head trauma | parasite brain migration | idiopathic epilepsy | canine geriatric vestibular syndrome (old dog vestibular disease) | facial nerve paralysis | brain abscess | neoplasia | thiamine deficiency | hypoglycemia | hepatic encephalopathy | hypocalcemia | lead poisoning | strychnine poisoning | ethylene glycol (antifreeze) poisoning | insecticide poisoning | metronidazole overdose | ivermectin overdose | hydrocephalus | hypomyelinogenesis | listeriosis | rabies | rabies (vaccine-induced) | canine distemper virus | canine herpes virus | canine parvo virus | protozoal encephalitis | mycotic encephalitis | bacterial meningitis | granulomatous meningoencephalitis | pug encephalitis | idiopathic tremor syndrome
Introduction: The nervous system in dogs is a vital, extremely complex system which allows rapid communication between all parts of the body. It is responsible for constant coordination between all other body systems. The nervous system is generally broken down into the central nervous system (CNS) and the peripheral nervous system. The central nervous system comprises of the brain and spinal cord, while the peripheral nervous system includes all the smaller nerves throughout the body and their points of contact with other organs.
The brain is made up of three different areas: the cerebrum, the cerebellum, and the brain stem. The cerebrum is the main part of the brain that rests inside the skull. The cerebrum is responsible for consciousness, voluntary movement of muscles, pain perception, vision, learning, and behavior. Any disorder or injury to the cerebrum can result in a frightening response called a seizure. The cerebellum is a smaller, round part of the brain which sits just behind the cerebrum. The cerebellum is responsible for the rate, range, and force of motion. It helps to fine tune muscle coordination. Finally, the brain stem is a long, somewhat cylinder-shaped organ sitting on its side just under the cerebrum and cerebellum. The brain stem shares some functions with the cerebrum, including consciousness and pain perception. It also aids in many involuntary processes in the body and is the part of the brain where ten special pairs of nerves enter the brain. These ten nerve pairs are called cranial nerves and are numbered III through XII (cranial nerves I and II directly enter the cerebrum instead of the brain stem). These cranial nerves are responsible for a variety of special senses and functions, including eye movement and position (III, IV, and VI), feeling and motor function to the face and head (V and VII), balance and hearing (VIII), taste and feeling to the back of the throat (IX), swallowing and some control over digestion and heart function (X), some muscular movement to shoulders and head (XI), and movement of the tongue (XII). Cranial nerve I aids in the sense of smell and cranial nerve II helps with sight/vision.
A specialized neurological examination is generally performed on all patients suffering from a disorder of the nervous system. The neurological examination gives extremely important information in determining which body functions are not working properly. Listing the abnormal findings on a neurological examination and applying an extensive knowledge on how the nervous system works, a veterinarian is able to determine the location of a disease within the nervous system. Some diseases are focal, which means affecting only one portion of the entire system, while others are generalized, which means a large portion of the system is malfunctioning. A neurological examination may take some time to perform. Specific tests for each function of the nervous system and many of the cranial nerves are often necessary and can give the doctor extremely valuable information in the diagnosis of a disease of the nervous system. See page B548 for additional details.
Major disease categories that affect the nervous system:
Diagnostic Testing for Diseases of the Brain:
In addition to the standard diagnostic testing procedures used for other body systems (physical examination, bloodwork, urine analysis, and standard x-rays), there are a variety of specialized tests that can be used for diagnosing diseases of the nervous system. These tests include the following:
Focal Diseases of the Brain
One of the most common reasons a dog may visit a veterinary hospital for a brain disorder is following head trauma. Damage to the brain during a traumatic event is usually very serious and often fatal. Patients that recover may suffer permanent brain damage for the remainder of their lives. The specific types of brain disorder that may forever follow a trauma patient include epileptic seizures, permanent paralysis of limbs or other muscles, and pronounced personality and behavioral changes.
The time in which a head trauma patient is transported to a care facility is critical. Pressure within the brain increases with most cases of severe head trauma. As the pressure increases within the brain, further damage is progressively done until the patient becomes comatose or dies. The head should be elevated during transport to help drain excess fluids from the brain.
A veterinarian caring for a patient with severe head trauma will be able to provide some aid in lowering pressure within the brain. Hyperventilating with pure oxygen, certain diuretics, and rapid-acting anti-inflammatory steroid therapy can all help to lower pressure within the brain. Anticonvulsants are given to help control seizures. Routine radiographs of the head may show skull fractures which should be surgically repaired if they are pressing in on brain tissues. Following a head injury, supportive care with frequent monitoring and several days of hospitalization may be required.
