Experimental encephalitis following peripheral inoculation of West Nile virus in mice of different ages.

Experimental arbovirus infections of mice provide a convenient model to study factors which determine the occurrence or severity of encephalitis following extraneural infection with certain neurotropic viruses. Varying doses of West Nile or Powassan viruses were inoculated by intraperitoneal or intramuscular routes into mice of varing ages; individual variables were manipulated to influence the outcome of infection. Three patterns of pathogenesis were delineated: (1) Fatal encephalitis, preceded by early viraemia, and invasion of the central nervous system. (2) Inapparent infection, with no detectable viraemia and no evidence of central nervous system invasion. (3) Subclinical encephalitis, usually preceded by trace viraemia, with minimal transient levels of virus in the brain. In this latter type of subclinical infection with a potentially lethal virus, the immune response probably plays an important role in recovery.