Equine Neurologic Pathology

This is an excerpt from Equine Disease Quarterly, funded by underwriters at Lloyd's, London, brokers, and their Kentucky agents.

Horses are afflicted with a number of central nervous system diseases. Diagnosis of these conditions can be challenging both in the live horse and in horses that died or were euthanized due to the neurologic disease. In a clinically affected horse, central nervous system (brain and spinal cord) disease may manifest as altered mental state or behavior, cranial nerve deficits, seizures, abnormal posture, ataxia, paresis or paralysis, muscle atrophy, incontinence, pain, recumbency, or coma. With these varied signs, a systematic neurologic examination is essential for diagnosis.

The goals of the neurologic examination are to establish whether a neurologic problem is truly present and to determine the anatomic location of the process. Likewise, the postmortem examination of a neurologic horse is important to evaluate the clinical diagnosis, establish a diagnosis in undiagnosed cases, provide information that might allow prevention or treatment in herd mates, and monitor for possible zoonotic diseases.

An approach used in the pathological examination of the brain and spinal cord (as well as other body systems) is pathologic pattern recognition. Recognition of the pattern of lesions helps to categorize the condition and allows for consideration of diseases known to produce the pattern. Using both macroscopic and microscopic examination, it can be determined if the lesions affect only a single area of the nervous system or are multifocal or diffuse. Likewise, involvement of specific anatomic structures and symmetry of the pathology are evaluated.

In horses, one pattern of pathology exhibits variably severe degeneration of all nerve fiber tracts and grey matter in a cervical spinal cord segment (see Figure 1 at www.ca.uky.edu/gluck/q_jan07.asp). Cross sections of spinal cord above and below this area show distinctive patterns of change. Above the site there is degeneration of the fiber tracts in the dorsal and dorsolateral funiculi, and below the primary site the degeneration is limited to the medial and ventromedial funiculi. This is a pattern of descending and ascending neuron degeneration.  When the axon of a neuron is damaged at some point, the portion of the fiber farthest away from the nerve cell body degenerates. The portion near the cell body can regenerate. The dorsal and lateral funiculi represent sensory fibers, and the ventral funiculi represent motor fibers. This pattern is seen when there is focal injury to the spinal cord. In horses the most common cause by far of this injury is cervical vertebral stenotic myelopathy. Other causes would include tumors, subluxations, and abscesses that exert pressure on the spinal cord.

A second pattern of neurologic pathology is characterized by lesions affecting only certain anatomical structures. In one example of this pattern, there is symmetrical degeneration of nerve cell bodies and fibers primarily in the thoracic portion of the spinal cord (nucleus thoracicus and dorsal spinocerebellar tract fibers in the lateral funiculus). Also, there is degeneration of a particular nucleus (cuneate nuclei) in the brain stem. This pattern is seen in the disease equine degenerative myelopathy. Another disease of horses, nigropallidal encephalomalacia, is associated with the ingestion of the plant yellow star thistle (Centaurea solstitialis). Toxicity from eating this poisonous plant causes a special pattern of pathology in which there is degeneration in certain basal nuclei in the brain stem.

Another neurologic pattern is an inflammatory pattern that consists of random, variably sized (often large), asymmetric areas of inflammation that affect the brain stem and spinal cord. Aggregates of primarily mononuclear cells surround blood vessels and infiltrate into adjacent brain or spinal cord tissue, resulting in degeneration. This pattern is consistent with the disease equine protozoal myeloencephalitis.

A final example is another inflammatory-type pathologic pattern that consists of a mild accumulation of mononuclear cells (multifocal to diffuse). These infiltrates are associated with blood vessels, and occasionally the cells invade the nerve tissue. This pattern suggests viral infection and can be seen with any viral encephalitis, including rabies. Neurologic equine herpesvirus type 1 infection varies slightly in its pattern. With EHV-1, vasculitis leads to thrombosis and degeneration of the surrounding nerve tissue.

Pathologic pattern recognition is a tool utilized by pathologists to assist in diagnosis of neurologic disease. A particular pattern can suggest a diagnosis even when the causative agent cannot be demonstrated.

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

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Equine Disease Quarterly

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