Prognosis of patients suffering traumatic brain injury varies depending on many factors. The outcome may be excellent to very poor. Generally a patient remaining comatose for more than 48 hours will have a poor prognosis. Patients which recover rapidly and respond well to treatment will usually have a good to excellent prognosis.
Occasionally the larval stage of some parasites may "accidentally" find its way into the brain of a dog. These parasites are commonly found in other areas of the body and apparently enter the brain through a blood vessel or directly through nearby tissues. The host brain is not a normal part of the life cycle of any of these parasites. When the brain is inadvertently accessed by these parasitic larvae, the result is usually fatal. Some parasite larvae which have been known to migrate through brain tissue include bot flies (Cuterebra), heartworms (Dirofilaria immitis), roundworms (Toxascaris), and tapeworms (Taenia).
Clinical signs of parasitic migration through the brain depend on which area of the brain is affected. Seizures, blindness, lethargy, behavioral or personality changes, and other clinical signs may be noted.
Diagnosis of this condition is usually based on necropsy, although as MRI and other specialized imaging techniques improve, the ability to diagnose this condition in a living animal becomes more common.
Treatment is usually unsuccessful, and the prognosis for parasite migration through the brain is usually considered very poor.
Idiopathic epilepsy is a common seizure disorder in dogs. Idiopathic epilepsy has no known cause, but it is suspected that there is a link between this condition and early traumatic head injury in the puppy. Difficult delivery during whelping or a minor blow to the head during the first weeks of life may later contribute to development of epilepsy. Epilepsy is also being shown in recent research to be an inheritable disease. Seizures occur because of an abnormal focus in the brain that becomes very excitable. Because it is generally a single-focus disease, epilepsy is listed among those diseases of the brain which are focal. Idiopathic epilepsy is placed into either a primary or secondary category. Epileptic seizures of either primary or secondary variety tend to be generalized "tonic-clonic" seizures.
A generalized tonic-clonic seizure tends to start with what is known as a "pre-ictal" phase. A pre-ictal phase is a short period of time in which the dog senses a seizure coming on. The dog may become very nervous and apprehensive and seek out human companionship during this phase. The seizure itself follows the pre-ictal phase, usually lasting less than a minute. The seizure patient will fall or slump to the ground as the seizure starts and quickly loses consciousness. The entire body then goes through a period of muscle spasms of varying degrees of severity. The neck usually tenses or arches and rapid spasmodic movement of the mouth, eye muscles, and limbs occurs. The dog may lose control of the bladder and/or bowels during the seizure. Muscle spasms may end quickly or gradually die off and the dog will then start into a period of time known as the "post-ictal" phase. It is during this period that the dog will gradually recover. The post-ictal phase may last anywhere from several minutes to several hours. If the seizure involved parts of the brain that control movement or sensation to certain parts of the body, abnormalities may be observed in the body’s function during the post-ictal phase. Temporary difficulty walking or blindness are some of the possible problems the patient may experience during the post-ictal recovery phase.
Primary epilepsy occurs more frequently in males than in females. Over 90% of primary epilepsy occurs in purebred dogs, and most often in Labrador retrievers and golden retrievers. Large-breed dogs tend to develop primary epileptic seizures more frequently than small-breed dogs. The first seizure in a dog diagnosed with primary epilepsy is usually seen between 1 and 5 years of age and there is generally a large time span (one month or more) between the first and second seizures. During the period of time between seizures, the dog will generally act as if nothing at all is wrong.
Secondary epilepsy tends to start in dogs that are either less than one or over seven years old. These patients tend to have epileptic seizures in response or reaction to some kind of stimulus. Any dog breed may develop secondary epilepsy and the period of time between the first and second seizures tends to be short (less than one month).
Diagnosis of idiopathic epilepsy in dogs is made by looking at the history and physical examination findings and eliminating other causes of seizures with a variety of tests. Bloodwork can show disorders of the metabolism or failure of certain organs that may lead to seizures in dogs. CSF (cerebrospinal fluid) analysis can show certain types of inflammation in the brain and spinal cord. CT scanning and MRI can show brain abscesses, tumors or other disorders within the head that may lead to seizures. Dogs with idiopathic epilepsy tend to have normal results on all these tests.
Treatment of idiopathic epilepsy is based on anticonvulsant drugs. It is known that the earlier an epileptic dog is started on drugs that prevent seizures, the easier and better will be the control of this disease for the rest of that dog’s life. Therefore, most veterinarians start treating epilepsy very early in the development of the disease. Drugs commonly used to help control seizures in dogs include the following:
The primary goal for treating epilepsy in dogs is to control (not necessarily elimination) the seizures, while maintaining a good quality of life for the patient and avoiding harmful drug side effects. Effective control of seizures may be defined as reducing the frequency of seizures to once per month or less, while not experiencing any bodily harm from long-term drug use (especially liver disease). It is also essential that the patient maintain a normal personality and quality of life. Like in people, epilepsy is not considered curable in the canine patient.
