Bioassay of nucleopolyhedrovirus of a silkworm, Bombyx mori L., (BmNPV) discriminating the pathogenicity caused by free virion from that caused by viral occlusion body

Abstract

A bioassay for nucleopolyhedrovirus of Bombyx mori L. (BmNPV) that discriminates the pathogenicity caused by the occlusion derived virus (designated as ODV) from that caused by the viral occlusion body has been developed. Suspensions of the ODV of BmNPV and the viral occlusion body were incubated with sodium ascorbate in various conditions and pathogenicity of the suspensions was bioassayed by using silkworms that had fed on a diet containing Polyoxin AL wettable powder (designated as Polyoxin AL), as an enhancer for BmNPV, prior to this bioassay. An incubation condition that inactivates the ODV but has no effect on the viral occlusion body was determined. The ODV released from the occlusion body by alkalinization was inactivated effectively in incubation with 9.1% (w/w) sodium ascorbate (pH=9.2) for 120 min at 26°C. In contrast, incubation at pH=7.4 without sodium ascorbate had little effect on the ODV. The viral occlusion body was not affected by either condition. Feces of the 4th stadium silkworm that were excreted after ingesting the viral occlusion body were collected and their pathogenicity was bioassayed after incubating them at pH=9.2 with 9.1% sodium ascorbate or at pH=7.4 without sodium ascorbate. Feces that were excreted by larvae just after ingesting the viral occlusion body contained pathogenicity caused only by the free virion, while feces excreted by larvae just before the liquefaction of the larval body by viral infection, which occurred 5 d after the viral inoculation, contained a high degree of pathogenicity caused by the viral occlusion body.