diagnostic testing for diseases for the brain | head trauma | parasite brain migration | idiopathic epilepsy | feline idiopathic vestibular disease | facial nerve paralysis | brain abscess | neoplasia | thiamine deficiency | hypoglycemia | hepatic encephalopathy | hypocalcemia | toxicities | hydrocephalus | cerebellar hypoplasia | listeriosis | rabies | rabies (vaccine-induced) | feline infectious peritonitis (fip) | protozoal encephalitis | bacterial meningitis
Introduction: The nervous system in cats 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 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 on performing a neurological test.
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 cat may be taken to 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 progressively occurs 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 cat. 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 relatively uncommon seizure disorder in cats. 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 kitten. Difficult delivery during birth or a minor blow to the head during the first weeks of life may later contribute to development of epilepsy. 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 cat senses a seizure coming on. The cat may become very nervous and apprehensive and go into a safe hiding place. 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 cat may lose control of the bladder and/or bowels during the seizure. Muscle spasms may end quickly or gradually die off and the cat will then start into a period of time known as the "post-ictal" phase. It is during this period that the cat 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. During the period of time between seizures, the cat will generally act as if nothing at all is wrong.
Primary epilepsy is generally diagnosed in younger cats, less than 4 years of age. Triggers or causes of epileptic seizures in primary epilepsy are usually not present.
Secondary epilepsy tends to start in young cats. These patients tend to have epileptic seizures in response or reaction to some kind of stimulus.
Diagnosis of idiopathic epilepsy in cats 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 cats. 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. Cats 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 cat is started on drugs that prevent seizures, the easier and better will be the control of this disease for the rest of that cat’s life. Therefore, most veterinarians start treating epilepsy very early in the development of the disease. Drugs commonly used to help control seizures in cats include the following:
The primary goal for treating epilepsy in cats is to control (but not necessarily eliminate) 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. Similar to people, epilepsy is not considered curable in the feline patient.
It is highly recommended that owners of cats with epilepsy keep detailed records of the 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.
Feline Idiopathic Vestibular Disease:
This is a disease that is included in the section on the 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 feline vestibular disease 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 uncommon disease can affect cats of any age, although it seems to be rare in cats less than one year old.
The suddenness of the disease and the shocking appearance of the clinical signs naturally cause many cats and their owners to panic. Clinical signs include a tilting of the head with difficulty walking; some cats 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. 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 idiopathic vestibular disease is based on history, physical and neurological examinations, and subsequent rapid improvement. No specific test exists for diagnosis of feline idiopathic vestibular disease. Bloodwork and head x-rays or MRI are often performed to rule out other diseases.
Idiopathic vestibular disease is treated with supportive care only. Intravenous fluid and nutritional therapy may be needed initially for more severe cases. Cats suffering from severe disorientation may benefit from sedatives such as diazepam or acepromazine. Steroids, such as prednisolone, have often been used as a treatment for nerve inflammation for this condition, but probably do nothing to improve outcome. Prognosis is usually very good to excellent in spite of the horrible appearance of this disease. Most cats will make a full recovery without specific treatment of any kind within 72 hours.
Paralysis of the muscles of the face results when the facial nerve (cranial nerve VII) malfunctions. This is a somewhat uncommon condition in the cat, 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 inner ear infections, nasopharyngeal polyps (abnormal growths relatively common in the cat), and head trauma. 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 inner ear infections or surgical removal of a polyp 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, prognosis depends upon the underlying cause.
Brain abscesses are uncommon in cats 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.
Cats 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.
Many cats with a brain abscess do not fully recover.
Brain tumors are much less common in cats than in dogs. There are a variety of tumor types that can occur in the feline brain, arising from differing populations of cells. Brain tumors occur in cats of any breed, gender, and age; however, older males seem to be at higher risk for menigiomas, the most common brain tumor type seen in cats. No feline breed seems to be at any higher risk for developing brain cancer.
Brain tumors in cats 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. The most common types of brain tumor occurring in the cat begins in the meninges, the fibrous capsule that covers and protects the brain and spinal cord. This tumor type is known as a meningioma. Only rarely do other types of brain tumors occur in cats. These rare tumors can arise from the supportive tissue inside the brain. Some of these cancers are called astrocytomas and oligodendrogliomas. Lymphoma in cats can affect the brain, usually in conjunction with other body systems. Specific causes of brain tumors in cats are not generally known, although previous head trauma, genetics, viral infections, and immune system abnormalities are all suspected.
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 cat with a brain tumor are often consistent with a "headache" and may include reluctance to be handled, hiding, and acting short-tempered. Some owners have reported diminished purring and playfulness in their pets for up to a year before a brain tumor was diagnosed. Seizures are a common clinical sign in cats 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. If a meningioma is located in an easily accessible area of the brain the cat, such as over the frontal lobes, surgical removal can be very successful.
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 cats with untreated brain tumors. Treatment with radiation therapy, surgery, corticosteroids, or in combination can increase the survival time to an average of up to two years 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 cat foods contain sufficient thiamine, and additional vitamin supplements are not generally needed. Some raw fish diets (principally salmon and tuna) may create thiamine deficiencies due to the presence of an enzyme in these meats that destroys thiamine.
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 cats. 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 (in diabetics, seizures may occur even before the blood glucose drops below 40 mg/dl). 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 cat 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 feline liver disease (page F475). Cats 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, weakness, vomiting, listlessness, and depression. Excessive salivation and bizarre behavioral changes will develop in cats with advanced disease.
