The nervous system is composed of billions of neurons with long, interconnecting processes that form complex integrated electrochemical circuits. It is through these neuronal circuits that animals experience sensations and respond appropriately.
Congenital defects of the CNS are, by definition, present at birth. Some congenital defects may be inherited, others may be caused by environmental factors (eg, toxic plants, nutritional deficiencies, viral infections); for many, the cause is unknown. In those animals born with a well-developed nervous system (foals, calves, lambs, pigs), the clinical signs of a congenital neurologic disorder may be recognizable at birth. Kittens and puppies are born with a less well-developed nervous system, and in those species, neurologic signs may not be apparent until they begin to walk.
Developmental myelin disorders of the nervous system can be broadly categorized into myelination that is decreased or absent (hypomyelinogenesis) or abnormal (dysmyelination—eg, leukodystrophy, myelinolysis). Myelin disorders can affect the entire nervous system or be localized to the central nervous system or the peripheral nervous system. Developmental myelin disorders should not be confused with demyelination, in which previously normal myelin breaks down and is lost. Signature pathological processes of nerves undergoing demyelination include Wallerian degeneration, axonal degeneration, and segmental and diffuse myelin degeneration. Often there is no treatment for degenerative conditions; however, supportive care may counter underlying causes of acquired disorders of myelin. Genetic testing is available for some degenerative conditions.
Diseases of the peripheral nerves and neuromuscular junction include degenerative diseases, inflammatory diseases, metabolic disorders, neoplasia, nutritional disorders, toxic disorders, trauma, and vascular diseases, some of which are congenital disorders.
Diseases of the spinal column and cord include congenital disorders, degenerative diseases, inflammatory and infectious diseases, neoplasia, nutritional disorders, trauma, toxic disorders, and vascular diseases. Many of these diseases are discussed in full in other chapters and are only briefly described here. For a discussion of congenital disorders related to the spinal column and cord, All.see page Congenital and Inherited Anomalies of the Nervous System.
The dysautonomias are a group of diseases with strikingly similar clinical and pathologic signs reported in a number of unrelated species, including horses, dogs, cats, rabbits, and hares. The disease is characterized by the degeneration of neurons in autonomic ganglia and clinical signs of autonomic nervous system dysfunction. The etiology is unknown in all species, and there is no effective treatment.
Facial paralysis is paralysis of the muscles affecting facial expression (eyelids, lips, ears, nose, etc). It can be caused by a lesion of the peripheral portion of the facial nerve or the facial nucleus in the brain stem. Diagnosis is based on clinical signs and use of tests to identify the specific cause. Treatment is directed at the underlying etiology.
Meningitis, encephalitis, and encephalomyelitis are terms used to describe inflammatory conditions of the meninges, brain, or brain and spinal cord, respectively. These inflammatory processes frequently occur concurrently, with the terms meningoencephalitis and meningoencephalomyelitis used. Although such conditions have long been associated with bacteria, viruses, fungi, rickettsial agents, and parasites, more recent findings in both people and animals are starting to also implicate chemical agents and immune-mediated processes, with genetic predispositions proposed for some of these latter conditions. Depending on the causal agent, the extent and speed of onset, and location of the inflammation, clinical signs can vary from subtle to dramatic, and therapies can have varied successes. Accurate diagnosis is dependent on quality neurologic (including ocular) examinations and supported by CSF analysis and imaging, in particular MRI. In unsuccessful cases, the importance of postmortem examinations cannot be underestimated, because historically, many zoonotic agents have manifested as meningoencephalitis in people and animals.
Motion sickness is characterized by signs referable to stimulation of the vestibular and autonomic nervous systems, including excessive salivation and vomiting. Affected animals may also yawn, whine, or show signs of uneasiness and apprehension; severely affected animals may also develop diarrhea. Motion sickness is seen during travel by land, sea, or air, and signs usually disappear when vehicular motion ceases.
Primary neoplasia of the nervous system includes tumors originating from the brain, spinal cord, and peripheral nerves. Clinical signs are related to location: causing seizures, affecting mentation, altering sensation, or producing dysfunction of enervated muscles. Most tumors are presumptively diagnosed with advanced imaging (ie, MRI or CT) and definitively diagnosed with biopsy. Therapy can include surgery, radiation therapy, chemotherapy, or a combination.
Paraneoplastic syndromes are nonmetastatic complications of cancer with effects distant from the primary tumor. They are unrelated to neurologic complications secondary to metabolic or nutritional disorders, infection, cerebrovascular incidents, or adverse effects of treatments. They can affect all parts of the nervous system, including the brain, cranial nerves, spinal cord, dorsal root ganglia, peripheral nerves, and the neuromuscular junction. Some are thought to be immunologically mediated through cross-reactivity by immune cells against antigens expressed by tumors and neural tissues (molecular mimicry), whereas others are related to the production of circulating hormones, peptides, or other substances that exert systemic effects.
Polioencephalomalacia is a common neurologic disease of ruminants. The main clinical signs reflect dysfunction of the cerebrum and include wandering, circling, cortical blindness, incoordination, head pressing, recumbency, nystagmus, and seizure activity. Some animals are found dead. Clinical diagnosis is often difficult and suspected based on the combination of neurologic signs, elimination of other diagnoses, and response to thiamine administration. Antemortem diagnostic tests include blood thiamine levels, erythrocyte transketolase activity, thiamine pyrophosphate, and, in cases of suspected sulfur-induced polioencephalomalacia, by determination of sulfur content in feed and water.
