An electron-microscopic study of avirulent and virulent Semliki forest virus in the brains of different ages of mice.

Ultrastructural studies of brains infected with avirulent and virulent strains of Semliki forest virus (SFV) were performed in 2-7, 14, 19, and 21-28 day old mice. Mature virus particles, dense clumps of fine granules, spherules and advanced stages of viral development i.e. cytopathic vacuoles, Type II (CPV II) are seen in the brains at all ages with the virulent strain which is pathogenic to all age group of mice. In the avirulent strain infection which is pathogenic to mice below 15 days old, no mature virus particles or advanced stages of viral development are seen in 19 day old and adult mice in spite of high virus titres. However, dense clumps of fine granules and spherules are seen which seem to have the capacity to develop into lethal highly infectious mature virus when reinoculated into 2-7 day old mice. It is suggested that the dense clumps of fine granules and spherules are very early viral forms, which are seen in all age groups of mice. The cut-off mechanism in pathogenicity of the avirulent strain occurs in mice around 14 days old. Though some mature virus particles and advanced viral developmental stages are seen in these mice they were much less frequent compared to baby mice. Lymphoblastic type cells are numerous in this age group. These cells are seen more frequently in all ages of mice infected with the avirulent strain as compared to the virulent strain. These mononuclear cells can have an immunological role and may play a part in limiting the process of viral maturation and hence preventing death. Polymorphs and macrophages are rare with the avirulent infection but are the predominant infiltrating cells in the virulent strain infections. With the avirulent infections the astrocytes show hypertrophy and intranuclear inclusions are seen in them.