It is highly recommended that owners of dogs with epilepsy keep detailed records of seizures to help the veterinarian know how well a treatment program is working for the patient.
Prognosis for most epileptics is good with early treatment and careful
control of the condition. Epilepsy can become fatal if a seizure continues
beyond a normal one-minute or less time frame. Generally, seizures that last 5
minutes or longer can cause permanent damage to the brain or other vital organs
and are considered to be a life-threatening emergency.
Canine Geriatric Vestibular Syndrome (old dog vestibular disease):
This is a disease that is included in the section on brain although technically it is considered to be a disorder of the peripheral nervous system (specifically a portion of cranial nerve VIII). The cause of geriatric vestibular syndrome is not completely known. Often thought (incorrectly) to be a "stroke," a sudden loss of balance is the primary clinical sign noted. Imbalance is often accompanied by nausea, vomiting, and abnormal eye movements. This syndrome is relatively common in older dogs, perhaps with a tendency to affect medium or larger sized breeds.
The suddenness of the disease and the shocking appearance of the clinical signs naturally cause many dogs and their owners to panic. Clinical signs include a tilting of the head with difficulty walking; some dogs are completely unable to stand or move at all, but rather fall or roll around on the ground trying to regain lost balance. The tilting of the head and the rolling are always in one direction if only one side of the head is affected. The tilting of the head will always be toward the affected side. On some occasions, both sides of the head will be affected together. If this happens, the rolling and head tilt may not be predictable. Abnormal eye movement called nystagmus generally occurs, with rolling or rapid shifting of the eyes from side to side. Nausea and vomiting often occur initially as a form of "motion sickness."
Diagnosis of geriatric vestibular syndrome is based on history, physical and neurological examinations, and subsequent rapid improvement. No specific test exists for diagnosis of canine geriatric vestibular disease. Bloodwork, including screening for thyroid disease, and head x-rays or MRI are often performed to rule out other diseases.
Geriatric vestibular disease is treated with supportive care only. Intravenous fluid and nutritional therapy may be needed initially for more severe cases. Dogs suffering from severe disorientation may benefit from sedatives. Steroids, such as prednisone, have often been used as a treatment for nerve inflammation for this condition, but probably do nothing to improve outcome. Many specialists caution against the use of steroids in older patients with possible kidney or liver disease.
Prognosis is usually very good to excellent in spite of the horrible
appearance of this disease. Most dogs will make a full recovery without specific
treatment of any kind within 72 hours.
Facial Nerve Paralysis:
Paralysis of the muscles of the face results when the facial nerve (cranial nerve VII) malfunctions. This is a somewhat common condition in the dog, resulting in sudden drooping of the eyelid, sagging of the lips, loss of eye blink, and drooling from the affected side of the mouth. One side of the face is usually affected without the other. Tear production is often reduced in the affected eye, making the patient susceptible to "dry eye" syndrome (keratoconjunctivitis sicca). Bell’s palsy in people is a very similar situation, in which the facial nerve becomes paralyzed, resulting in distortion of the face.
Diagnosis of facial nerve paralysis is easily made through physical examination findings by a veterinarian. Determining the cause of the paralysis may be more difficult, however. Facial nerve paralysis can result from other disease conditions such as hypothyroidism, Cushing’s disease, and lead poisoning. A thorough examination, followed by tests for thyroid and adrenal gland function and possible testing for various toxicities, may be needed. If no underlying cause can be found, the condition is said to be idiopathic– no known cause.
There is no specific treatment for facial nerve paralysis. Treating underlying causes such as hypothyroidism may help resolve the condition to some extent. To some degree, idiopathic facial nerve paralysis will often resolve on its own within two or three months. Treatment of problematic effects of the disease, such as dry eye syndrome, are necessary during the recovery period.
Prognosis for facial nerve paralysis is fair to good for most cases of
idiopathic disease; the animal’s general well-being is usually not affected.
When associated with other more serious conditions, such as Cushing’s disease,
prognosis depends upon the underlying cause.
Brain Abscess:
Brain abscesses are uncommon in the dog and usually arise because of an ongoing ear, respiratory tract, or mouth infection spreading to the brain directly or through blood vessels. Seizures, vision loss, disorientation, fever, lack of appetite and energy may be seen in patients with brain abscesses. If the abscess affects the brain stem, dysfunction of cranial nerves may be noted upon physical examination (see page B548).
Diagnosis of a brain abscess is best made with the help of cerebrospinal fluid (CSF) analysis and CT scanning or MRI. The abscess often appears on CT or MRI much like a brain tumor would, so the combining of these tests with CSF evaluation will help confirm a brain abscess. CSF will contain higher protein and white blood cell levels than normal. The white blood cells will often be found with bacteria or other infectious organisms within their cell membranes.
