removed, the brain and spinal cord are examined and samples taken for bacteriology, virology, or toxicology. The remainder is usually immersed in formalin for fixation prior to processing for microscopic examination. As part of a complete necropsy examination, the other body systems are also examined and appropriate samples retained.

A database search over a three-year period from 2000-2002 revealed that 8,833 equine necropsies were performed at the University of Kentucky Livestock Disease Diagnostic Center (LDDC). Of this total, 565, or 6.4%, had diagnoses referable to the nervous system. In a prior Equine Disease Quarterly report (Vol.4, No. 2, 1996) encompassing 34 months (Jan. 1993 to Oct. 1995), 397 (8.7%) of a total of 4,559 equine necropsies had neurologic diagnoses. This comparison implies a constant occurrence of neurologic diseases on a percentage basis, even though the overall equine caseload has increased considerably since 1995.

The majority of equine neurologic cases examined at the LDDC could be classified as developmental or infectious/inflammatory diseases (Figure 1). All of the cases in the developmental classification were cervical stenotic myelopathy (cervical vertebral malformation-malarticulation) cases. There were 158 cases of cervical stenotic myelopathy over the three-year period, making it the most common neurologic disease, accounting for about one-third of all equine neurologic cases. Horses with cervical stenotic myelopathy ranged in age from less than 1 year to 8 years old, with 67% of the horses being yearlings. In keeping with the general population, 79% of the cases were Thoroughbreds; however, a total of 6 different breeds were represented.

The infectious/inflammatory disease group had 161 cases. Included in this group were 23 cases diagnosed as equine protozoal myeloencephalitis. With the spread of West Nile virus to Kentucky, 35 cases were diagnosed as West Nile virus infection in 2002. This category also included cases of encephalitis, myelitis, and meningitis in which conclusive causes were not found. These likely would include additional cases of viral encephalitis and equine protozoal myeloencephalitis that could not be definitively diagnosed due to insufficient lesion development, condition of the tissue, or the lack of microbiological confirmation. During the 3-year period, 191 horses were tested for rabies and none of the horses were positive. Five cases of rabies were diagnosed in other species.

One hundred and fifteen cases were associated with trauma to the nervous system. These included cases of hemorrhage into the central nervous system (67 cases) and fractures (29 cases). Fractures of the skull accounted for 13 cases and vertebral fractures for 16 cases. All segments of the spinal column were involved, but fractures were most common in the cervical spine.

Degenerative diseases included 52 cases of non-specific encephalopathy and myelopathy, four cases of leukoencephalomalacia associated with moldy corn ingestion, and one case of equine motor neuron disease. Congenital anomalies accounted for 24 cases of the neurologic diagnoses. Within this group were a variety of conditions including hydrocephalus, anencephaly, and encephalocele.

Tumors of the central nervous system were very rare, with only a cholesteatoma (actually a granuloma, not at tumor) and a single case of metastatic melanoma diagnosed during the three years. No primary tumors of the nervous system were diagnosed.

Cervical stenotic myelopathy is a major cause of neurologic disease losses in Kentucky and points to the need for better strategies to prevent and manage this disorder.

Dr. Neil M. Williams, (859) 253-0571, nmwillia@uky.edu
University of Kentucky, Livestock Disease Diagnostic Center, Lexington, Kentucky.