An in-depth report on the causes, diagnosis, treatment, and prevention of viral encephalitis.
West Nile virus
Encephalitis, an inflammation of the brain, is rare but can be caused by many different viruses. The main causes of viral encephalitis are:
- Herpes viruses. among which herpes simplex virus is particulary important.
Loss of consciousness
Because encephalitis can be dangerous, it needs to be diagnosed promptly. Patients are treated immediately, even before diagnostic tests identify the specific virus that caused the illness. If herpes is a possible cause, the standard treatment is the antiviral drug acyclovir. Once the cause has been determined, other drugs may be administered. Unfortunately, however, many types of encephalitis, such as the ones caused by West Nile virus and other arboviruses, do not respond to antiviral drugs.
The best way to prevent becoming infected with a mosquito-borne 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 from your property.
2012 Record Year for West Nile Virus
According to the U.S. Centers for Disease Control, more cases of West Nile virus were reported in 2012 than in any other year in nearly a decade. A third of all cases occurred in Texas. Researchers are studying the virus and trying to understand what makes it so unpredictable and difficult to control. Fortunately, only a small percentage of West Nile virus infections cause encephalitis. Unfortunately, there is currently no treatment available.
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 well-publicized 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 (neuropathy )
The damage may cause long-term mental 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 Associated with 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. There is no treatment for encephalitis caused by arboviruses.
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. Herpes simplex is the most common type of herpes-associated encephalitis. It can cause severe brain damage, but can be treated with antiviral medication..
Less common viral causes of encephalitis include enteroviruses, adenoviruses, and viruses associated with childhood diseases such as measles, mumps, and rubella.
[For more information, see the Causes section in this report.]
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.
There are two ways that viruses can infect brain cells and cause encephalitis:
- Primary encephalitis is when the virus directly affects the brain or spinal cord. The resulting inflammation can occur in one area (focal) or can occur throughout the brain (diffuse).
- Secondary encephalitis. also called post-infectious encephalitis, is when the virus first attacks another part of the body and the infection then spreads to the brain.
The herpes virus family includes at least 8 distinct viruses that cause infections in humans. These viruses and infections include varicella-zoster virus (the cause of chickenpox and shingles), Epstein-Barr virus (the cause of mononucleosis), cytomegalovirus, and herpes virus 6. Although any herpes virus can cause encephalitis, the herpes simplex virus is the most important cause of encephalitis.
Herpes simplex virus (HSV) is responsible for 5 – 10% of encephalitis cases worldwide. Herpes simplex encephalitis (HSE) tends to be most severe when it affects children and older people. There are two distinct types of the herpes simplex virus:
- Herpes simplex virus 1 (HSV-1) causes most cases of adult herpes encephalitis. HSV-1 is the main cause of oral herpes infections but it can also cause genital herpes.
- Herpes simplex virus 2 (HSV-2) causes most cases of encephalitis in newborn infants. HSV-2 is the main cause of genital herpes.
Unlike arbovirus encephalitis, herpes simplex encephalitis is treatable, but treatment (typically intravenous acyclovir) must be administered within the first few days of symptom onset. If left untreated, herpes simplex encephalitis can be fatal. [For more information, see In-Depth Report #52: Herpes simplex . ]
Arboviruses, including the West Nile virus, are transmitted by blood-sucking insects such as mosquitoes and ticks. Most of the time, these viral infections initially develop in birds, which function as the reservoir of infection. Insects that feed on the infected blood from a diseased bird pick up the virus, and transmit it when they bite a susceptible host (such as an animal or a human). The insects that play a role in this disease-transmission process are referred to as vectors.
Arboviruses multiply in blood-sucking vectors. In fact, the word arbovirus is an acronym for ar thropod-bo rne virus. Mosquitoes, ticks, and many other insects are classified as arthropods. Mosquitoes are the most common vector for arboviruses.
In general, the virus first passes through an insect before infecting a person. These infections are not transmitted through casual contact from one person (or animal) to another. (However, a small number of West Nile virus cases have occurred through blood transfusions, organ transplantation, and possibly breastfeeding.) Only a small percentage of people who are infected by an arbovirus develop encephalitis.
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, which is also transmitted by mosquito, 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.
Common Forms of Mosquito-Borne Arbovirus Encephalitis