What is encephalitis?

Causes and Symptoms

Encephalitis is sometimes classified according to the causative agent or the anatomic structures affected. Limbic encephalitis, for example, affects structures in the brain

known as the limbic system. Exposure to lead often produces cerebral inflammation and edema (swelling) and is referred to as lead encephalitis. If the agent is bacterial, then the disease is referred to as bacterial encephalitis. In amebic encephalitis, patients with weakened immune systems become infected through certain protozoa (Acanthamoeba) found in water and moist soil.

The principal cause of encephalitis, however, is viral. In primary encephalitis, the virus directly invades the brain and spinal cord. Secondary encephalitis occurs as an aftereffect of such airborne diseases as measles and influenza (postinfectious) or of certain vaccinations (postvaccinal).

Though many viruses can produce encephalitis, only a limited number tend to recur. They fall into three major groups: enteroviruses, arboviruses, and nonarthropod viruses. Nonarthropod viruses, transmitted without an insect vector, include the very common herpes simplex virus 1 (HSV-1).

Enteroviruses infect the gastrointestinal tract and are spread by a fecal-oral route. Hands come into contact with feces or bodily fluids in which the virus is present. If unwashed, the hands can transfer the virus to the mouth. Once ingested, the virus replicates in the intestines and then moves to the nervous system.

Arboviruses, which are responsible for epidemics, are spread by mosquitoes (such as in eastern equine encephalitis) and ticks (as in Powassan encephalitis). For natural reasons related to their vectors (carriers), these infections, at least in northerly climes, peak in late summer.

If a mosquito ingests a blood meal from an infected vertebrate, over a period of one to three weeks, the virus replicates in the mosquito’s gut and then moves to its salivary glands. When the mosquito bites a human, the virus lurks in the person’s visceral organs and then passes by means of the blood to the nervous system. A possible route to the central nervous system (CNS) is through the brain capillaries. Infection of neurons, and glial cells, which constitute the non-nervous tissue of the brain and spinal cord, follows, leading to cell dysfunction and death. The body’s own immune response, which includes infusing white blood cells into the cerebrospinal fluid, contributes to the brain edema and inflammation.

In diagnosis, physicians use blood and deoxyribonucleic acid (DNA) tests and analyze the cerebrospinal fluid for a too-high count of white blood cells and elevated protein levels and fluid pressure. Neuroimaging and electroencephalograms (EEGs), which record electrical activity in the brain, are used to eliminate other possibilities, such as clotting because of the rupture of a blood vessel (hematoma). Isolation of the virus itself, with some exceptions, is difficult. Biopsy of brain tissue for evidence of the virus has largely been replaced by less invasive procedures.

Symptoms may occur within a few hours or over the course of several days and initially are nonspecific, which complicates the diagnosis. Although they vary depending on the virus and the extent and length of infection, symptoms generally include fever, headache, muscle ache, stiff neck, respiratory symptoms, sensitivity to light, abdominal pain, vomiting, dizziness, an altered level of consciousness that may range from lethargy to coma, personality changes that may progress to behavior that appears psychotic, intellectual deficit, and a host of neurological deficiencies, such as tremors, loss of muscular coordination, partial paralysis, and ocular (eye) fixation.

Treatment and Therapy

For some types of encephalitis, such as Japanese encephalitis, effective vaccines exist. In bacterial cases, antibiotics are prescribed. In patients in whom the herpes simplex virus is implicated, the antiviral acyclovirin is useful. In general, however, treatment, often initially in an intensive care unit (ICU), is supportive and designed to control complications. For example, steroids are sometimes administered to reduce brain swelling and anticonvulsants, if seizures occur.

The disease runs its course in one to two weeks. Mortality rates depend on the type of virus and the age of the patient, the very young and elderly being more vulnerable. Most cases of encephalitis are mild. In eastern equine encephalitis, the mortality rate is about 33 percent; in western equine encephalitis, it is about 3 percent in older patients and as high as 30 percent in younger patients. Residual symptoms after recovery vary, again according to the agent and extent of infection. In eastern equine encephalitis, 80 percent of patients suffer neurologic aftereffects.

Perspective and Prospects

Some historians of medicine believe that viral encephalitis appeared early in the Mediterranean area. The evidence is indirect, with the mention in the Hippocratic corpus (fifth century BCE and later) of genital and labial lesions consistent with the herpes virus, a leading cause of the disease. It was not until the nineteenth and twentieth centuries that the numerous agents and vectors for encephalitis were successfully identified, such as the rabies
virus (isolated by Louis Pasteur’s dog experiments), the spirochete of syphilis, and more recently Human immunodeficiency virus (HIV).

A firm connection between the great influenza
pandemic of 1918 and the repeated global outbreaks of encephalitis lethargica in the 1920s was not established until 1982. In the 1990s, aspirin therapy in children’s influenza was implicated in the sometimes fatal brain edema known as Reye syndrome.

Research has focused on oral antiviral drug therapy. Interferon alpha-2b therapy and ribovarin, related to the vitamin B complex, have been tested on patients with West Nile virus, but their value has not been conclusively established. Emphasis therefore has remained on prevention: proper vaccinations and, for arboviruses, mosquito spraying campaigns, application of effective insect repellents, and limited outside exposure during the early evening hours.

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