Encephalitis
1990 Case Definition

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 infection with other neurotropic viruses. Symptoms may include headache, confusion or other alterations in sensorium, nausea, or vomiting. Signs may include evidence of elevated intracranial pressure or meningeal irritation, cranial nerve palsies, paresis or paralysis, altered reflexes, or convulsions. (See Aseptic Meningitis and Encephalitis, primary.)

Laboratory Criteria For Diagnosis

• Fourfold or greater rise in serum antibody titer, OR
• Isolation of virus from or demonstration of viral antigen in tissue, blood, cerebrospinal fluid (CSF), or other body fluid, OR
• Specific Immunoglobulin M (IgM) antibody in CSF

Probable

A clinically compatible illness occurring during a period when arbovirus transmission is likely to occur, and with the following supportive serology: a stable (twofold or greater 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, greater than or equal to 160 by neutralization, or a positive serologic result by enzyme immunoassay (EIA)

Confirmed

A clinically compatible illness that is laboratory confirmed

Comments

The time of year in which arboviral transmission is likely to occur depends on the geographic location of exposure, the specific cycle of virus transmission, and local climatic conditions.

Arboviruses causing encephalitis include the following:

• St. Louis encephalitis
• Western equine encephalitis
• Eastern equine encephalitis
• California encephalitis (includes infections from the following viruses: LaCrosse, Jamestown Canyon, Snowshoe Hare, Trivittatus, and California viruses)
• Powassan encephalitis
• Other central nervous system infections transmitted by mosquitoes, ticks, or midges (Venezuelan equine encephalitis, Cache Valley encephalitis)

Clinical Description

Encephalitis or meningoencephalitis that follows or occurs in combination with other viral illnesses that are not central nervous system illnesses, or after vaccine is administered. Symptoms may be due to hypersensitivity reaction. Primary encephalitis is excluded.

Confirmed

A clinically compatible illness diagnosed by a physician as postinfectious (or parainfectious) encephalitis

Comments

Laboratory studies are important in clinical diagnosis but are not required for reporting purposes.

Clinical Description

An illness in which encephalitis is the major manifestation. Symptoms are due to direct invasion and replication of the infectious agent in the central nervous system, resulting in objective clinical evidence of cerebral or cerebellar dysfunction. Postinfectious (or parainfectious) encephalitis is excluded.

Confirmed

A clinically compatible illness diagnosed by a physician as primary encephalitis

Comments

Laboratory studies are important in clinical diagnosis but are not required for reporting purposes.
Primary encephalitis is a category used for reporting to the National Notifiable Diseases Surveillance System (NNDSS). This category includes arboviral encephalitis and primary encephalitis of unspecified cause.