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Encephalitis or Meningitis, Arboviral (includes
California serogroup, Eastern equine, St. Louis, Western equine, West Nile,
Powassan)
2001 Case Definition
Clinical description
Arboviral infections may be asymptomatic or may result
in illnesses of variable severity sometimes associated with central
nervous system (CNS) involvement. When the CNS is affected, clinical
syndromes ranging from febrile headache to aseptic meningitis to
encephalitis may occur, and these are usually indistinguishable from
similar syndromes caused by other viruses. Arboviral meningitis is
characterized by fever, headache, stiff neck, and pleocytosis. Arboviral
encephalitis is characterized by fever, headache, and altered mental
status ranging from confusion to coma with or without additional
signs of brain dysfunction (e.g., paresis or paralysis, cranial nerve
palsies, sensory deficits, abnormal reflexes, generalized convulsions,
and abnormal movements).
Laboratory criteria for diagnosis
- Fourfold or greater change in virus-specific serum
antibody titer, or
- Isolation of virus from or demonstration of specific
viral antigen or genomic sequences in tissue, blood, cerebrospinal
fluid (CSF), or other body fluid, or
- Virus-specific immunoglobulin M (IgM) antibodies demonstrated
in CSF by antibody-capture enzyme immunoassay (EIA), or
- Virus-specific IgM antibodies demonstrated in serum
by antibody-capture EIA and confirmed by demonstration of virus-specific
serum immunoglobulin G (IgG) antibodies in the same or a later specimen
by another serologic assay (e.g., neutralization or hemagglutination
inhibition).
Case classification
Probable: an encephalitis
or meningitis case occurring during a period when arboviral transmission
is likely, and with the following supportive serology: 1) a single
or stable (less than or equal to twofold change) but elevated titer
of virus-specific serum antibodies; or 2) serum IgM antibodies detected
by antibody-capture EIA but with no available results of a confirmatory
test for virus-specific serum IgG antibodies in the same or a later
specimen.
Confirmed: an encephalitis or meningitis case
that is laboratory confirmed
Comment
Because closely related arboviruses exhibit serologic cross-reactivity,
positive results of serologic tests using antigens from a single arbovirus
can be misleading. In some circumstances (e.g., in areas where two or more
closely related arboviruses occur, or in imported arboviral disease cases),
it may be epidemiologically important to attempt to pinpoint the infecting
virus by conducting cross-neutralization tests using an appropriate battery
of closely related viruses. This is essential, for example, in determining
that antibodies detected against St. Louis encephalitis virus are not the
result of an infection with West Nile (or dengue) virus, or vice versa,
in areas where both of these viruses occur.
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 six encephalitides/meningitides
printed in bold are nationally reportable to CDC):
St. Louis encephalitis/meningitis
West Nile encephalitis/meningitis
Powassan encephalitis/meningitis
Eastern equine encephalitis/meningitis
Western equine encephalitis/meningitis
California serogroup viral encephalitis/meningitis (includes
infections with the following viruses: La Crosse, Jamestown Canyon,
snowshoe hare, trivittatus, Keystone, and California encephalitis viruses)
Other viral CNS infections transmitted by mosquitos,
ticks, or midges (e.g., Venezuelan equine encephalitis/meningitis and
Cache Valley encephalitis/meningitis)
See also:
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