It was first shown by Theiler (1) that repeated passage of the French strain of yellow fever virus through the brain tissues of white mice leads to some change in the virus so that it no longer causes the visceral lesions of yellow fever when inoculated into Macacus rhesus. These observations have been confirmed and extended by Sellards (2), Sawyer, Kitchen, and Lloyd (3), and Lloyd and Penna (4). On the other hand yellow fever virus of the Asibi strain is almost uniformly fatal to M. rhesus when inoculated into these animals.
There are here presented the results observed following inoculation of the two above mentioned viruses into M. rhesus, both simultaneously and at intervals up to forty-eight hours—the viscerotropic virus always being inoculated first. The details of seventeen experiments, in which forty-eight test animals were used, are set forth in table 1, and the results are summarized in table 2.
The studies and observations on which this paper is based were conducted with the support and under the auspices of the International Health Division of the Rockefeller Foundation.