It was found that 58 per cent of C. laniger injected with 1,000 LD50 of yellow fever virus circulated virus following the inoculation. The virus content of the serum of the infected animals was found to be small in most instances, the majority of titers being less than 1/100. The susceptibility of some of the animals was shown to depend on the amount of virus inoculated.
Over 90 per cent of the animals circulating virus developed neutralizing antibodies. Frequently these antibodies were present in small amounts, and their detection was possible only in neutralization tests employing small test doses. However, since no evidence of non-specific viricidal activity was found in normal woolly opossum sera, it was considered that a survival ratio of 2/6 or greater indicated the presence in the serum of neutralizing antibodies. Studies on the persistence of neutralizing antibodies in the serum revealed that a significant number of animals lost these antibodies during the course of a year after inoculation.
Reinoculation of previously infected animals with large doses of yellow fever virus showed that occasionally animals would completely lose their immunity and circulate virus following the second dose of virus. In some cases it was found that although the animal had lost detectable humoral immunity, no virus could be isolated from the blood, which demonstrated that the intracerebral neutralization test result is not always a reliable indication of the immune status of the animal. Animals with neutralizing antibody at the time of inoculation in no instance circulated virus, thus establishing the specificity of the neutralization test.
The epidemiological implications of the data are discussed.