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NOTE: A surveillance case definition is a set of uniform criteria used to define a disease for public health surveillance. Surveillance case definitions enable public health officials to classify and count cases consistently across reporting jurisdictions. Surveillance case definitions are not intended to be used by healthcare providers for making a clinical diagnosis or determining how to meet an individual patient’s health needs.


  • California serogroup encephalitis
  • Eastern equine encephalitis
  • St. Louis encephalitis
  • Western equine encephalitis

Clinical Description

Arboviral infection may result in a febrile illness of variable severity associated with neurologic symptoms ranging from headache to aseptic meningitis or encephalitis. Arboviral encephalitis cannot be distinguished clinically from other central nervous system (CNS) infections. Symptoms can include headache, confusion or other alteration in sensorium, nausea, and vomiting. Signs may include fever, meningismus, cranial nerve palsies, paresis or paralysis, sensory deficits, altered reflexes, convulsions, abnormal movements, and coma of varying degree.

Laboratory Criteria For Diagnosis

  • Fourfold or greater change in serum antibody titer, OR
  • Isolation of virus from or demonstration of viral antigen or genomic sequences in tissue, blood, cerebrospinal fluid (CSF), or other body fluid, OR
  • Specific immunoglobulin M (IgM) antibody by enzyme immunoassay (EIA) antibody captured in CSF or serum. Serum IgM antibodies alone should be confirmed by demonstration of immunoglobulin G antibodies by another serologic assay (e.g., neutralization or hemagglutination inhibition).

Case Classification


A clinically compatible case occurring during a period when arboviral transmission is likely, and with the following supportive serology: a stable (less than or equal to twofold change) elevated antibody titer to an arbovirus (e.g., greater than or equal to 320 by hemagglutination inhibition, greater than or equal to 128 by complement fixation, greater than or equal to 256 by immunofluorescence, and greater than or equal to 160 by neutralization, or greater than or equal to 400 by enzyme immunoassay IgM).


A clinically compatible case that is laboratory confirmed


The seasonality of arboviral transmission is variable and depends on the geographic location of exposure, the specific cycles of viral transmission, and local climatic conditions. Reporting should be etiology-specific (see below; the first four encephalitides are nationally reportable to CDC):

  1. St. Louis encephalitis
  2. Western equine encephalitis
  3. Eastern equine encephalitis
  4. California encephalitis (includes infections from the following viruses: LaCrosse, Jamestown Canyon, Snowshoe Hare, Trivittatus, Keystone, and California encephalitis viruses)
  5. Powassan encephalitis
  6. West Nile encephalitis
  7. Other CNS infections transmitted by mosquitoes, ticks, or midges (e.g., Venezuelan equine encephalitis and Cache Valley encephalitis)

Revised September 1996.

Related Case Definition(s)