The epidemiology of encephalitis is a composite of the epidemiologic features of the multiple individual etiologies associated with this syndrome. Understanding the epidemiology is confounded by difficulties in both clearly defining a case and determining its etiology. Encephalitis, or inflammation of brain tissue, can occur from various insults to the central nervous system (CNS) including direct invasion by an infectious agent; an abnormal immune response to infection, vaccines, or antigens present at sites outside the CNS; or a combination of factors. In addition, the symptoms of brain dysfunction, the clinical hallmark of encephalitis, can also be associated with a number of noninfectious entities including autoimmune diseases, tumors, paraneoplastic syndromes, metabolic disorders, and toxins.
The many causes of encephalitis and nonencephalitic brain disease, the fact that the many different causes often cannot be distinguished clinically, and the difficulty in defining its etiology have frustrated our understanding of this syndrome. Encephalitis is, however, associated with substantial morbidity, long-term disability, and death and, consequently, is an important problem for which better treatment and prevention strategies could be of substantial benefit. This chapter will focus on overall epidemiologic patterns. Information on specific agents can be found in more detail in the virus-specific sections.
Various case definitions of encephalitis in general and for encephalitides due to specific etiologies such as smallpox vaccination, arbovirus infections, and rabies infection, have been developed [1, 2, 3, 4]. These definitions include clinical signs and symptoms that indicate involvement of the parenchyma of the brain (e.g. altered consciousness, personality changes, focal neurologic findings such as weakness or paralysis, and seizures) and results of laboratory studies (e.g., examination of cerebral spinal fluid [CSF]), brain imaging, and/or electroencephalography (EEG) studies that suggest brain disease and inflammation.