Dogs with brain abscesses are usually very sick and must be treated very aggressively. Antibiotics are the main treatment used for brain abscesses. Antibiotic drugs must be chosen carefully, however, and be based upon culture and sensitivity testing of the CSF, if possible. Not all antibiotics will cross from the bloodstream into the brain, and it is important to carefully select an appropriate drug that can penetrate the area involved. Steroid therapy may be helpful at first to minimize inflammation into surrounding brain tissue, but prolonged use will slow the body’s immune response.
Because it requires identifying the exact location of the abscess and should be performed only by a surgical specialist, treatment of a brain abscess with surgical drainage is very limited.
Most dogs with a brain abscess do not fully recover.
Neoplasia:
Brain tumors are more common in dogs than in other domesticated animals. There are a variety of tumor types that can occur in the canine brain, arising from differing populations of cells. Brain tumors occur in dogs of any breed, gender, and age; however, dogs over 5 years of age are at higher risk. Boxers, golden retrievers, Doberman pinschers, Scottish terriers, and Old English sheepdogs are breeds in which brain tumors may occur more commonly.
Brain tumors in dogs can be either benign or malignant; however, in a vital organ such as the brain, tumors of either type may be equally likely to cause problems or death. Tumors can originate in the brain (often called primary tumors). These primary brain tumors are usually slow-growing. The tumors in the brain can also be secondary, having spread to the brain from another cancer elsewhere in the body. One of the most common types of brain tumor occurring in the dog begin in the supporting structure of the nervous tissue. Astrocytomas and oligodendrogliomas arise from glial cells, which make up the matrix protecting and surrounding the brain’s nerve cells. Glial cell tumors are more common in the brachycephalic ("short-nosed") breeds. Meningiomas are another common type of brain tumor in dogs, arising from the protective membrane-like tissue which surrounds the brain itself. Meningiomas are more common in the dolichocephalic ("long-nosed") breeds. A few other types of brain tumors occur much less frequently, such as medulloblastomas and ventricular tumors. Both can occur in young dogs. Specific causes of brain tumors in dogs are not generally known, although previous head trauma, genetics, viral infections, and abnormalities of the immune system are all factors that may play a role.
Clinical signs of a brain tumor usually begin very gradually and are slowly progressive. In people with brain tumors, the earliest recognition of a problem is often a persistent headache. Early changes in the behavior of a dog with a brain tumor are often consistent with a "headache" such as reluctance to be handled, hiding, and acting short-tempered. Seizures are a common clinical sign in dogs with a brain tumor. Weakness, blindness, circling, and hearing loss are also observed in some cases. With time, the brain tumor will cause inflammation and swelling within the head, resulting eventually in a loss of consciousness and death.
Diagnosis of a brain tumor is rather difficult. A thorough physical examination with an accompanying neurological examination must be performed by a veterinarian. Routine bloodwork and x-rays of the chest, abdomen, and head may also be included. Analysis of the cerebrospinal fluid is often used as a supportive test. Sometimes cancer cells can be found within the fluid itself. CT scanning or MRI imaging tests are necessary for determining the exact location of the tumor within the brain and the extent of the tumor growth. An actual diagnosis of the tumor type is based on a biopsy. Often, the actual biopsy is not performed prior to treatment because of the risks to the patient and the cost of such a procedure.
Treatment of a brain tumor is usually centered around limiting the effects of the tumor on the patient. Radiation therapy, chemotherapy, and corticosteroid therapy may be recommended to help reduce tumor size and secondary tissue inflammation. Anticonvulsant drugs may be used to control seizures if needed. Surgery is an available option for some brain tumors, but often includes a significant risk to the patient.
Prognosis for a patient with a brain tumor varies somewhat depending on tumor
type, aggressiveness, and location, but usually carries a poor outcome without
treatment. Average survival times as reported in studies are about two to three
months for dogs with untreated brain tumors. Treatment with radiation therapy,
surgery, corticosteroids, or in combination can increase the survival time to an
average of nine months to a year from the time of diagnosis.
Generalized Diseases of the Brain
Thiamine is one of the several vitamin B compounds that is essential for proper functioning of the body. This particular vitamin B compound cannot be manufactured by the body and must be eaten in sufficient quantity in the diet. Fortunately, nearly all commercial dog foods contain sufficient thiamine, and additional vitamin supplements are not generally needed. Some raw fish diets (principally salmon and tuna) may create thiamine deficiencies.
Clinical signs of thiamine deficiencies usually begin suddenly and progress rapidly. Loss of balance with abnormal eye movements, lethargy, muscular tenseness or contraction may occur. The disease is fatal if not treated.