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. Control of dietary protein with the use of a high quality protein in low quantities can also be helpful in cats. Surgical correction of the shunt is an option in many cats, but is a very expensive and difficult surgery. In general, management of portosystemic shunts in cats with medical therapy alone is less successful than it is in dogs.
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 determine the cause of the hypocalcemia and then work to correct the underlying problem. 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.
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. This condition is more common in dogs than it is in cats. Not all cats that suffer from hydrocephalus exhibit problems. In the normal feline brain, cerebrospinal fluid is produced and absorbed by specialized cells. Cats 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. 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.
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 cat may never be as bright or learn as quickly as its littermates. Severe cases carry a guarded prognosis and many do not survive.
The cerebellum is a small round extension of the brain which sits just inside the base of the skull. The primary function of the cerebellum is to help control and regulate movement, especially as it relates to distance and force. Some cats are born with an underdeveloped cerebellum. This failure of proper development may occur as a result of viral infections during certain stages of development, may be genetic, or may not have any known cause. One, several or all kittens born in a single litter may be affected. When the cerebellum fails to develop, the animal suffers from constant tremors, and may have difficulty performing simple tasks. The tremor seen is commonly called cerebellar or "intention" tremor, since it seems to worsen as the animal begins to perform an action, such as putting its head down to eat. Both head and limbs are affected by intention tremor. The condition does not worsen with time, but generally does not improve either.
Diagnosis of cerebellar hypoplasia is based on history, clinical signs, physical examination, and elimination of other diseases.
No treatment exists for cerebellar hypoplasia. The outcome is usually fair, however, since most cats suffering from this condition can adjust under the right circumstances and live a relatively normal life.
Infectious Diseases
Listeria monocytogenes is a gram-positive bacteria that can occasionally infect the brain in cats. The bacteria normally live in soil and contaminated environments. Transmission is through the mouth, as the cat 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 lessened the frequency of rabies in cats. However, because rabies vaccination for dogs has been more focused in general than it has been for cats, the actual ratio of cats diagnosed with rabies in comparison to dogs has increased.
Attenuated live-virus rabies vaccines can result in the actual disease in cats. Killed rabies vaccines have never been known to cause this type of problem and as such are used almost exclusively in cats to vaccinate against rabies. A cat 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 cats have not been shown to shed the virus, and are probably not capable of transmitting rabies to others. About two thirds of cats that develop vaccine-induced rabies are also infected with feline leukemia virus. Like the natural rabies infections, no treatment is known.
Feline Infectious Peritonitis (FIP) (see page F226):
Because so many body systems may be affected, feline infectious peritonitis virus may result in a variety of clinical signs. The brain and meninges (capsule-like tissue which covers the brain and spinal cord) may become very inflamed in over half of cats with FIP infection. Many of these cats that develop inflammation of the brain and meninges show clinical signs of convulsions, difficulty walking, head tilting, and behavior changes. Diagnosis of FIP continues to be somewhat of a challenge; however, recent advances in diagnostic testing have made it easier to reach a definitive result. No treatment exists for FIP infection, and the disease is invariably fatal to cats. Vaccination for FIP, keeping pets away from wild or stray animals and other preventative measures are the principle recommendations for avoiding FIP infection for cats.
Feline Immunodeficiency Virus (FIV) (see page F224):
One in three cats affected with FIV may develop direct brain injury. Depression, behavior changes, and loss of litterbox training occur commonly in affected cats. Routine testing of cats for FIV involves screening the blood for antibodies formed against the virus. The mere presence of antibodies in the blood against FIV is suggestive, but not fully diagnostic of the virus, especially in cases where brain involvement is the only problem. Cerebrospinal fluid should be collected in these cases and also screened for the presence of antibodies against FIV. Several drugs have been used in treating FIV positive cats with brain involvement. However, the long-term outcome for such cases is considered incurable and eventually fatal.
Feline Leukemia Virus (FeLV) (see page F228):
Malfunction of the central nervous system is one of many variations of disease caused by FeLV in cats. Most common is the development of cancer in the brain or spinal cord leading to difficulty walking/paralysis, seizures, behavioral changes including aggression, and loss of litterbox training. Lymphosarcoma cancer is the aggressive cancer type associated with FeLV infection. In other FeLV cases, neurological signs are found, but there is no evidence of cancer growth. These cats are diagnosed with "FeLV neurologic syndrome." Neurologic disease in FeLV positive cats, whether associated with lymphosarcoma or not, is a very serious problem. Chemotherapy drugs may be used to help treat cancer formation. Corticosteroids can be helpful very temporarily in some cases whether cancer is present or not, but can cause more harm than good in the long run as these drugs further suppress a failing immune system. The outcome of most FeLV infections is poor, especially if the central nervous system becomes involved. Vaccination and avoidance of exposure to the virus continue to be recommended as the most effective methods of prevention against FeLV.
Two or more protozoal organisms can result in brain infections in cats. 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. Most infected cats 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 cats. Clinical signs of toxoplasmosis in cats 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 cats.
Diagnosis of protozoal infections of the nervous system may be somewhat difficult. Fecal examinations are helpful in diagnosing toxoplasmosis in cats. 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 cats. 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.
Mycotic (fungal) Encephalitis:
Fungal infections of the brain can occur in cats of any age or breed. Cryptococcus neoformans is the most commonly identified fungal organism infecting the nervous system of cats. 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 cats, the prognosis is very guarded.
Bacterial infection of the membranes that cover the brain (meninges) are uncommon but can occur in cats. Bite wound injuries and infections in the blood stream from other areas of the body can lead to bacterial meningitis. Affected cats 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 cats 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.