CHANGES IN VIRUS-INFECTIVITY TITER IN HEMOLYMPH AND MIDGUT DURING COURSE OF A CYTOPLASMIC POLYHEDROSIS IN SILKWORM

The infectivity titer of cytoplasmic-polyhedrosis virus in the silkworm, Bombyx mori, was investigated by a serial dilution method on larval hemolymph and midgut epithelium of the larvae reared at 25°C after the virus injection on the 1st day of the fifth instar. The virus-infectivity titers of the supernatant of homogenates of the midgut epithelium showed four typical phases, i.e., a decreasing, an eclipse, an increasing, and a stationary phase in the ed50-time curve, which was similar to the growth curves of many other viruses. In the case of hemolymph, however, although the infectivity titers were maintained at relatively high levels throughout the course of the polyhedrosis, no decreasing or increasing phase was observed. The small polyhedra were observed with a light microscope for the first time in the midgut cytoplasm 15 hr after the virus injection, and the polyhedra became larger and more numerous during the following 48 hr. This fact indicates that the midgut epithelium is one of the main sites of multiplication of the cytoplasmic-polyhedrosis virus of the silkworm. © 1969.