Diagnosis of thiamine deficiency is based on a good physical exam and evaluation of the patient’s diet. Vitamin B compounds can be measured in the bloodstream for confirmation.
Treatment of thiamine deficiency focuses on correcting the nutritional imbalance. Vitamin B injections are given to stabilize the patient, with oral vitamin supplementation for several weeks following. Proper dietary monitoring will help prevent further relapses.
Prognosis for patients suffering from thiamine deficiency is excellent with rapid treatment and proper dietary monitoring thereafter.
Hypoglycemia, or low blood sugar, is a relatively common cause of brain imbalance in dogs. A large variety of disease processes can cause hypoglycemia. Many of these are listed below:
The brain is very dependent on its source of blood glucose to function. Blood glucose levels below 50 mg/dl are considered to be abnormally low. When blood glucose levels drop below 30 mg/dl, the patient may experience seizures. Other clinical signs of hypoglycemia include mental dullness, weakness (often profound), stumbling, blindness, and coma.
Diagnosis of hypoglycemia is very easy to make with a rapid blood test. Identifying the underlying cause of hypoglycemia may be more difficult. Additional bloodwork, x-rays, ultrasound evaluations, and tissue biopsies may be required for a complete diagnosis.
Treatment of hypoglycemia should be rapid, by administration of dextrose (sugar) by mouth, if possible, or by injection. At home, a pet may be given corn syrup in small quantities by mouth as an emergency measure until professional help is available. Even if the dog has lost its swallow reflex, the sugar will be absorbed through the gums into the bloodstream and start to treat the problem. Treating hypoglycemia is usually simple and effective. Prognosis depends on the cause of the hypoglycemia and often the long-term outlook is very good if the underlying problem can be controlled.
Some types of liver disease will cause a syndrome called hepatic encephalopathy to occur. This condition is discussed in the section on canine liver disease (page F475). Dogs with portosystemic shunts are especially prone to developing this particular problem. Toxic waste products usually removed from the bloodstream by the liver make it to the brain in higher quantities than normal. These toxic products cause disturbances in the brain’s functioning processes, leading to seizures, circling, aimless wandering, blurred vision or complete blindness, weakness, and stumbling.
A thorough evaluation of the patient with liver-specific testing will help the veterinarian determine the cause of the hepatic encephalopathy. Treatment should focus on the underlying cause. However, there are specific medications that can be used to lessen the effects of hepatic encephalopathy. These include specific antibiotics to reduce the bacteria in the intestines which are responsible for producing most of the toxic waste products that cause problems in the brain. Enemas may also help to keep the large intestine and colon free from these naturally-occurring bacteria. Other medications such as lactulose can be given to help alleviate the clinical signs.
Calcium is a very important element necessary for normal function of many tissues in the body. The nervous and muscular systems are two that depend on adequate calcium levels within the body to function properly. Calcium levels may drop within the body for various reasons. Some of the more common reasons are included in the following list:
Clinical signs of hypocalcemia include weakness and trembling with an altered mental status, sometimes leading to unconsciousness, seizures, and coma. Physical examination and laboratory work can help the veterinarian to determine the cause of the hypocalcemia and then work to correct the underlying problem. Probably the most commonly recognized cause is post-partum hypocalcemia, caused by rapid shifting of body calcium stores to produce milk in a lactating female. Correction of hypocalcemia is accomplished by administering calcium. This can be as simple as administering calcium carbonate oral tablets in mild cases, to intravenous calcium administration. Calcium must be administered very slowly when given intravenously and is usually best done under careful supervision using a programmed infusion pump. Prognosis of hypocalcemia depends on the underlying cause, but is generally considered very favorable if the problem can be controlled.
Toxicities:
Several poisonous substances or drugs used at high doses are very harmful to the brain. Fortunately, the body has a built-in protection on a microscopic level called the "blood-brain barrier." The blood-brain barrier is made up of specialized membrane cells that regulate what can pass from the blood into the brain cells. Most other tissues throughout the body do not have such an effective protection against substances entering through the bloodstream. Without this blood-brain barrier as a protection, the list of poisonous substances to the brain would be much longer than it is currently. Because many drugs cannot cross the blood-brain barrier, a veterinarian must be aware of this when treating infections or other diseases within the brain. In order to have the desired effect, selecting antibiotics or other medications requires a knowledge of what drugs and what doses are known to penetrate through the blood-brain barrier. Some well-known toxic substances affecting the brain include the following:
Developmental Defects
Hydrocephalus is a condition in which cerebrospinal fluid accumulates within the small spaces in the brain and causes compression of the normal brain tissue against the bones of the skull. The severity of this disease depends on the degree of compression of the normal tissue and other factors. Not all dogs that suffer from hydrocephalus exhibit problems. In the normal canine brain, cerebrospinal fluid is produced and absorbed by specialized cells. Dogs suffering from hydrocephalus as a birth defect seem to absorb the fluid at a slower rate than it is being produced, resulting in excess fluid and pressure within the brain. This disease is most common in the toy and flat-nosed breeds including the Yorkshire terrier, Boston terrier, Manchester terrier, toy poodle, Chihuahua, Pomeranian, Lhasa apso, English bulldog, Pekingese, and Shih tzu. Affected animals may have an open fontanelle (soft spot in the top of the head).
