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Humans are infected via
the bite of a WNV-infected mosquito, and over the past three years the
impact on human health in the United States has steadily increased with
4,071 cases and 274 deaths reported in 2002. The majority of human infections
do not result in illness, with only 20% of infections developing into
West Nile fever. Human disease is characterized by a febrile, flu-like
illness with quick onset, usually 3-6 days, and moderate to high fever
lasting 3-5 days. Other typical symptoms include fever, headache, and
body aches, occasionally with a skin rash on the trunk of the body and
swollen lymph glands. The symptoms of severe infection (West Nile encephalitis
or meningitis) include headache, high fever, neck stiffness, stupor, disorientation,
coma, tremors, convulsions, muscle weakness, and paralysis. Until the
Romanian epidemic of 1996, no neurologic involvement had been observed
among patients. It is now typical for severe clinical disease to present
with neurologic manifestations.
The very young, elderly, and those with weakened
immune systems are most at risk of developing clinical disease - median
age among this year's fatalities was 78 (range: 24-99 years). The majority
of infections are asymptomatic, less than one-percent (1 in 150) results
in severe clinical illness. It is believed that infection and the subsequent
development of antibodies leads to lifelong immunity. The virus is not
directly transmitted from person-to-person, but indirect transmission
through the transfusion of infected blood, transplantation of infected
organs, and consumption of infected breast milk has been observed.
There is no specific treatment for West Nile encephalitis;
intensive supportive therapy is provided to patients with clinical illness.
The National Institutes of Health has funded research for the development
of a vaccine as well as antiviral therapy. A Cambridge, Massachusetts
biotechnology firm, Acambis, has developed a WNV candidate vaccine based
on the existing vaccine for Yellow Fever. The vaccine, ChimeriVax-West
Nile®, is in the pre-clinical stages of human trials, more than a
year from FDA approval and licensing. Research into antiviral drugs for
the treatment of WNV infection is taking place and there are several investigational
new drugs in the pipeline. Researchers are also screening large numbers
of approved drugs in order to determine whether existing antivirals are
effective against WNV.
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