In 2007, 3,510 cases of West Nile virus were reported to the U.S. Centers for Disease Control. States with the highest number of reported cases included Colorado, California, and North Dakota. Of the reported cases, two-thirds were in the form of West Nile fever, and one-third were diagnosed as West Nile neuroinvasive disease (encephalitis and meningitis). However, the high proportion of neuroinvasive disease cases is due to the fact that serious cases of West Nile virus are more likely to be reported to health authorities than mild cases. In general, less than 1% of people who become infected with West Nile virus develop encephalitis.
Most people (80%) who are infected with West Nile virus do not have any symptoms. About 20% of people develop mild symptoms that include fever, headache, body aches, and nausea and vomiting. These symptoms can last from a few days to a few weeks. For the minority of people who develop neuroinvasive disease, symptoms can include high fever, headache, neck stiffness, muscle weakness, and convulsions. While West Nile neuroinvasive disease is rare, its neurological complications such as paralysis can be permanent.
Prevention
West Nile virus is carried by mosquitoes and is most common during the summer and early fall. The best way to prevent becoming infected with West Nile virus is to avoid being bitten by a mosquito. Use insect repellant when you go outside, especially during the peak mosquito hours of dusk and dawn. Remove mosquito-breeding environments (such as standing water in flower pots) from your property. Scientists are testing several different vaccines to protect against West Nile virus, but it will be many years before they are commercially available.
Introduction
Encephalitis is a rare but potentially life-threatening inflammation of the brain that can occur in people of all ages. The most common cause of encephalitis is infection by a virus. In very rare cases, encephalitis can also be caused by bacterial infection, parasites, or complications from other infectious diseases. This report focuses on viral encephalitis.
Encephalitis: Viral Infection of the Brain
Many viruses can cause encephalitis. The West Nile virus, for example, has been responsible for high-profile outbreaks in the U.S. Most people exposed to encephalitis-causing viruses have no symptoms. Others may experience a mild flu-like illness, but do not develop full-blown encephalitis.
In severe cases, the infection can have devastating effects, including:
- Swelling of the brain (cerebral edema)
- Bleeding within the brain (intercerebral hemorrhage)
- Nerve damage
The damage may cause long-term cognitive or physical problems, depending on the specific areas of the brain affected.
Other Viral Infections of the Central Nervous System. Viral infection and inflammation can affect multiple areas of the central nervous system, and is categorized by its location:
- Meningitis: infection of the meninges (the membranes that surround the brain and spinal cord)
- Meningoencephalitis: infection of both the brain and meninges
- Encephalomyelitis: infection of the brain and spinal cord
Specific Viruses Implicated in Encephalitis
Encephalitis caused by viruses in the United States generally fall into the following groups:
- Arboviruses are the primary cause of acute encephalitis (sudden-onset encephalitis caused by direct infection). Arboviruses, short for "arthropod-borne viruses," are spread by mosquitoes and ticks.
- Enteroviruses, such as coxsackievirus.
- Herpes viruses are the other major cause of encephalitis in the U.S. This virus family includes herpes simplex, Epstein-Barr, cytomegalovirus, and varicella-zoster.
- In rare cases, secondary encephalitis can develop following childhood viral diseases such as measles, mumps, and rubella.
[For more information, see the Causes section in this report.]
How Viruses Can Infect the Central Nervous System
Encephalitis can develop shortly after an initial viral infection, or it can develop when a virus that was lying dormant in the body suddenly reactivates. Viruses are simple, but powerful infectious organisms.
- The virus infects a person (host) by penetrating a cell membrane and ejecting its genetic material (its DNA or RNA) into the cell.
- The viral DNA or RNA takes control of important cell processes, telling the cell to make more viruses.
- The cell ruptures, releasing new viral particles that infect other cells.
There are two ways that viruses can infect brain cells:
- The virus silently invades the body. There are no initial symptoms. The virus is carried by the bloodstream to the nerve cells of the brain, where they gather and multiply. Viruses that enter the brain in this manner are often widely scattered throughout the brain. This is called diffuse encephalitis.
- A virus first infects other tissue and then invades brain cells. Viruses that are transmitted from other tissues usually cause focal infection, meaning they produce extensive damage in only a small area of the brain.
