Infections of congenitally athymic (nude) and normal mice with avirulent and virulent strains of Venezuelan encephalitis virus.

Two strains of Venezuelan encephalitis virus that are avirulent for normal BALB/c mice inoculated subcutaneously were also avirulent for infected congenitally athymic (nude) mice of the same strain. Viremias were of similar magnitudes and durations in normal and nude mice. Brain concentrations were higher in nude mice with the one avirulent strain tested, although the periods of detectable virus in brains were similar. No lesions were found in brains, spleens, or lymph nodes by ordinary histopathological examination. Viral neutralizing antibody titers in plasmas at 1 to 3 weeks after infection were lower and more transient in nude than in normal mice, and implantations of thymic tissues into nude mice partially restored their neutralizing antibody responses. Concentrations of spleen cells producing antibodies that lysed sheep erythrocytes 4 days after inoculation of erythrocytes and avirulent virus into nude mice were above the levels of uninfected nude mice. These concentrations were similar in infected and uninfected normal mice. In contrast, two mouse-virulent strains of Venezuelan encephalitis virus killed nude mice faster than normal mice after subcutaneous inoculation. Yet concentrations and durations of virus in bloods and brains were not consistently different between nude and normal mice. There were perivascular monocytes in brains and slight architectural alterations of spleens and lymph nodes. Concentrations of spleen cells producing antibodies hemolytic for sheep erythrocytes 4 days after inoculation with erythrocytes were depressed in nude and normal mice by infection with virulent strains.