Route of Transmission of St. Louis Encephalitis Virus in Mice. ∗

Abstract

Histopathological studies by Webster and Fite 1 suggest that the virus isolated from the St. Louis outbreak of encephalitis, travels by way of the central nervous system in mice. The present report confirms and extends these findings.

In order to determine the portal by which mice could be infected, they were given virus by the nasal and gastro-intestinal routes. Three series of mice each consisting of 4 to 6 animals, were given a concentrated suspension of the virus in their food over 2 to 3 days. Not one of these mice became infected. Three animals, upon whom a laparotomy was performed, were injected intrastomachally with 0.2 cc. of a thick virus suspension. None of these showed evidence of the disease.

Mice could be infected regularly with the intracerebral virus, within an incubation period of 6-8 days, by placing 0.03 cc. of a 1% suspension into one nostril, while 0.1% failed to give regular infection. The virus was transmitted serially by the intranasal route for 28 passages, using a 10% suspension, but its infectivity, by this route, did not increase.

After 28 passages the virus was infective by the intranasal route, at approximately a 1:250 dilution.

In order to trace the route of infection from the nasopharynx, the blood, parenchymatous tissues and various areas of the cerebrospinal axis were tested for the presence of virus after intranasal instillation of virus at various stages of the incubation period and of the disease. Two experiments were carried on simultaneously; one in which various organs and areas of the cerebrospinal axis were suspended in distilled water and one in which 10% normal monkey serum, which enhances the infectivity of the virus, was employed as diluent. In testing any area, a suspension made of a pooling of 2 to 3 organs in that area was injected intracerebrally and intraperitoneally Control animals receiving intranasal inoculations of the same virus were included in east test and always showed symptoms.