The Central Nervous System
The brain and spinal cord comprise the central nervous system. The adult human brain weighs about 3 pounds (1.4 kilograms). There are two major parts of the brain:
- The higher and larger forebrain (the cerebrum)
- The lower and smaller brain stem
The Cerebrum
The cerebrum is the uppermost and largest part of the brain. It is the most highly developed section of the brain. There cerebrum has several components:
The Cerebral Cortex. The cortex is the outermost layer of the cerebrum. It is made of gray and white matter:
- Gray matter is a thin sheet of nerve cells that cover the surface of the brain.
- White matter is a bundle of insulated nerve fibers that underlies the cortex and makes up the core of the cerebral hemispheres.
The Hemispheres. The two hemispheres control higher brain functions, such as memory, learning, decision making, and processing input from the senses. They are each divided into four lobes, which regulate different brain functions:
- Frontal lobe: This is the brain's "gatekeeper." It controls higher motor functions, including speech, and governs concentration, attention, inhibition, judgment, and personality traits.
- Parietal lobe: Processes information from the senses and controls walking, posture, and head and eye movements.
- Occipital lobe: Responsible for interpreting visual input from the eyes.
- Temporal lobe: Responsible for interpreting auditory input from the ears. Also regulates how language is interpreted and retrieves information for memory storage.
The Basal Ganglia. The basal ganglia are clusters of gray matter within each of the lobes. They are important for coordinating voluntary muscle movement, balance, and posture.
The Limbic System. The limbic system is located deep in the cerebrum and controls interpretation of smell, instinctive behavior, emotions, and drives.
Brain Stem
The brain stem is responsible for all vital functions. It is divided into the following areas, which are responsible for specific functions:
- Medulla: sleep, breathing, heartbeat, digestion, activation of higher forebrain functions
- Pons: sleep, breathing, motor control, activation of higher forebrain functions
- Cerebellum: movement coordination
- Midbrain: walking, posture, head, eye movement
- Hypothalamus: body temperature, appetite, sexual behavior, reproductive hormones
- Thalamus: communication with higher forebrain
The Spinal Cord
The spinal cord extends out of the base of the skull through the vertebrae of the spinal column. It is continuous with the brain. Thirty-one pairs of nerves extend from the sides of the spinal cord to other parts of the body (the peripheral nervous system).
The Meninges and Cerebrospinal Fluid
The meninges are three membranes that enclose the brain and spinal cord. They contain cerebrospinal fluid, which protects the central nervous system from pressure and injury.
Causes
Arboviruses
Arboviruses, including the West Nile virus, are transmitted by blood-sucking insects such as mosquitoes and ticks. Most of the time, the viral infections initially develop in birds. Insects that feed on the infected blood from a diseased bird (or reservoir ) carry the virus, and transmit it when they bite a susceptible host (such as an animal or a human). Because these insects play a role in the disease-transmission process, they are referred to as vectors.
Arboviruses multiply in blood-sucking vectors, nearly always mosquitoes. There is no evidence that these infections can be transmitted casually from one infected person or animal directly to another uninfected person without passing through a mosquito (or tick) first. (Although, a small number of West Nile virus cases have occurred through blood transfusions, organ transplantation, and possibly breast-feeding.) It should be stressed that only about 10% of people who are infected by an arbovirus develop encephalitis and that only about 1% of those infected show symptoms.
Arboviruses that cause encephalitis are primarily found in three virus families: Togaviridae, Bunyaviridae, and Flaviviridae. In the United States, the main mosquito-borne encephalitis strains are: Eastern equine, Western equine, St. Louis, La Crosse, and West Nile. Equine encephalitis causes disease in both humans and, as its name implies, horses. Powassan encephalitis is a less common tick-borne flavivirus that occurs primarily in the northern United States. Japanese encephalitis is the most common form of viral encephalitis to occur outside of the United States. It is endemic in rural areas in east, south, and southwest Asia, especially China and Korea. Venezuelan equine encephalitis is found in South and Central America.
Different arboviruses cause different forms of encephalitis. Although the overall disease is the same, there are subtle differences in symptoms and the type of brain damage they produce.
