A change in an animal’s ability to sense its environment can be caused by disease in either the central nervous system or the peripheral nervous system. The primary signs of nervous system disorders include behavioral changes, seizures, tremors, pain, numbness, lack of coordination, and weakness or paralysis of one or more legs. The effects of an injury on sensory and motor functions depend on its location and severity.
A spinal cord injury can cause loss of feeling and paralysis beyond the level of the injury. Mild spinal cord injuries can result in clumsy movement and mild weakness of the limbs. Moderate spinal cord injuries can cause a greater weakness of the limbs. In severe spinal cord injuries, a complete loss of movement (paralysis) and feeling can occur. However, not all spinal cord injuries cause paralysis. For example, injury to the spinal cord in the lower back can result, not in limb paralysis, but in loss of bladder control.
Brain injuries result in different effects, again depending on which part of the brain is affected. Injuries to the brain stem can cause a loss of balance, weakness of the limbs, hyperactive reflexes, stupor, or coma. Injuries to the cerebellum can result in a lack of coordination of the head and legs, tremors, and a loss of balance. Injuries to the cerebrum can cause complete or partial blindness, loss of the sense of smell, seizures, coma, stupor, pacing or circling behavior, and inability to recognize an owner.
Some injuries to the nervous system can cause damage that is not evident until 24 to 48 hours after the injury occurs. Longterm damage is usually caused by swelling or internal bleeding of the vessels in the brain. Strokes caused by clogged arteries or high blood pressure are rare in animals.
In addition to the effects of injuries, nervous system disorders can include birth defects, infections, inflammatory conditions, poisoning, metabolic disorders, nutritional deficiencies, degenerative diseases, or cancer.
Most birth defects, often called congenital disorders, are obvious at birth or shortly after. Some genetic diseases cause the neurons to degenerate slowly and irreversibly in the first year of life. In other inherited diseases, such as epilepsy, the animal may not show any signs for 2 to 3 years.
Infections of the nervous system are caused by specific viruses or microorganisms. Other inflammatory conditions, such as certain types of meningitis, can be caused by the body’s own immune system becoming activated to act on "self" rather than a foreign invader. These conditions are known as autoimmune disorders. Various chemicals can cause a toxic reaction in the nervous system. These include certain pesticides and herbicides, rat poisons, antifreeze, and sedatives. Botulism, tetanus, and tick bites, as well as coral and tiger snake venom, can also affect the nervous system and cause paralysis.
Some metabolic disorders affect the function of the nervous system, including low blood sugar, shortness or loss of breath, liver disease, and kidney failure. Thyroid gland abnormalities can also cause neurologic signs.