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)