Knowledge of the pathogenesis of arboviral encephalitis is incomplete. The viruses multiply at the site of the mosquito bite and in local lymph nodes, producing viremia. Levels of viremia are very low and transitory, yet the ratio of overt encephalitis to mild or inapparent infection can be 1:25 or greater. It is not known how these viruses cross the blood-brain barrier so readily. The viruses replicate in nerve cells and cause extensive destruction of brain tissue in severe cases. The process is halted with the appearance of neutralizing antibody. Mortality ranges from about 2% with LaCrosse, to 35% to 50% with eastern equine encephalitis. Disabilities occur in those who recover, ranging from 5% to more than 50% of cases, depending largely on the kind of virus and the age of the patient, the very young and the elderly suffering the most. The kinds of disability include emotional instability, mental retardation, epilepsy, blindness, deafness, and paralysis of one side of the body.
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