Grasserie: The Virus That Bloats Silkworms

The Most Recognisable Silkworm Disease

Grasserie is a viral disease of silkworms caused by a nuclear polyhedrosis virus, and infected larvae are unmistakable: shiny, bloated, and eventually splitting open.

What the Virus Does

Nuclear polyhedrosis viruses belong to the baculovirus family, which infects insects. They replicate inside the nuclei of the host's cells and produce protein crystals — the polyhedra the disease is named for — that package the virus particles and let them survive outside a host.

The infected larva swells as tissue breaks down internally. The characteristic gloss is the skin stretched thin over that fluid. When it ruptures, the contents are dense with infectious material.

Why It Spreads So Well

Those protein-crystal packages are durable, which means a single ruptured larva can contaminate a rearing tray, the leaves, and everything the worms touch. In commercial sericulture that is the whole problem — the disease is not subtle, but by the time it is obvious the exposure has already happened.

The Same Virus Family, Used Deliberately

Here is the turn. Baculoviruses are also one of biotechnology's standard tools. The baculovirus expression system uses these insect viruses to make insect cells manufacture chosen proteins — the same machinery that kills silkworms in a rearing house, harnessed to produce vaccines and research proteins.

The silkworm sits on both sides of that: a species devastated by its own baculovirus, and a species used industrially as a protein factory.

Related: the four silkworm diseases · raising silkworms at home