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The California Encephalitis Project was initiated in June 1998 to identify the causes and characterize the clinical and epidemiologic features of encephalitis in California. Testing for >or=13 agents, including herpesviruses, enteroviruses, arboviruses, Bartonella species, Chlamydia species, and Mycoplasma pneumoniae, was performed at the Viral and Rickettsial Disease Laboratory (Richmond, California). Epidemiologic and clinical information collected for each case guided further testing. From June 1998 through December 2000, 334 patients who met our case definition of encephalitis were enrolled. A confirmed or probable viral agent of encephalitis was found in 31 cases (9%), a bacterial agent was found in 9 cases (3%), and a parasitic agent was found in 2 cases (1%). A possible etiology was identified in 41 cases (12%). A noninfectious etiology was identified in 32 cases (10%), and a nonencephalitis infection was identified in 11 (3%). Despite extensive testing and evaluation, the etiology of 208 cases (62%) remained unexplained.

These results support the need for therapies effective against rhinovirus as a means to decrease asthma exacerbations.

Analysis of the National Hospital Discharge Survey data for 1988-1997 revealed a substantial disease burden associated with encephalitis in the United States: on average, there were nearly 19,000 hospitalizations (7.3 hospitalizations per 100,000 population), 230,000 hospital days, and 1400 deaths annually. For most encephalitis-associated hospitalizations (59.5%), the etiologic agent was unknown or not recorded; the most common etiologic categories among known causes were "viral" (38.2%) and "other infectious" (34.1%). The most common individual diagnoses with known agents were herpetic and toxoplasmic encephalitides (each associated with an annual average of 2100 hospitalizations). Human immunodeficiency virus infection was listed among discharge diagnoses for 15.6% of hospitalizations. Rates of encephalitis-associated hospitalization were highest for children <1 year old and persons > or =65 years old. The etiology of encephalitis was unknown for persons > or =65 years old significantly more often than it was for younger persons. The average cost of an encephalitis-associated hospitalization, as determined by the Healthcare Cost and Utilization Project for 1997, was $28,151, for an annual national cost of hospitalization of $650 million.