Abstract
Anopheles quadrimaculatus mosquitoes, fed on mice in which encephalitis virus (St. Louis type) is present in the blood stream, take up and retain the virus for the duration of their lives. The titre of the virus in mosquitoes 4 hours after engorging on mice with a maximum blood stream infection represents about 10,000 lethal mouse intracerebral doses per mosquito. This titre drops during the following 2 weeks to about 100 lethal doses per mosquito, but from the 3rd week to the death of the mosquito, usually increases to approximately the original level and remains there. The titre of virus in mosquitoes which have engorged on mouse blood containing smaller quantities of virus exhibits the same drop and subsequent rise to the original level. The virus-containing mosquitoes did not infect mice or monkeys by biting.