Bovine spongiform encephalopathy (BSE) is a progressive, fatal, infectious neurologic disease that is due to the accumulation of misfolded proteins, termed prions, in the central nervous system of cattle. Two main presentation forms can be distinguished: classical BSE, which appears in cattle after oral exposure to prions; and atypical BSE, which is believed to appear spontaneously in aged animals. BSE causes progressive neurologic signs such as ataxia and hyperesthesia and is always fatal. There are no known vaccines or treatments to control the disease. Confirmatory diagnosis is based on postmortem detection of misfolded prion proteins in the brain. BSE is a zoonotic disease, causing variant Creutzfeldt-Jakob disease (vCJD) in humans.
Chronic wasting disease is a transmissible spongiform encephalopathy that affects deer and other cervids, primarily in North America. It is a fatal, progressive neurodegenerative disorder and affects both wild and farmed animals. The primary signs are significant weight loss, ataxia, and hypersalivation. Diagnosis is made by ELISA and Western blot, with confirmation by immunohistochemistry. There are no treatments or vaccines, so control in farmed animals relies on depopulation of affected herds.
Scrapie is a degenerative, fatal disease of the CNS of sheep and goats. Clinical signs, when they are present, often include ataxia and recumbency. Scrapie cannot be treated and is best diagnosed by microscopic postmortem exam. Selective breeding for genetic resistance, surveillance, and depopulation are the primary means of controlling this disease.
Equine arboviral encephalomyelitis is due to infection with arthropod-borne viruses typically belonging to the families Togaviridae (genus Alphavirus) or Flaviviridae (genus Flavivirus). Initial clinical findings are nonspecific and include fever, anorexia, and stiffness. Subsequent diffuse encephalomyelitis causes clinical signs consistent with forebrain and cranial nerve dysfunction. In endemic areas, a presumptive diagnosis is based on vaccination history, clinical presentation, and case progression; however, serologic testing and necropsy are required for definitive diagnosis of viral infection and subsequent encephalomyelitis.
Louping ill, caused by louping ill virus and transmitted by ticks, causes a frequently fatal encephalitis that results in highly variable neurologic signs and has no specific treatment or available vaccine. Antemortem diagnosis is usually by serology using hemagglutination inhibition, which can also identify acute infections by differentiating IgM from IgG predominance. Postmortem, diagnosis is by histologic and immunohistochemical examination of the brain and also by PCR.
Pseudorabies is an acute, often fatal, viral disease with a worldwide distribution. Swine are the primary host, but other species are also occasionally infected. Clinical signs include reproductive failure and CNS and respiratory signs in growing pigs. The diagnosis is suspected based on clinical signs and confirmed by serology, PCR, or viral isolation. There is no specific treatment, but highly effective vaccines are available. The disease is reportable and has been successfully eradicated from commercial swine in the USA.
Rabies is an acute, progressive encephalomyelitis caused by lyssaviruses. This zoonosis occurs worldwide in mammals, with dogs, bats, and wild carnivores as the principal reservoirs. Typical clinical signs include acute behavioral changes and progressive paralysis. The disease is fatal once clinical signs appear; however, appropriate and timely administration of local wound care, immune globulin, and vaccination can prevent disease in unvaccinated humans after exposure. Vaccines are available for domestic animals, wildlife, and humans to prevent rabies and to help control transmission in reservoir populations.
Teschovirus encephalomyelitis (TE) is a sporadic disease of swine caused by neurotropic infection with Teschovirus A (TV A). The severity of clinical signs correlates with the severity and distribution of lesions within the spinal cord and brain. The disease was initially reported in 1929 in the district of Teschen, Czech Republic (Teschen disease), and it was later recognized in England (Talfan disease) and Denmark (benign enzootic paresis). Severe outbreaks are usually associated with highly pathogenic strains of Teschovirus A1, whereas less-severe outbreaks are frequently associated with other genotypes (ie, TV A2, A3, A4, A5, A6 and A11). Currently, commercial vaccines are unavailable for prevention of teschovirus encephalomyelitis, and treatment of affected animals is frequently palliative, consisting of supportive care. No outbreaks of CNS disease have been associated with Teschovirus B.
Sporadic bovine encephalomyelitis is caused by infection of cattle and buffalo with Chlamydia pecorum. Infection occurs mainly in calves 6 months old. Nonspecific early signs (eg, fever, depression, anorexia, diarrhea) may be followed by peritonitis, decreased tail tone, incontinence, staggering, circling, and other neurologic signs. Diagnosis is based on clinical signs, bacterial culture, and PCR. Supportive care and antibiotics are the primary treatments.
Equine protozoal myeloencephalitis (EPM) is caused by CNS infection of equids with either of the apicomplexan protozoa Sarcocystis neurona or Neospora hughesi. Common clinical signs are asymmetric ataxia and weakness of limbs and regional neurogenic muscle atrophy. Less common signs are obtundation, seizures, facial paralysis, head tilt, and other signs of cranial nerve dysfunction. Serologic support for the diagnosis is obtained using serum:CSF titer ratios for ELISA or indirect fluorescent antibody tests. EPM is treated with antiprotozoal drugs and immunomodulators.
Tick paralysis (toxicity) is an acute, progressive, symmetrical, ascending motor paralysis caused by salivary neurotoxin(s) produced by certain species of ticks. With some species, other signs of systemic "single organ" toxicity (eg, cardiac, airway, bladder, lung, esophagus, etc) may be seen separate to or within the classic paretic-paralysis presentation. People (usually children) and a wide variety of other mammals, birds, and reptiles may be affected. Human cases of tick paralysis caused by the genera Ixodes, Dermacentor, and Amblyomma have been reported from Australia, North America, Europe, and South Africa; these three plus Rhipicephalus, Haemaphysalis, Otobius, and Argas have been associated with paralysis to varying degrees in animals.