Encephalitis is an inflammation of the brain, usually caused by a direct viral infection or a hypersensitivity reaction to a virus or foreign protein. Theinflammation is a reaction of the body's immune system to infection or invasion. During the inflammation, the brain's tissues become swollen. Thecombination of the infection and the immune reaction to it can cause headacheand a fever, as well as more severe symptoms in some cases.
There are more than a dozen viruses that can cause encephalitis, spread by either human-to-human contact or by animal bites. Encephalitis may occurwith several common viral infections of childhood. Viruses and viral diseases that may cause encephalitis include:
- Epstein-Barr virus (EBV)
- Cytomegalovirus infection
- Herpes simplex
- Herpes zoster(shingles)
- Herpes B
- Mosquito-borne viruses (arboviruses).
Mosquitoes spread viruses responsible for equine encephalitis (eastern and western types), St. Louis encephalitis, California encephalitis, and Japanese encephalitis. Lyme disease, spread by ticks, can cause encephalitis, as can Colorado tick fever. Rabies is most often spread by animal bites from dogs, cats, mice, raccoons, squirrels, and bats and may cause encephalitis.
Equine encephalitis is carried by mosquitoes that do not normally bite humansbut do bite horses and birds. It is occasionally picked up from these animals by mosquitoes that do bite humans. Japanese encephalitis and St. Louis encephalitis are also carried by mosquitoes. The risk of contracting a mosquito-borne virus is greatest in mid- to late summer, when mosquitoes are most active, in those rural areas where these viruses are known to exist. Eastern equine encephalitis occurs in eastern and southeastern United States; western equine and California encephalitis occur throughout the West; and St. Louis encephalitis occurs throughout the country. Japanese encephalitis does not occur in the United States, but is found throughout much of Asia. The viruses responsible for these diseases are classified as arbovirus and these diseases are collectively called arbovirus encephalitis.
Herpes simplex encephalitis, the most common form of sporadic encephalitis inwestern countries, is a disease with significantly high mortality. It occursin children and adults and both sides of the brain are effected. It is theorized that brain infection is caused by the virus moving from a peripheral location to the brain via two nerves, the olfactory and the trigeminal (largestnerves in the skull).
Herpes simplex encephalitis is responsible for 10% of all encephalitis casesand is the main cause of sporadic, fatal encephalitis. In untreated patients,the rate of death is 70% while the mortality is 15-20% in patients who havebeen treated with acyclovir. The symptoms of herpes simplex encephalitis arefever, rapidly disintegrating mental state, headache, and behavioral changes.
The symptoms of encephalitis range from very mild to very severe and may include:
- Lethargy (sleepiness, decreasedalertness, and fatigue)
- Nausea and vomiting
- Visual disturbances
- Decreased consciousness (drowsiness, confusion, delirium, unconsciousness)
- Stiff neck
Symptoms may progress rapidly, changing from mild to severe within several days or even several hours.
Diagnosis of encephalitis includes careful questioning to determine possibleexposure to viral sources. Tests which can help confirm the diagnosis and rule out other disorders include:
- Blood tests. These are to detect antibodies to viral antigens, and foreign proteins.
- Cerebrospinal fluidanalysis (spinal tap). This detects viral antigens, and provides culturespecimens for the virus or bacteria that may be present in the cerebrospinalfluid.
- Electroencephalogram (EEG)
- CT and MRI scans.
A brain biopsy (surgical gathering of a small tissue sample) may be recommended in some cases where treatment to date has been ineffective and the cause of the encephalitis is unclear. Definite diagnosis by biopsy may allow specific treatment that would otherwise be too risky.
Choice of treatment for encephalitis will depend on the cause. Bacterial encephalitis is treated with antibiotics. Viral encephalitis is usually treated with antiviral drugs including acyclovir, ganciclovir, foscarnet, ribovarin, and AZT. Viruses that respond to acyclovir include herpes simplex, the most common cause of sporadic (non-epidemic) encephalitis in the United States.
The symptoms of encephalitis may be treated with a number of different drugs.Corticosteroids, including prednisone and dexamethasone, are sometimes prescribed to reduce inflammation and brain swelling. Anticonvulsant drugs, including dilantin and phenytoin, are used to control seizures. Fever may be reduced with acetaminophen or other fever-reducing drugs.
A person with encephalitis must be monitored carefully, since symptoms may change rapidly. Blood tests may be required regularly to track levels of fluidsand salts in the blood.
Encephalitis symptoms may last several weeks. Most cases of encephalitis aremild, and recovery is usually quick. Mild encephalitis usually leaves no residual neurological problems. Overall, approximately 10% of those with encephalitis die from their infections or complications such as secondary infection.Some forms of encephalitis have more severe courses, including herpes encephalitis, in which mortality is 15-20% with treatment, and 70-80% without. Antiviral treatment is ineffective for eastern equine encephalitis, and mortalityis approximately 30%.
Because encephalitis is due to infection, it may be prevented by avoiding theinfection. Minimizing contact with others who have any of the viral illnesslisted above may reduce the chances of becoming infected. Most infections arespread by hand-to-hand or hand-to-mouth contact; frequent hand washing may reduce the likelihood of infection if contact cannot be avoided.
Mosquito-borne viruses may be avoided by preventing mosquito bites. Mosquitoes are most active at dawn and dusk, and are most common in moist areas with standing water. Minimizing exposed skin and use of mosquito repellents on other areas can reduce the chances of being bitten.
Vaccines are available against some viruses, including polio, herpes B, Japanese encephalitis, and equine encephalitis. Rabies vaccine is available for animals; it is also given to people after exposure. Japanese encephalitis vaccine is recommended for those traveling to Asia and staying in affected rural areas during transmission season.