Epidemiologically, the viruses are maintained in nature by birds as natural hosts
and reservoirs, mosquitoes as vectors, and humans as incidental or accidental hosts. With
reference to St. Louis encephalitis, the virus is transmitted to birds in the United States by
Culex mosquitoes, C. pipiens and C. quinquefasciatus in Midwestern and Eastern States,
by C. nigripalpus in Florida and by C. tarsalus in the Great Plains and further west.
Clinical diagnosis still depends on recognition of constitutional signs and symptoms
including fever, and various neurological abnormalities, especially severe febrile
headache, aseptic meningitis, mental clouding, movement disorders, and coma with or
without worsening course to eventual death. Virus isolation, antibody development and
newer molecular immunologic techniques for early diagnosis help identify the specific
pathogen accurately.
Lymphocytic Choriomeningitis
The maxim that “Chance favors the prepared mind” is attributed to Louis Pasteur.
Charles Armstrong’s discovery of the virus of lymphocytic choriomeningitis (21)
demonstrated the veracity of this maxim exquisitely. The discovery occurred while
Armstrong continued to pass serially infected monkey brain tissue to other monkeys. In
the transmission from monkey to monkey of infectious materials from a fatal case of the
1933 epidemic of St. Louis encephalitis, Armstrong encountered a virus apparently quite
distinct from the strains previously isolated by himself, Muckenfuss, and McCordock (8),
and subsequently in white mice by Webster and Fite (9). This virus, differing from any
virus with which Armstrong was familiar, he designated as the virus of “experimental
lymphocytic chriomeningitis”, based on the pathological changes in brains produced by