Clinical signs of hydrocephalus include vision problems, stumbling, seizures, difficulty learning, and sometimes a deviation of the eyes downward and away from each other, known as the "setting sun sign," with each eye appearing as a sun setting below the horizon. Many affected dogs have a prominent, dome-shaped forehead.
Diagnosis of hydrocephalus is made by imaging techniques, such as MRI and CT scanning. If an open fontanelle exists, routine x-ray films and ultrasound of the skull may be helpful. Most of the time, the diagnosis is made based on clinical signs, physical examination, and history.
Treatment of hydrocephalus varies based on need. If clinical signs are mild, simple corticosteroid therapy may be remarkably helpful in improving absorption of cerebrospinal fluid. Diuretics such as furosemide may also be helpful in reducing the amount of CSF produced in the brain, but are not generally recommended for long-term use. If clinical signs are severe and not controllable with medication, surgical drainage and shunt placement is possible, but carries a very high risk for the patient. In general, prognosis for hydrocephalus patients is fair to good, although the dog may never be as bright or learn as quickly as its littermates. Severe cases carry a guarded prognosis and many do not survive.
Hypomyelinogenesis is a birth defect occurring in several dog breeds, including chow chows, Samoyeds, Weimaraners, Dalmations, and schnauzers. Myelin is a protective sheath covering most nerve tissues. It acts as an insulation to the electrical impulse along a nerve and gives speed to the nerve cell’s ability to communicate to other nerve cells. In hypomyelinogenesis, the myelin sheath does not form properly. It seems to mostly affect the spinal cord, but can also occur in the brain stem and cerebellum. Head tremors and clumsiness when walking are most commonly seen when affected puppies first learn to walk. Muscles develop normally, and limb weakness does not tend to occur. Several puppies in a litter may be affected together. No treatment currently exists to correct this defect, but many puppies may recover most function. Occasionally, an affected puppy may actually become normal with time.
Infectious Diseases
Listeria monocytogenes is a gram-positive bacteria that can occasionally infect the brain in dogs. The bacteria normally live in soil and contaminated environments. Transmission is through the mouth, as the dog eats or plays with contaminated material. The bacteria travel to the brain through nerve cells, where they cause inflammation in the brain stem. Often clinical signs are centered around the cranial nerves, especially V, VII, X, and XI. Weakness, stumbling, head tilting, twisting of the neck muscles ("torticollis"), and circling are common neurologic signs. Diagnosis of listeriosis is made with a standard approach to most diseases of the brain, with physical and neurological examinations, bloodwork, and cerebrospinal fluid evaluation. Treatment is with aggressive antibiotic therapy and supportive care. Most patients will make a full recovery if treated early.
Rabies is a fatal disease caused by the rabies virus which affects mainly the brain. Any warm-blooded animal species may become infected with rabies, including humans. The virus is transmitted into an animal through a bite wound. From there, it travels through nerves to the brain. The time it takes the virus to make this journey depends on many factors, including the distance to travel. It may take several weeks or months to reach the brain and begin causing clinical signs. Effective vaccination programs over the past 50 years have made rabies in dogs a relatively rare problem.
Attenuated live-virus rabies vaccines can result in the actual disease in dogs. Killed rabies vaccines have never been known to cause this type of problem and as such are used almost exclusively in dogs to vaccinate against rabies. A dog that develops rabies as a result of a live-virus vaccine will gradually become paralyzed. The paralysis often starts at the vaccination site about 2-3 weeks after the vaccine is given. It then spreads towards the head, presumably following the virus as it travels to the brain. These infected dogs have not been shown to shed the virus, and are probably not capable of transmitting rabies to others. Like the natural rabies infections, no treatment is known.
Canine Distemper Virus (see page F102):
Because so many body systems may be affected, canine distemper virus may result in a variety of clinical signs. The brain is one organ that may become severely damaged by this virus. Distemper infections in dogs can result in invasion and destruction of the supportive structure of the brain ("white matter") and cause severe nerve inflammation and death. Puppies less than one year of age at the time of infection with distemper virus seem to be most at risk of developing brain inflammation. Effective vaccines and vaccination programs have dramatically reduced the incidence of canine distemper infections.
