CLINICAL DESCRIPTION

Most West Nile virus (WNV) infections are mild and often clinically unapparent. Approximately 20% of those infected develop a generally mild illness, WNV fever syndrome, characterized by fever, headache, malaise, nausea, body aches, and sometimes associated with a rash and swollen lymph glands. The symptoms generally last 3 to 7 days, however, some patients have symptoms that persist for weeks to months.

Approximately 1 in 150 infections will result in severe neurological disease, neuroinvasive WNV, characterized by encephalitis, meningitis or acute flaccid paralysis. Clinical manifestations may include fever, headache, stiff neck, altered mental status (ranging from confusion to coma) and additional signs of brain dysfunction (e.g., weakness, paralysis, nerve palsies, abnormal reflexes, convulsions and abnormal movements). Case fatality rates for neuroinvasive WNV range from 3% - 15%, with advanced age the most significant risk factor for death. Patients older than 70 years of age are at particularly high risk for death.

The incubation period is 5 to 15 days.

CASE CLASSIFICATION

1. CONFIRMED
   
   A clinically compatible case which is laboratory confirmed by:
   
   • A fourfold or greater change in serum antibody titer, or
   • Isolation of West Nile virus from, or demonstration of viral antigen or genomic sequences in tissue, blood, cerebrospinal fluid (CSF), or other body fluid, or
   • West Nile virus-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 West Nile virus-specific serum neutralizing antibodies in the same or a later specimen.
     
     
2. PROBABLE

   A clinically compatible case occurring during a period when arboviral transmission is likely (July through October) with serum IgM antibodies detected by antibody-capture EIA but without a confirmatory test to demonstrate West Nile virus-specific serum neutralizing antibodies in the same or a later specimen.
   
   

3. POSSIBLE
   
   Not used

What should I know about West Nile Virus?