Cytopathological studies on the multiplication of a small flacherie virus (SFV) in the midgut epithelium of silkworm, Bombyx mori L

A small flacherie virus (SFV) was isolated from a regular flacherie virus (FV) in the diseased silkworm larvae and partially purified. Then, silkworm larvae were inoculated per os by feeding the SFV suspension on the mulberry leaves. On the 7th day after inoculation, the infected larvae were dissected out and their tissues were processed for the optical and electron microscope histopathology. The optical microscope observations revealed that the infected nuclei of columnar cells had swollen and contained a few inclusions which were stained intensely with Feulgen. In other cells, such as goblet cells of the midgut, fat cells and epidermal cells, no remarkable histopathological changes were observed. It was demonstrated by electron microscope observations that the virogenic stroma where SFV particles were produced were formed in the nuclei of columnar cells. Although SFV multiplied independently of FV multiplication in the columnar cell, it was frequently observed that SFV and FV infected doubly in a columnar cell, the former in the nucleus was about 19nm in diameter and the latter in the cytoplasm was about 25nm in diameter. From the results, it was suggested that SFV is a virus species different from FV.