Encephalitis

Encephalitis is a severe inflammation of the brain characterized by intense lymphocytic infiltration of brain tissue and the leptomeninges. This process causes cerebral edema, degeneration of the brain’s ganglion cells, and diffuse nerve cell destruction.

Encephalitis is usually caused by a mosquito-borne or, in some areas, a tickborne virus. Transmission by means other than arthropod bites may occur through ingestion of infected goat’s milk and accidental injection or inhalation of the virus.

Causes

Encephalitis usually results from infection with arboviruses that are specific to rural areas. In urban areas, encephalitis is most frequently caused by enteroviruses (coxsackievirus, poliovirus, and echovirus). Other causes include herpesvirus, mumps virus, adenoviruses, and demyelinating diseases after measles, varicella, rubella, or vaccination. (See Types of encephalitis.)

Complications

Potential complications associated with viral encephalitis include bronchial pneumonia, urine retention, urinary tract infection, pressure ulcers, and coma. Epilepsy, parkinsonism, and mental deterioration may also occur.

Assessment Findings

Depending on the severity of the infection, all forms of viral encephalitis have similar clinical features. The severity of arbovirus encephalitis may range from subclinical to rapidly fatal necrotizing disease. Herpes encephalitis also produces signs and symptoms that vary from subclinical to acute and often fatal fulminating disease.

If encephalitis is the primary illness, the patient may be acutely ill when he or she seeks treatment because the nonspecific symptoms that occur before the onset of acute neurologic symptoms aren’t recognized as signs of encephalitis. Thus, the patient history may include reports of systemic symptoms, such as headache, muscle stiffness, malaise, sore throat, and upper respiratory tract symptoms that existed for several days before the onset of neurologic symptoms.

Signs and symptoms include an altered level of consciousness, from lethargy or drowsiness to stupor, seizures, confusion, hallucinations, tremors, cranial nerve palsies, exaggerated deep tendon reflexes, absent superficial reflexes, and paresis or paralysis of the extremities. The patient may complain of a stiff neck when the head is bent forward.

Other signs include fever, nausea, and vomiting. If the cerebral hemispheres are involved, assessment findings may include aphasia, involuntary movements identified on inspection, ataxia, sensory defects such as disturbances of taste and smell, and poor memory retention.

Diagnostic Tests

Serologic assays, such as immunoglobulin (Ig) M antibody-capture enzyme-linked immunosorbent assay (MAC-ELISA) and IgG ELISA may be diagnostic. Early in the infection, IgM antibody is more specific, whereas later IgG is more reactive.
Blood analysis or, rarely, cerebrospinal fluid (CSF) analysis identifies the virus and confirms the diagnosis.
Serologic studies in herpes encephalitis may show rising titers of complement-fixing antibodies. In some types of encephalitis, serologic blood tests may be diagnostic.
Lumbar puncture detects elevated CSF pressure in all forms of encephalitis. Despite inflammation, CSF analysis findings often reveal clear fluid. White blood cell count and protein levels in CSF are slightly elevated, but the glucose level remains normal.
Electroencephalography reveals abnormalities such as generalized slowing of waveforms.

Types of Encephalitis

Four main virus agents cause most cases of encephalitis in the United States: eastern equine encephalitis (EEE), western equine encephalitis (WEE), St. Louis encephalitis, and La Crosse encephalitis, all of which are transmitted by mosquitoes. Since 1999, encephalitis due to West Nile virus (WNV) has also been seen in the United States. Most cases of arboviral encephalitis occur from June through September, when the weather is wet and arthropods are most active. In milder parts of the country, where arthropods are active late into the year, cases are seen into the winter months. No vaccines are available for these U.S.-based diseases. However, a Japanese encephalitis vaccine is available for those who will be traveling to Japan, a tickborne encephalitis vaccine is available for those who will be traveling to Europe, and an equine vaccine is available for EEE, WEE, and Venezuelan equine encephalitis (VEE). Public health measures often require spraying of insecticides to kill larvae and adult mosquitoes as well as controlling standing water that can be used as breeding sites for mosquitoes.

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