Canine Herpes Virus (see page F105):
Canine herpesvirus infection in small puppies between one and three weeks of age can result in fatal inflammation of the brain as well as failure of multiple other organ systems. Puppies usually acquire this disease from their mother as they pass through the birth canal. They may also become infected through nasal or vaginal secretions after birth. Once one puppy is infected, it spreads the virus among its littermates. The virus takes 4-7 days to start causing problems, which then rapidly develop into a fatal disease. This virus is one major cause of "fading puppy syndrome" which tends to affect multiple puppies in a litter. No vaccine exists for prevention of canine herpesvirus. Tissue hemorrhage is commonly seen with bruising of the skin and gums. Death usually occurs in puppies less than 3 weeks of age. If a puppy is infected between three and five weeks of age, the puppy has a better chance of survival. Most often the diagnosis is made through a necropsy of a deceased puppy. Littermates of the deceased puppy can then be treated for CHV with intraperitoneal injections (directly given into the abdomen) of serum taken from a dog known to have survived herpesvirus infection. This treatment may be helpful in making the disease less severe for affected puppies. Female dogs that have lost entire litters of puppies to herpesvirus infection may still be able to successfully have healthy litters in the future if she provides sufficient antibody protection through her colostrum. However, this possibility is not certain and a risk is taken when breeding a dog known to have been infected with CHV.
Canine Parvo Virus (see page F108):
Brain inflammation (encephalitis) has been seen in puppies infected with parvo virus. The entire brain, including cerebrum, cerebellum, and brain stem may become inflamed in such cases. When this does happen, the patient does not usually survive the infection. Fortunately, most puppies suffering from parvo virus infection do not experience encephalitis.
Two different coccidian protozoal organisms can result in brain infections in dogs. Neosporum caninum is a parasite that lives within the cells of its host where it forms infective cysts. These cysts then infect a new host that eats the infected tissue of a previous host. Neosporum caninum is particularly adapted to the canine nervous system, although other body systems are also affected. Most infected patients fortunately show no clinical signs and may carry the parasite without becoming sick. Suppression of the immune system with chemotherapy or corticosteroid drugs may bring the parasite out of dormancy and result in active illness. Those that do become sick may develop itchy skin, liver disease, and inflammation of the eyes. Nervous system signs are the most commonly seen and include muscle stiffness and wasting, stumbling (especially in the hind legs), and paralysis.
Toxoplasma gondii is another coccidian parasite that can infect the nervous system of dogs. This protozoa is more commonly associated with cats, but dogs can become affected as well. Unlike cats, dogs are not known to shed Toxoplasma gondii eggs in their stools. Clinical signs of toxoplasmosis in dogs are similar to those seen with Neosporum caninum and include muscle pain, inflammation of the eyes, and difficulty walking. Toxoplasmosis may also cause liver inflammation and pneumonia in dogs.
Diagnosis of protozoal infections of the nervous system may be somewhat difficult. To positively identify the organism, a tissue biopsy containing the parasite can be examined. Such biopsies are usually obtained after the animal has died. Occasionally, evaluation of cerebrospinal fluid may provide a diagnosis, especially with Neosporum caninum. Blood serology tests can be very helpful in providing a diagnosis. The typical clinical signs with the neurological examination will often cause a veterinarian to suspect infection with a protozoal parasite, and response to the proper treatment may help to confirm the preliminary diagnosis.
Antibiotics with activity against protozoal parasites are the treatment of choice for Toxoplasma gondii. Clindamycin is the drug of choice for treating toxoplasmosis in dogs. Sulfa antibiotics, penicillins, and clindamycin have all been used to treat Neosporum caninum, but no specific drug has been identified as a treatment of choice. Clindamycin is probably the most promising therapy for Neosporum caninum.
Fungal infections of the brain can occur in dogs of any age or breed. Cryptococcus neoformans is the most commonly identified fungal organism infecting the nervous system of dogs. Other fungal organisms include Coccidioidomyces immitis, Histoplasma capsulatum, Blastomyces dermatitides, and Aspergillus species. The severity of clinical signs will vary with each organism, but all can cause serious disease. Depression, difficulty walking, weakness, paralysis, blindness, and confusion are common clinical signs with fungal brain infections. The specific type of fungus involved may be seen in cerebrospinal fluid evaluations. CSF can also be used to measure antibodies produced against fungal organisms. Blood serology testing may also specifically identify the type of fungal infection. Treatment involves the use of antifungal drugs, including ketoconazole, itraconazole, and amphotericin B for prolonged periods of time. Because there are only a few reports of complete recovery from fungal infections of the brain in dogs, the prognosis is very guarded.
Bacterial infection of the membranes that cover the brain (meninges) are uncommon but can occur in dogs. Bite wound injuries and infections in the blood stream from other areas of the body can lead to bacterial meningitis. Affected dogs are usually very sensitive to being touched almost everywhere on the body and show specific severe pain when the neck is moved or turned. Infected animals usually have a fever and are depressed and lethargic. Some dogs may have difficulty walking. A history of inner ear or sinus infection, or a bite injury near the spine may lead to a suspicion of bacterial meningitis. Any type of bacteria may be involved in an infection of the meninges. Specific bacteria may be identified and cultured from cerebrospinal fluid for a diagnosis. Treatment includes supportive care and long-term antibiotics. Antibiotic selection should be based on the results of culture from the cerebrospinal fluid. The prognosis is generally good if the disease is treated early and aggressively.
Miscellaneous Brain Disorders
Granulomatous Meningoencephalitis (GME):
GME is a group of diseases in which a specific type of inflammation occurs in either small areas of the brain or is widely dispersed throughout the brain. The inflammation does not appear to be associated with any type of infection. Abnormal forms of the same types of cells that make up GME are also found in cases of lymphoma of the brain, one of the several types of brain cancer. GME also shows many similarities to cancer in its growth and response to treatment. GME seems to occur more frequently in older toy breeds, especially poodles. Females seem to be more affected than males. Ocular, focal, and disseminated forms of GME are known to exist. The ocular form causes sudden blindness, but progresses slowly to other parts of the nervous system. The focal form will cause clinical signs very similar to a growing mass like a brain tumor. Depending on the specific location of the area of inflammation, clinical signs may include seizures, stumbling, behavioral changes, deficits in the cranial nerves, or paralysis. The final form is called disseminated, meaning spread throughout. The areas of inflammation occur through many areas in the brain and cause a variety of clinical signs. This form is the most aggressive and often leads to meningitis with severe neck pain and fever. The cause of GME in dogs is completely unknown at this time.
Diagnosis of granulomatous meningoencephalitis can only be made by histopathology of a biopsy specimen from the brain. Since this is such a risky and expensive procedure, many cases of GME are given a presumptive diagnosis based on a combination of other tests. Physical and neurological examinations, routine bloodwork and x-ray films of the head, evaluation of cerebrospinal fluid, and imaging of the brain (CT scanning or MRI) can provide a great deal of information pointing to a diagnosis of GME. Cerebrospinal fluid analysis is especially helpful, as it will usually show high numbers of the abnormal cell type which occurs in GME.
Treatment consists of suppression of the immune system. Corticosteroid therapy (prednisone or prednisolone) is helpful, sometimes in combination with azathioprine or cytoxan. Radiation therapy has been very helpful in many cases as well. Treatment is not considered curative, however. Temporary remission of disease may last for months to years, but the disease is eventually fatal.
Pugs can develop a form of GME specific to the breed with some slight differences. Pug encephalitis tends to occur in younger dogs, sometimes less than a year old, and is apparently a hereditary problem. All areas of the brain and brain stem are affected. Sometimes the cerebellum may be affected also.
Clinical signs include seizures, stumbling, behavioral changes, circling, tilting of the head, and abnormal eye movements. Tremors of the head may be noted if the cerebellum is involved. Diagnosis is usually made after the patient has died, but can be highly suspected based on the clinical picture and CSF evaluation. Treatment with corticosteroids may help temporarily, but the disease is eventually fatal in all cases.
This disease has been called several different names, including idiopathic nonsuppurative encephalitis and white dog shaker syndrome. Idiopathic tremor syndrome is probably the most descriptive of the clinical picture. White dog shaker syndrome is an older term used at a time when it was apparent that the disease seemed to occur in small breed dogs that tend to have white hair coats such as the Maltese, West Highland white terrier, Bichon Frise, and white poodles.
The predominant clinical sign is the uncontrollable shaking of the entire body, usually coming on rather suddenly. The eyes tend to have rapid, random movements that are also uncontrollable. It has not yet been determined where in the brain the problem occurs because no abnormalities are found in the nervous system after death. A defect in the physiology of the brain, probably in the molecules that communicate from one nerve cell to another (neurotransmitters) is likely the cause. Because all specific tests of the nervous system including CSF evaluation and MRI imaging are completely normal, the diagnosis of idiopathic tremor syndrome is difficult. The diagnosis is based on the history, clinical signs, neurological examination, and otherwise normal test findings. If treated early, it appears this disease can be cured in some dogs. Corticosteroid therapy (prednisone or prednisolone) is used at gradually decreasing doses over several weeks. Some patients may require lifelong therapy, but these seem to be the minority of cases. Recovery is complete in most cases.